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Potential energy surfaces
The concept of potential energy surface was very important in the development of TST. The foundation of this concept was laid by René Marcelin in 1913. He theorized that the progress of a chemical reaction could be described as a point in a potential energy surface with coordinates in atomic momenta and distances.
In 1931, Henry Eyring and Michael Polanyi constructed a potential energy surface for the reaction below. This surface is a three-dimensional diagram based on quantum-mechanical principles as well as experimental data on vibrational frequencies and energies of dissociation.
H + H2 → H2 + H
A year after the Eyring and Polanyi construction, Hans Pelzer and Eugene Wigner made an important contribution by following the progress of a reaction on a potential energy surface. The importance of this work was that it was the first time that the concept of col or saddle point in the potential energy surface was discussed. They concluded that the rate of a reaction is determined by the motion of the system through that col.
Kramers theory of reaction rates
By modeling reactions as Langevin motion along a one dimensional reaction coordinate, Hendrik Kramers was able to derive a relationship between the shape of the potential energy surface along the reaction coordinate and the transition rates of the system. The formulation relies on approximating the potential energy landscape as a series of harmonic wells. In a two state system, there will be three wells; a well for state A, an upside-down well representing the potential energy barrier, and a well for state B.
In the overdamped (or "diffusive") regime, the transition rate from state A to B is related to the resonant frequency of the wells via
where is the frequency of the well for state A, is the frequency of the barrier well, is the viscous damping, is the energy of the top of the barrier, is the energy of bottom of the well for state A, and is the temperature of the system times the Boltzmann constant.
For general damping (overdamped or underdamped), there is a similar formula. | Transition state theory | Wikipedia | 431 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Justification for the Eyring equation
One of the most important features introduced by Eyring, Polanyi and Evans was the notion that activated complexes are in quasi-equilibrium with the reactants. The rate is then directly proportional to the concentration of these complexes multiplied by the frequency (kBT/h) with which they are converted into products. Below, a non-rigorous plausibility argument is given for the functional form of the Eyring equation. However, the key statistical mechanical factor kBT/h will not be justified, and the argument presented below does not constitute a true "derivation" of the Eyring equation.
Quasi-equilibrium assumption
Quasi-equilibrium is different from classical chemical equilibrium, but can be described using a similar thermodynamic treatment. Consider the reaction below
{A} + {B} <=> {[AB]^\ddagger} -> {P}
where complete equilibrium is achieved between all the species in the system including activated complexes, [AB]‡ . Using statistical mechanics, concentration of [AB]‡ can be calculated in terms of the concentration of A and B.
TST assumes that even when the reactants and products are not in equilibrium with each other, the activated complexes are in quasi-equilibrium with the reactants. As illustrated in Figure 2, at any instant of time, there are a few activated complexes, and some were reactant molecules in the immediate past, which are designated [ABl]‡ (since they are moving from left to right). The remainder of them were product molecules in the immediate past ([ABr]‡).
In TST, it is assumed that the flux of activated complexes in the two directions are independent of each other. That is, if all the product molecules were suddenly removed from the reaction system, the flow of [ABr]‡ stops, but there is still a flow from left to right. Hence, to be technically correct, the reactants are in equilibrium only with [ABl]‡, the activated complexes that were reactants in the immediate past.
Plausibility argument
The activated complexes do not follow a Boltzmann distribution of energies, but an "equilibrium constant" can still be derived from the distribution they do follow. The equilibrium constant K‡ for the quasi-equilibrium can be written as
.
So, the chemical activity of the transition state AB‡ is
.
Therefore, the rate equation for the production of product is
,
where the rate constant k is given by
. | Transition state theory | Wikipedia | 506 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Here, k‡ is directly proportional to the frequency of the vibrational mode responsible for converting the activated complex to the product; the frequency of this vibrational mode is . Every vibration does not necessarily lead to the formation of product, so a proportionality constant , referred to as the transmission coefficient, is introduced to account for this effect. So k‡ can be rewritten as
.
For the equilibrium constant K‡ , statistical mechanics leads to a temperature dependent expression given as
().
Combining the new expressions for k‡ and K‡, a new rate constant expression can be written, which is given as
.
Since, by definition, ΔG‡ = ΔH‡ –TΔS‡, the rate constant expression can be expanded, to give an alternative form of the Eyring equation:
.
For correct dimensionality, the equation needs to have an extra factor of (c⊖)1–m for reactions that are not unimolecular:
,
where c⊖ is the standard concentration 1 mol⋅L−1 and m is the molecularity.
Inferences from TST and relationship with Arrhenius theory
The rate constant expression from transition state theory can be used to calculate the ΔG‡, ΔH‡, ΔS‡, and even ΔV‡ (the volume of activation) using experimental rate data. These so-called activation parameters give insight into the nature of a transition state, including energy content and degree of order, compared to the starting materials and has become a standard tool for elucidation of reaction mechanisms in physical organic chemistry. The free energy of activation, ΔG‡, is defined in transition state theory to be the energy such that holds. The parameters ΔH‡ and ΔS‡ can then be inferred by determining ΔG‡ = ΔH‡ – TΔS‡ at different temperatures. | Transition state theory | Wikipedia | 370 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Because the functional form of the Eyring and Arrhenius equations are similar, it is tempting to relate the activation parameters with the activation energy and pre-exponential factors of the Arrhenius treatment. However, the Arrhenius equation was derived from experimental data and models the macroscopic rate using only two parameters, irrespective of the number of transition states in a mechanism. In contrast, activation parameters can be found for every transition state of a multistep mechanism, at least in principle. Thus, although the enthalpy of activation, ΔH‡, is often equated with Arrhenius's activation energy Ea, they are not equivalent. For a condensed-phase (e.g., solution-phase) or unimolecular gas-phase reaction step, Ea = ΔH‡ + RT. For other gas-phase reactions, Ea = ΔH‡ + (1 − Δn‡)RT, where Δn‡ is the change in the number of molecules on forming the transition state. (Thus, for a bimolecular gas-phase process, Ea = ΔH‡ + 2RT.) | Transition state theory | Wikipedia | 238 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
The entropy of activation, ΔS‡, gives the extent to which transition state (including any solvent molecules involved in or perturbed by the reaction) is more disordered compared to the starting materials. It offers a concrete interpretation of the pre-exponential factor A in the Arrhenius equation; for a unimolecular, single-step process, the rough equivalence A = (kBT/h) exp(1 + ΔS‡/R) (or A = (kBT/h) exp(2 + ΔS‡/R) for bimolecular gas-phase reactions) holds. For a unimolecular process, a negative value indicates a more ordered, rigid transition state than the ground state, while a positive value reflects a transition state with looser bonds and/or greater conformational freedom. It is important to note that, for reasons of dimensionality, reactions that are bimolecular or higher have ΔS‡ values that depend on the standard state chosen (standard concentration, in particular). For most recent publications, 1 mol L−1 or 1 molar is chosen. Since this choice is a human construct, based on our definitions of units for molar quantity and volume, the magnitude and sign of ΔS‡ for a single reaction is meaningless by itself; only comparisons of the value with that of a reference reaction of "known" (or assumed) mechanism, made at the same standard state, is valid.
The volume of activation is found by taking the partial derivative of ΔG‡ with respect to pressure (holding temperature constant): . It gives information regarding the size, and hence, degree of bonding at the transition state. An associative mechanism will likely have a negative volume of activation, while a dissociative mechanism will likely have a positive value.
Given the relationship between equilibrium constant and the forward and reverse rate constants, , the Eyring equation implies that
.
Another implication of TST is the Curtin–Hammett principle: the product ratio of a kinetically-controlled reaction from R to two products A and B will reflect the difference in the energies of the respective transition states leading to product, assuming there is a single transition state to each one:
().
(In the expression for ΔΔG‡ above, there is an extra term if A and B are formed from two different species SA and SB in equilibrium.) | Transition state theory | Wikipedia | 494 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
For a thermodynamically-controlled reaction, every difference of RT ln 10 ≈ (1.987 × 10−3 kcal/mol K)(298 K)(2.303) ≈ 1.36 kcal/mol in the free energies of products A and B results in a factor of 10 in selectivity at room temperature (298 K), a principle known as the "1.36 rule":
().
Analogously, every 1.36 kcal/mol difference in the free energy of activation results in a factor of 10 in selectivity for a kinetically-controlled process at room temperature:
().
Using the Eyring equation, there is a straightforward relationship between ΔG‡, first-order rate constants, and reaction half-life at a given temperature. At 298 K, a reaction with ΔG‡ = 23 kcal/mol has a rate constant of k ≈ 8.4 × 10−5 s−1 and a half life of t1/2 ≈ 2.3 hours, figures that are often rounded to k ~ 10−4 s−1 and t1/2 ~ 2 h. Thus, a free energy of activation of this magnitude corresponds to a typical reaction that proceeds to completion overnight at room temperature. For comparison, the cyclohexane chair flip has a ΔG‡ of about 11 kcal/mol with k ~ 105 s−1, making it a dynamic process that takes place rapidly (faster than the NMR timescale) at room temperature. At the other end of the scale, the cis/trans isomerization of 2-butene has a ΔG‡ of about 60 kcal/mol, corresponding to k ~ 10−31 s−1 at 298 K. This is a negligible rate: the half-life is 12 orders of magnitude longer than the age of the universe. | Transition state theory | Wikipedia | 391 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Limitations
In general, TST has provided researchers with a conceptual foundation for understanding how chemical reactions take place. Even though the theory is widely applicable, it does have limitations. For example, when applied to each elementary step of a multi-step reaction, the theory assumes that each intermediate is long-lived enough to reach a Boltzmann distribution of energies before continuing to the next step. When the intermediates are very short-lived, TST fails. In such cases, the momentum of the reaction trajectory from the reactants to the intermediate can carry forward to affect product selectivity. An example of such a reaction is the ring closure of cyclopentane biradicals generated from the gas-phase thermal decomposition of 2,3-diazabicyclo[2.2.1]hept-2-ene.
Transition state theory is also based on the assumption that atomic nuclei behave according to classical mechanics. It is assumed that unless atoms or molecules collide with enough energy to form the transition structure, then the reaction does not occur. However, according to quantum mechanics, for any barrier with a finite amount of energy, there is a possibility that particles can still tunnel across the barrier. With respect to chemical reactions this means that there is a chance that molecules will react, even if they do not collide with enough energy to overcome the energy barrier. While this effect is negligible for reactions with large activation energies, it becomes an important phenomenon for reactions with relatively low energy barriers, since the tunneling probability increases with decreasing barrier height.
Transition state theory fails for some reactions at high temperature. The theory assumes the reaction system will pass over the lowest energy saddle point on the potential energy surface. While this description is consistent for reactions occurring at relatively low temperatures, at high temperatures, molecules populate higher energy vibrational modes; their motion becomes more complex and collisions may lead to transition states far away from the lowest energy saddle point. This deviation from transition state theory is observed even in the simple exchange reaction between diatomic hydrogen and a hydrogen radical.
Given these limitations, several alternatives to transition state theory have been proposed. A brief discussion of these theories follows.
Generalized transition state theory
Any form of TST, such as microcanonical variational TST, canonical variational TST, and improved canonical variational TST, in which the transition state is not necessarily located at the saddle point, is referred to as generalized transition state theory. | Transition state theory | Wikipedia | 496 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Microcanonical variational TST
A fundamental flaw of transition state theory is that it counts any crossing of the transition state as a reaction from reactants to products or vice versa. In reality, a molecule may cross this "dividing surface" and turn around, or cross multiple times and only truly react once. As such, unadjusted TST is said to provide an upper bound for the rate coefficients. To correct for this, variational transition state theory varies the location of the dividing surface that defines a successful reaction in order to minimize the rate for each fixed energy. The rate expressions obtained in this microcanonical treatment can be integrated over the energy, taking into account the statistical distribution over energy states, so as to give the canonical, or thermal rates.
Canonical variational TST
A development of transition state theory in which the position of the dividing surface is varied so as to minimize the rate constant at a given temperature.
Improved canonical variational TST
A modification of canonical variational transition state theory in which, for energies below the threshold energy, the position of the dividing surface is taken to be that of the microcanonical threshold energy. This forces the contributions to rate constants to be zero if they are below the threshold energy. A compromise dividing surface is then chosen so as to minimize the contributions to the rate constant made by reactants having higher energies.
Nonadiabatic TST
An expansion of TST to the reactions when two spin-states are involved simultaneously is called nonadiabatic transition state theory (NA-TST).
Semiclassical TST
Using vibrational perturbation theory, effects such as tunnelling and variational effects can be accounted for within the SCTST formalism.
Applications | Transition state theory | Wikipedia | 348 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
Enzymatic reactions
Enzymes catalyze chemical reactions at rates that are astounding relative to uncatalyzed chemistry at the same reaction conditions. Each catalytic event requires a minimum of three or often more steps, all of which occur within the few milliseconds that characterize typical enzymatic reactions. According to transition state theory, the smallest fraction of the catalytic cycle is spent in the most important step, that of the transition state. The original proposals of absolute reaction rate theory for chemical reactions defined the transition state as a distinct species in the reaction coordinate that determined the absolute reaction rate. Soon thereafter, Linus Pauling proposed that the powerful catalytic action of enzymes could be explained by specific tight binding to the transition state species Because reaction rate is proportional to the fraction of the reactant in the transition state complex, the enzyme was proposed to increase the concentration of the reactive species.
This proposal was formalized by Wolfenden and coworkers at University of North Carolina at Chapel Hill, who hypothesized that the rate increase imposed by enzymes is proportional to the affinity of the enzyme for the transition state structure relative to the Michaelis complex. Because enzymes typically increase the non-catalyzed reaction rate by factors of 106-1026, and Michaelis complexes often have dissociation constants in the range of 10−3-10−6 M, it is proposed that transition state complexes are bound with dissociation constants in the range of 10−14 -10−23 M. As substrate progresses from the Michaelis complex to product, chemistry occurs by enzyme-induced changes in electron distribution in the substrate. Enzymes alter the electronic structure by protonation, proton abstraction, electron transfer, geometric distortion, hydrophobic partitioning, and interaction with Lewis acids and bases. Analogs that resemble the transition state structures should therefore provide the most powerful noncovalent inhibitors known.
All chemical transformations pass through an unstable structure called the transition state, which is poised between the chemical structures of the substrates and products. The transition states for chemical reactions are proposed to have lifetimes near 10−13 seconds, on the order of the time of a single bond vibration. No physical or spectroscopic method is available to directly observe the structure of the transition state for enzymatic reactions, yet transition state structure is central to understanding enzyme catalysis since enzymes work by lowering the activation energy of a chemical transformation. | Transition state theory | Wikipedia | 485 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
It is now accepted that enzymes function to stabilize transition states lying between reactants and products, and that they would therefore be expected to bind strongly any inhibitor that closely resembles such a transition state. Substrates and products often participate in several enzyme catalyzed reactions, whereas the transition state tends to be characteristic of one particular enzyme, so that such an inhibitor tends to be specific for that particular enzyme. The identification of numerous transition state inhibitors supports the transition state stabilization hypothesis for enzymatic catalysis.
Currently there is a large number of enzymes known to interact with transition state analogs, most of which have been designed with the intention of inhibiting the target enzyme. Examples include HIV-1 protease, racemases, β-lactamases, metalloproteinases, cyclooxygenases and many others.
Adsorption on surfaces and reactions on surfaces
Desorption as well as reactions on surfaces are straightforward to describe with transition state theory. Analysis of adsorption to a surface from a liquid phase can present a challenge due to lack of ability to assess the concentration of the solute near the surface. When full details are not available, it has been proposed that reacting species' concentrations should be normalized to the concentration of active surface sites, an approximation called the surface reactant equi-density approximation (SREA). | Transition state theory | Wikipedia | 273 | 9217017 | https://en.wikipedia.org/wiki/Transition%20state%20theory | Physical sciences | Kinetics | Chemistry |
A ceramic capacitor is a fixed-value capacitor where the ceramic material acts as the dielectric. It is constructed of two or more alternating layers of ceramic and a metal layer acting as the electrodes. The composition of the ceramic material defines the electrical behavior and therefore applications. Ceramic capacitors are divided into two application classes:
Class 1 ceramic capacitors offer high stability and low losses for resonant circuit applications.
Class 2 ceramic capacitors offer high volumetric efficiency for buffer, by-pass, and coupling applications.
Ceramic capacitors, especially multilayer ceramic capacitors (MLCCs), are the most produced and used capacitors in electronic equipment that incorporate approximately one trillion (1012) pieces per year.
Ceramic capacitors of special shapes and styles are used as capacitors for RFI/EMI suppression, as feed-through capacitors and in larger dimensions as power capacitors for transmitters.
History
Since the beginning of the study of electricity non-conductive materials such as glass, porcelain, paper and mica have been used as insulators. These materials some decades later were also well-suited for further use as the dielectric for the first capacitors.
Even in the early years of Marconi's wireless transmitting apparatus, porcelain capacitors were used for high voltage and high frequency application in the transmitters. On the receiver side, the smaller mica capacitors were used for resonant circuits. Mica dielectric capacitors were invented in 1909 by William Dubilier. Prior to World War II, mica was the most common dielectric for capacitors in the United States. | Ceramic capacitor | Wikipedia | 345 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Mica is a natural material and not available in unlimited quantities. So in the mid-1920s the deficiency of mica in Germany and the experience in porcelain—a special class of ceramic—led in Germany to the first capacitors using ceramic as dielectric, founding a new family of ceramic capacitors. Paraelectric titanium dioxide (rutile) was used as the first ceramic dielectric because it had a linear temperature dependence of capacitance for temperature compensation of resonant circuits and can replace mica capacitors. In 1926 these ceramic capacitors were produced in small quantities with increasing quantities in the 1940s. The style of these early ceramics was a disc with metallization on both sides contacted with tinned wires. This style predates the transistor and was used extensively in vacuum-tube equipment (e.g., radio receivers) from about 1930 through the 1950s.
But this paraelectric dielectric had relatively low permittivity so that only small capacitance values could be realized. The expanding market of radios in the 1930s and 1940s create a demand for higher capacitance values but below electrolytic capacitors for HF decoupling applications. Discovered in 1921, the ferroelectric ceramic material barium titanate with a permittivity in the range of 1,000, about ten times greater than titanium dioxide or mica, began to play a much larger role in electronic applications.
The higher permittivity resulted in much higher capacitance values, but this was coupled with relatively unstable electrical parameters. Therefore, these ceramic capacitors only could replace the commonly used mica capacitors for applications where stability was less important. Smaller dimensions, as compared to the mica capacitors, lower production costs and independence from mica availability accelerated their acceptance.
The fast-growing broadcasting industry after the Second World War drove deeper understanding of the crystallography, phase transitions and the chemical and mechanical optimization of the ceramic materials. Through the complex mixture of different basic materials, the electrical properties of ceramic capacitors can be precisely adjusted. To distinguish the electrical properties of ceramic capacitors, standardization defined several different application classes (Class 1, Class 2, Class 3). It is remarkable that the separate development during the War and the time afterwards in the US and the European market had led to different definitions of these classes (EIA vs IEC), and only recently (since 2010) has a worldwide harmonization to the IEC standardization taken place. | Ceramic capacitor | Wikipedia | 512 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The typical style for ceramic capacitors beneath the disc (at that time called condensers) in radio applications at the time after the War from the 1950s through the 1970s was a ceramic tube covered with tin or silver on both the inside and outside surface. It included relatively long terminals forming, together with resistors and other components, a tangle of open circuit wiring.
The easy-to-mold ceramic material facilitated the development of special and large styles of ceramic capacitors for high-voltage, high-frequency (RF) and power applications.
With the development of semiconductor technology in the 1950s, barrier layer capacitors, or IEC class 3/EIA class IV capacitors, were developed using doped ferroelectric ceramics. Because this doped material was not suitable to produce multilayers, they were replaced decades later by Y5V class 2 capacitors.
The early style of the ceramic disc capacitor could be more cheaply produced than the common ceramic tube capacitors in the 1950s and 1970s. An American company in the midst of the Apollo program, launched in 1961, pioneered the stacking of multiple discs to create a monolithic block. This "multi-layer ceramic capacitor" (MLCC) was compact and offered high-capacitance capacitors. The production of these capacitors using the tape casting and ceramic-electrode cofiring processes was a great manufacturing challenge. MLCCs expanded the range of applications to those requiring larger capacitance values in smaller cases. These ceramic chip capacitors were the driving force behind the conversion of electronic devices from through-hole mounting to surface-mount technology in the 1980s. Polarized electrolytic capacitors could be replaced by non-polarized ceramic capacitors, simplifying the mounting.
In 1993, TDK Corporation succeeded in displacing palladium bearing electrodes with much cheaper nickel electrodes, significantly reducing production costs and enabling mass production of MLCCs.
, more than 1012 MLCCs are manufactured each year. Along with the style of ceramic chip capacitors, ceramic disc capacitors are often used as safety capacitors in electromagnetic interference suppression applications. Besides these, large ceramic power capacitors for high voltage or high frequency transmitter applications are also to be found. | Ceramic capacitor | Wikipedia | 473 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
New developments in ceramic materials have been made with anti-ferroelectric ceramics. This material has a nonlinear antiferroelectric/ferroelectric phase change that allows increased energy storage with higher volumetric efficiency. They are used for energy storage (for example, in detonators).
Application classes, definitions
The different ceramic materials used for ceramic capacitors, paraelectric or ferroelectric ceramics, influences the electrical characteristics of the capacitors. Using mixtures of paraelectric substances based on titanium dioxide results in very stable and linear behavior of the capacitance value within a specified temperature range and low losses at high frequencies. But these mixtures have a relatively low permittivity so that the capacitance values of these capacitors are relatively small.
Higher capacitance values for ceramic capacitors can be attained by using mixtures of ferroelectric materials like barium titanate together with specific oxides. These dielectric materials have much higher permittivities, but at the same time their capacitance value are more or less nonlinear over the temperature range, and losses at high frequencies are much higher. These different electrical characteristics of ceramic capacitors requires to group them into "application classes". The definition of the application classes comes from the standardization. As of 2013, two sets of standards were in use, one from International Electrotechnical Commission (IEC) and the other from the now-defunct Electronic Industries Alliance (EIA).
The definitions of the application classes given in the two standards are different. The following table shows the different definitions of the application classes for ceramic capacitors:
Manufacturers, especially in the US, preferred Electronic Industries Alliance (EIA) standards. In many parts very similar to the IEC standard, the EIA RS-198 defines four application classes for ceramic capacitors.
The different class numbers within both standards are the reason for a lot of misunderstandings interpreting the class descriptions in the datasheets of many manufacturers. The EIA ceased operations on February 11, 2011, but the former sectors continue to serve international standardization organizations.
In the following, the definitions of the IEC standard will be preferred and in important cases compared with the definitions of the EIA standard. | Ceramic capacitor | Wikipedia | 460 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Class 1 ceramic capacitors
Class 1 ceramic capacitors are accurate, temperature-compensating capacitors. They offer the most stable voltage, temperature, and to some extent, frequency. They have the lowest losses and therefore are especially suited for resonant circuit applications where stability is essential or where a precisely defined temperature coefficient is required, for example in compensating temperature effects for a circuit. The basic materials of class 1 ceramic capacitors are composed of a mixture of finely ground granules of paraelectric materials such as titanium dioxide (), modified by additives of zinc, zirconium, niobium, magnesium, tantalum, cobalt and strontium, which are necessary to achieve the capacitor's desired linear characteristics.
The general capacitance temperature behavior of class 1 capacitors depends on the basic paraelectric material, for example . The additives of the chemical composition are used to adjust precisely the desired temperature characteristic.
Class 1 ceramic capacitors have the lowest volumetric efficiency among ceramic capacitors. This is the result of the relatively low permittivity (6 to 200) of paraelectric materials. Therefore, class 1 capacitors have capacitance values in the lower range.
Class 1 capacitors have a temperature coefficient that is typically fairly linear with temperature. These capacitors have very low electrical losses with a dissipation factor of approximately 0.15%. They undergo no significant aging processes and the capacitance value is nearly independent of the applied voltage. These characteristics allow applications for high Q filters, in resonant circuits and oscillators (for example, in phase-locked loop circuits).
The EIA RS-198 standard codes ceramic class 1 capacitors with a three character code that indicates temperature coefficient. The first letter gives the significant figure of the change in capacitance over temperature (temperature coefficient α) in ppm/K. The second character gives the multiplier of the temperature coefficient. The third letter gives the maximum tolerance from that in ppm/K. All ratings are from 25 to 85 °C: | Ceramic capacitor | Wikipedia | 441 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
In addition to the EIA code, the temperature coefficient of the capacitance dependence of class 1 ceramic capacitors is commonly expressed in ceramic names like "NP0", "N220" etc. These names include the temperature coefficient (α). In the IEC/EN 60384-8/21 standard, the temperature coefficient and tolerance are replaced by a two digit letter code (see table) in which the corresponding EIA code is added.
For instance, an "NP0" capacitor with EIA code "C0G" will have 0 drift, with a tolerance of ±30 ppm/K, while an "N1500" with the code "P3K" will have −1500 ppm/K drift, with a maximum tolerance of ±250 ppm/K. Note that the IEC and EIA capacitor codes are industry capacitor codes and not the same as military capacitor codes.
Class 1 capacitors include capacitors with different temperature coefficients α. Especially, NP0/CG/C0G capacitors with an α ±0•10−6 /K and an α tolerance of 30 ppm are technically of great interest. These capacitors have a capacitance variation dC/C of ±0.54% within the temperature range −55 to +125 °C. This enables accurate frequency response over a wide temperature range (in, for example, resonant circuits). The other materials with their special temperature behavior are used to compensate a counter temperature run of parallel connected components like coils in oscillator circuits. Class 1 capacitors exhibit very small tolerances of the rated capacitance.
Class 2 ceramic capacitors
Class 2 ceramic capacitors have a dielectric with a high permittivity and therefore a better volumetric efficiency than class 1 capacitors, but lower accuracy and stability. The ceramic dielectric is characterized by a nonlinear change of capacitance over the temperature range. The capacitance value also depends on the applied voltage. They are suitable for bypass, coupling and decoupling applications or for frequency discriminating circuits where low losses and high stability of capacitance are less important. They typically exhibit microphony. | Ceramic capacitor | Wikipedia | 471 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Class 2 capacitors are made of ferroelectric materials such as barium titanate () and suitable additives such as aluminium silicate, magnesium silicate and aluminium oxide. These ceramics have very high permittivity (200 to 14,000), allowing an extreme electric field and therefore capacitance within relatively small packages — class 2 capacitors are significantly smaller than comparable class 1 capacitors. However, the permittivity is nonlinear with respect to field strength, meaning the capacitance varies significantly as the voltage across the terminals increases. Class 2 capacitors also exhibit poor temperature stability and age over time.
Due to these traits, class 2 capacitors are typically used in applications where only a minimum value of capacitance (as opposed to an accurate value) is required, such as the buffering/filtering of inputs and outputs of power supplies, and the coupling of electric signals.
Class 2 capacitors are labeled according to the change in capacitance over the temperature range. The most widely used classification is based on the EIA RS-198 standard and uses a three-digit code. The first character, a letter, denotes the coldest operating temperature; the second character, a numeral, denotes the hottest temperature; and the third character, another letter, denotes the maximum allowed capacitance change over the capacitor's entire specified temperature range:
For instance, a Z5U capacitor will operate from +10 °C to +85 °C with a capacitance change of at most +22% to −56%. An X7R capacitor will operate from −55 °C to +125 °C with a capacitance change of at most ±15%. | Ceramic capacitor | Wikipedia | 360 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Some commonly used class 2 ceramic capacitor materials are listed below:
X8R (−55/+150, ΔC/C0 = ±15%),
X7R (−55/+125 °C, ΔC/C0 = ±15%),
X6R (−55/+105 °C, ΔC/C0 = ±15%),
X5R (−55/+85 °C, ΔC/C0 = ±15%),
X7S (−55/+125, ΔC/C0 = ±22%),
Z5U (+10/+85 °C, ΔC/C0 = +22/−56%),
Y5V (−30/+85 °C, ΔC/C0 = +22/−82%),
The IEC/EN 60384 -9/22 standard uses another two-digit-code.
In most cases it is possible to translate the EIA code into the IEC/EN code. Slight translation errors occur, but normally are tolerable.
X7R correlates with 2X1
Z5U correlates with 2E6
Y5V similar to 2F4, aberration: ΔC/C0 = +30/−80% instead of +30/−82%
X7S similar to 2C1, aberration: ΔC/C0 = ±20% instead of ±22%
X8R no IEC/EN code available
Because class 2 ceramic capacitors have lower capacitance accuracy and stability, they require higher tolerance.
For military types the class 2 dielectrics specify temperature characteristic (TC) but not temperature-voltage characteristic (TVC). Similar to X7R, military type BX cannot vary more than 15% over temperature, and in addition, must remain within +15%/-25 % at maximum rated voltage. Type BR has a TVC limit of +15%/-40%. | Ceramic capacitor | Wikipedia | 417 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Class 3 ceramic capacitors
Class 3 barrier layer or semiconductive ceramic capacitors have very high permittivity, up to 50,000 and therefore a better volumetric efficiency than class 2 capacitors. However, these capacitors have worse electrical characteristics, including lower accuracy and stability. The dielectric is characterized by very high nonlinear change of capacitance over the temperature range. The capacitance value additionally depends on the voltage applied. As well, they have very high losses and age over time.
Barrier layer ceramic capacitors are made of doped ferroelectric materials such as barium titanate (). As this ceramic technology improved in the mid-1980s, barrier layer capacitors became available in values of up to 100 μF, and at that time it seemed that they could substitute for smaller electrolytic capacitors.
Because it is not possible to build multilayer capacitors with this material, only leaded single layer types are offered in the market.
Due to advancements in multilayer ceramic capacitors enabling superior performance in a smaller package, barrier layer capacitors as a technology are now considered obsolete and no longer standardized by the IEC.
Construction and styles
Ceramic capacitors are composed of a mixture of finely ground granules of paraelectric or ferroelectric materials, appropriately mixed with other materials to achieve the desired characteristics. From these powder mixtures, the ceramic is sintered at high temperatures. The ceramic forms the dielectric and serves as a carrier for the metallic electrodes. The minimum thickness of the dielectric layer, which today (2013) for low voltage capacitors is in the size range of 0.5 micrometers is limited downwards by the grain size of the ceramic powder. The thickness of the dielectric for capacitors with higher voltages is determined by the dielectric strength of the desired capacitor.
The electrodes of the capacitor are deposited on the ceramic layer by metallization. For MLCCs alternating metallized ceramic layers are stacked one above the other. The outstanding metallization of the electrodes at both sides of the body are connected with the contacting terminal. A lacquer or ceramic coating protects the capacitor against moisture and other ambient influences. | Ceramic capacitor | Wikipedia | 468 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Ceramic capacitors come in various shapes and styles. Some of the most common are:
Multilayer ceramic chip capacitor (MLCC), rectangular block, for surface mounting
Ceramic disc capacitor, single layer disc, resin coated, with through-hole leads
Feedthrough ceramic capacitor, used for bypass purposes in high-frequency circuits. Tube shape, inner metallization contacted with a lead, outer metallization for soldering
Ceramic power capacitors, larger ceramic bodies in different shapes for high voltage applications
Multi-layer ceramic capacitors (MLCC)
Manufacturing
An MLCC can be thought of as consisting of many single-layer capacitors stacked together into a single package. The starting material for all MLCC chips is a mixture of finely ground granules of paraelectric or ferroelectric raw materials, modified by accurately determined additives. The composition of the mixture and the size of the powder particles, as small as 10 nm, reflect the manufacturer's expertise.
A thin ceramic foil is cast from a suspension of the powder with a suitable binder. Rolls of foil are cut into equal-sized sheets, which are screen printed with a metal paste layer, which will become the electrodes. In an automated process, these sheets are stacked in the required number of layers and solidified by pressure. Besides the relative permittivity, the size and number of layers determines the later capacitance value. The electrodes are stacked in an alternating arrangement slightly offset from the adjoining layers so that they each can later be connected on the offset side, one left, one right. The layered stack is pressed and then cut into individual components. High mechanical precision is required, for example, to produce a 500 or more layer stack of size "0201" (0.5 mm × 0.3 mm).
After cutting, the binder is burnt out of the stack. This is followed by sintering at temperatures between , producing the final, mainly crystalline, structure. This burning process creates the desired dielectric properties. Burning is followed by cleaning and then metallization of both end surfaces. Through the metallization, the ends and the inner electrodes are connected in parallel and the capacitor gets its terminals. Finally, each capacitor is electrically tested to ensure functionality and adequate performance, and packaged in a tape reel. | Ceramic capacitor | Wikipedia | 478 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Miniaturizing
The capacitance formula (C) of a MLCC capacitor is based on the formula for a plate capacitor enhanced with the number of layers:
where ε stands for dielectric permittivity; A for electrode surface area; n for the number of layers; and d for the distance between the electrodes.
A thinner dielectric or a larger electrode area each increase the capacitance value, as will a dielectric material of higher permittivity.
With the progressive miniaturization of digital electronics in recent decades, the components on the periphery of the integrated logic circuits have been scaled down as well. Shrinking an MLCC involves reducing the dielectric thickness and increasing the number of layers. Both options require huge efforts and are connected with a lot of expertise.
In 1995 the minimum thickness of the dielectric was 4 μm. By 2005 some manufacturers produced MLCC chips with layer thicknesses of 1 μm. , the minimum thickness is about 0.5 μm. The field strength in the dielectric increased to 35 V/μm.
The size reduction of these capacitors is achieved reducing powder grain size, the assumption to make the ceramic layers thinner. In addition, the manufacturing process became more precisely controlled, so that more and more layers can be stacked.
Between 1995 and 2005, the capacitance of a Y5V MLCC capacitor of size 1206 was increased from 4.7 μF to 100 μF. Meanwhile, (2013) a lot of producers can deliver class 2 MLCC capacitors with a capacitance value of 100 μF in the chip-size 0805.
MLCC case sizes
MLCCs don't have leads, and as a result they are usually smaller than their counterparts with leads. They don't require through-hole access in a PCB to mount and are designed to be handled by machines rather than by humans. As a result, surface-mount components like MLCCs are typically cheaper. | Ceramic capacitor | Wikipedia | 418 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
MLCCs are manufactured in standardized shapes and sizes for comparable handling. Because the early standardization was dominated by American EIA standards the dimensions of the MLCC chips were standardized by EIA in units of inches. A rectangular chip with the dimensions of 0.06-inch length and 0.03-inch width is coded as "0603". This code is international and in common use. JEDEC (IEC/EN), devised a second, metric code. The EIA code and the metric equivalent of the common sizes of multilayer ceramic chip capacitors, and the dimensions in mm are shown in the following table. Missing from the table is the measure of the height "H". This is generally not listed, because the height of MLCC chips depends on the number of layers and thus on the capacitance. Normally, however, the height H does not exceed the width W.
NME and BME metallization
Originally, MLCC electrodes were constructed out of noble metals such as silver and palladium which can withstand high sintering temperatures of without readily oxidizing. These noble metal electrode (NME) capacitors offered very good electrical properties.
However, a surge in prices of noble metals in the late 1990s greatly increased manufacturing costs; these pressures resulted in the development of capacitors that used cheaper metals like copper and nickel. These base metal electrode (BME) capacitors possessed poorer electrical characteristics; exhibiting greater shrinkage of capacitance at higher voltages and increased loss factor.
The disadvantages of BME were deemed acceptable for class 2 capacitors, which are primarily used in accuracy-insensitive, low-cost applications such as power supplies. NME still sees use in class 1 capacitors where conformance to specifications are critical and cost is less of a concern.
MLCC capacitance ranges
Capacitance of MLCC chips depends on the dielectric, the size and the required voltage (rated voltage). Capacitance values start at about 1pF. The maximum capacitance value is determined by the production technique. For X7R that is 47 μF, for Y5V: 100 μF. | Ceramic capacitor | Wikipedia | 448 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The picture right shows the maximum capacitance for class 1 and class 2 multilayer ceramic chip capacitors. The following two tables, for ceramics NP0/C0G and X7R each, list for each common case size the maximum available capacitance value and rated voltage of the leading manufacturers Murata, TDK, KEMET, AVX. (Status April 2017)
Low-ESL styles
In the region of its resonance frequency, a capacitor has the best decoupling properties for noise or electromagnetic interference. The resonance frequency of a capacitor is determined by the inductance of the component. The inductive parts of a capacitor are summarized in the equivalent series inductance, or ESL. (Note that L is the electrical symbol for inductance.) The smaller the inductance, the higher the resonance frequency.
Because, especially in digital signal processing, switching frequencies have continued to rise, the demand for high frequency decoupling or filter capacitors increases. With a simple design change the ESL of an MLCC chip can be reduced. Therefore, the stacked electrodes are connected on the longitudinal side with the connecting terminations. This reduces the distance that the charge carriers flow over the electrodes, which reduces inductance of the component.
For example, an 0.1 μF X7R MLCC in a 0805 package resonates at 16 MHz. The same capacitor with leads on its long sides (i.e. an 0508) has a resonance frequency of 22 MHz.
Another possibility is to form the device as an array of capacitors. Here, several individual capacitors are built in a common housing. Connecting them in parallel, the resulting ESL as well as ESR values of the components are reduced.
X2Y decoupling capacitor | Ceramic capacitor | Wikipedia | 383 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
A standard multi-layer ceramic capacitor has many opposing electrode layers stacked inside connected with two outer terminations. The X2Y ceramic chip capacitor however is a 4 terminal chip device. It is constructed like a standard two-terminal MLCC out of the stacked ceramic layers with an additional third set of shield electrodes incorporated in the chip. These shield electrodes surround each existing electrode within the stack of the capacitor plates and are low ohmic contacted with two additional side terminations across to the capacitor terminations. The X2Y construction results in a three-node capacitive circuit that provides simultaneous line-to-line and line-to-ground filtering.
Capable of replacing 2 or more conventional devices, the X2Y ceramic capacitors are ideal for high frequency filtering or noise suppression of supply voltages in digital circuits, and can prove invaluable in meeting stringent EMC demands in dc motors, in automotive, audio, sensor and other applications.
The X2Y footprint results in lower mounted inductance. This is particularly of interest for use in high-speed digital circuits with clock rates of several 100 MHz and upwards. There the decoupling of the individual supply voltages on the circuit board is difficult to realize due to parasitic inductances of the supply lines. A standard solution with conventional ceramic capacitors requires the parallel use of many conventional MLCC chips with different capacitance values. Here X2Y capacitors are able to replace up to five equal-sized ceramic capacitors on the PCB. However, this particular type of ceramic capacitor is patented, so these components are still comparatively expensive.
An alternative to X2Y capacitors may be a three-terminal capacitor. | Ceramic capacitor | Wikipedia | 362 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Mechanical susceptibility
Ceramics are brittle, and MLCC chips surface-mount soldered to a circuit board are often vulnerable to cracking from thermal expansion or mechanical stresses like depanelization, more so than leaded through-hole components.
The cracks can come from automated machine assembly line, or from high current in the circuit.
Vibration and shock forces on the circuit board are more or less transmitted undampened to the MLCC and its solder joints; excessive force may cause the capacitor to crack (flex crack). Excess solder in the joints are undesirable as they may magnify the forces that the capacitor is subject to.
The capability of MLCC chips to withstand mechanical stress is tested by a so-called substrate bending test, where a PCB with a soldered MLCC is bent by a punch by 1 to 3 mm. Failure occurs if the MLCC becomes a short-circuit or significantly changes in capacitance.
Bending strengths of MLCC chips differ by the ceramic material, the size of the chip, and the physical construction of the capacitors. Without special mitigation, NP0/C0G class 1 ceramic MLCC chips reach a typical bending strength of 2 mm while larger types of X7R, Y5V class 2 ceramic chips achieved only a bending strength of approximately 1 mm. Smaller chips, such as the size of 0402, reached in all types of ceramics larger bending strength values.
With special design features, particularly at the electrodes and terminations, the bending strength can be improved. For example, an internal short circuit arises by the contact of two electrodes with opposite polarity, which will be produced at the break of the ceramic in the region of the terminations. This can be prevented when the overlap surfaces of the electrodes are reduced. This is achieved e.g. by an "Open Mode Design" (OMD). Here a break in the region of the terminations only reduce the capacitance value a little bit (AVX, KEMET). | Ceramic capacitor | Wikipedia | 422 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
With a similar construction called "Floating Electrode Design" (FED) or "Multi-layer Serial Capacitors" (MLSC), also, only capacitance reduction results if parts of the capacitor body break. This construction works with floating electrodes without any conductive connection to the termination. A break doesn't lead to a short, only to capacitance reduction.
However, both structures lead to larger designs with respect to a standard MLCC version with the same capacitance value.
The same volume with respect to standard MLCCs is achieved by the introduction of a flexible intermediate layer of a conductive polymer between the electrodes and the termination called "Flexible Terminations" (FT-Cap) or "Soft Terminations". In this construction, the rigid metallic soldering connection can move against the flexible polymer layer, and thus can absorb the bending forces, without resulting in a break in the ceramic.
Some automotive capacitors are specified to adhere to AEC-Q200 and/or VW 80808.
RFI/EMI suppression with X- and Y capacitors
Suppression capacitors are effective interference reduction components because their electrical impedance decreases with increasing frequency, such that at higher frequencies they appear as short circuits to high-frequency electrical noise and transients between the lines, or to ground. They therefore prevent equipment and machinery (including motors, inverters, and electronic ballasts, as well as solid-state relay snubbers and spark quenchers) from sending and receiving electromagnetic and radio frequency interference as well as transients in across-the-line (X capacitors) and line-to-ground (Y capacitors) connections. X capacitors effectively absorb symmetrical, balanced, or differential interference. Y capacitors are connected in a line bypass between a line phase and a point of zero potential, to absorb asymmetrical, unbalanced, or common-mode interference. | Ceramic capacitor | Wikipedia | 402 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
EMI/RFI suppression capacitors are designed so that any remaining interference or electrical noise does not exceed the limits of EMC directive EN 50081. Suppression components are connected directly to mains voltage for 10 to 20 years or more and are therefore exposed to potentially damaging overvoltages and transients. For this reason, suppression capacitors must comply with the safety and non-flammability requirements of international safety standards such as
Europe: EN 60384-14,
USA: UL 1414, UL 1283
Canada: CSA C22.2, No.1, CSA C22.2, No.8
China: CQC (GB/T 14472-1998)
RFI capacitors that fulfill all specified requirements are imprinted with the certification mark of various national safety standards agencies. For power line applications, special requirements are placed on the non-flammability of the coating and the epoxy resin impregnating or coating the capacitor body. To receive safety approvals, X and Y powerline-rated capacitors are destructively tested to the point of failure. Even when exposed to large overvoltage surges, these safety-rated capacitors must fail in a fail-safe manner that does not endanger personnel or property.
most ceramic capacitors used for EMI/RFI suppression were leaded ones for through-hole mounting on a PCB, the surface-mount technique is becoming more and more important. For this reason, in recent years a lot of MLCC chips for EMI/RFI suppression from different manufacturers have received approvals and fulfill all requirements given in the applicable standards.
Ceramic power capacitors
Although the materials used for large power ceramic capacitors mostly are very similar to those used for smaller ones, ceramic capacitors with high to very high power or voltage ratings for applications in power systems, transmitters and electrical installations are often classified separately, for historical reasons. The standardization of ceramic capacitors for lower power is oriented toward electrical and mechanical parameters as components for use in electronic equipment. The standardization of power capacitors, contrary to that, is strongly focused on protecting personnel and equipment, given by the local regulating authority. | Ceramic capacitor | Wikipedia | 454 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
As modern electronic equipment gained the ability to handle power levels that were previously the exclusive domain of "electrical power" components, the distinction between the "electronic" and "electrical" power ratings has become less distinct. In the past, the boundary between these two families was approximately at a reactive power of 200 volt-amps, but modern power electronics can handle increasing amounts of power.
Power ceramic capacitors are mostly specified for much higher than 200 volt-amps. The great plasticity of ceramic raw material and the high dielectric strength of ceramics deliver solutions for many applications and are the reasons for the enormous diversity of styles within the family of power ceramic capacitors. These power capacitors have been on the market for decades. They are produced according to the requirements as class 1 power ceramic capacitors with high stability and low losses or class 2 power ceramic capacitors with high volumetric efficiency.
Class 1 power ceramic capacitors are used for resonant circuit application in transmitter stations. Class 2 power ceramic capacitors are used for circuit breakers, for power distribution lines, for high voltage power supplies in laser-applications, for induction furnaces and in voltage-doubling circuits. Power ceramic capacitors can be supplied with high rated voltages in the range of 2 kV up to 100 kV.
The dimensions of these power ceramic capacitors can be very large. At high power applications the losses of these capacitors can generate a lot of heat. For this reason some special styles of power ceramic capacitors have pipes for water-cooling.
Electrical characteristics
Series-equivalent circuit
All electrical characteristics of ceramic capacitors can be defined and specified by a series equivalent circuit composed out of an idealized capacitance and additional electrical components, which model all losses and inductive parameters of a capacitor. In this series-equivalent circuit the electrical characteristics of a capacitors is defined by
C, the capacitance of the capacitor,
Rinsul, the insulation resistance of the dielectric, not to be confused with the insulation of the housing
RESR, the equivalent series resistance, which summarizes all ohmic losses of the capacitor, usually abbreviated as "ESR".
LESL, the equivalent series inductance, which is the effective self-inductance of the capacitor, usually abbreviated as "ESL". | Ceramic capacitor | Wikipedia | 489 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The use of a series equivalent circuit instead of a parallel equivalent circuit is defined in IEC/EN 60384-1.
Capacitance standard values and tolerances
The "rated capacitance" CR or "nominal capacitance" CN is the value for which the capacitor has been designed. The actual capacitance depends on the measuring frequency and the ambient temperature. Standardized conditions for capacitors are a low-voltage AC measuring method at a temperature of 20 °C with frequencies of
Class 1 ceramic capacitors
CR ≤ 100 pF at 1 MHz, measuring voltage 5 V
CR > 100 pF at 1 kHz, measuring voltage 5 V
Class 2 ceramic capacitors
CR ≤ 100 pF at 1 MHz, measuring voltage 1 V
100 pF < CR ≤ 10 μF at 1 kHz, measuring voltage 1 V
CR > 10 μF at 100/120 Hz, measuring voltage 0.5 V
Capacitors are available in different, geometrically increasing preferred values as specified in the E series standards specified in IEC/EN 60063. According to the number of values per decade, these were called the E3, E6, E12, E24, etc. series. The units used to specify capacitor values includes everything from picofarad (pF), nanofarad (nF), microfarad (μF) and farad (F).
The percentage of allowed deviation of the capacitance from the rated value is called capacitance tolerance. The actual capacitance value must be within the tolerance limits, or the capacitor is out of specification. For abbreviated marking in tight spaces, a letter code for each tolerance is specified in IEC/EN 60062.
The required capacitance tolerance is determined by the particular application. The narrow tolerances of E24 to E96 will be used for high-quality class 1 capacitors in circuits such as precision oscillators and timers. For applications such as non-critical filtering or coupling circuits, for class 2 capacitors the tolerance series E12 down to E3 are sufficient. | Ceramic capacitor | Wikipedia | 440 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Temperature dependence of capacitance
Capacitance of ceramic capacitors varies with temperature. The different dielectrics of the many capacitor types show great differences in temperature dependence. The temperature coefficient is expressed in parts per million (ppm) per degree Celsius for class 1 ceramic capacitors or in percent (%) over the total temperature range for class 2 capacitors.
Frequency dependence of capacitance
Most discrete capacitor types have greater or smaller capacitance changes with increasing frequencies. The dielectric strength of class 2 ceramic and plastic film diminishes with rising frequency. Therefore, their capacitance value decreases with increasing frequency. This phenomenon is related to the dielectric relaxation in which the time constant of the electrical dipoles is the reason for the frequency dependence of permittivity. The graph on the right hand side shows typical frequency behavior for class 2 vs class 1 capacitors.
Voltage dependence of capacitance
Capacitance of ceramic capacitors may also change with applied voltage. This effect is more prevalent in class 2 ceramic capacitors. The ferroelectric material depends on the applied voltage. The higher the applied voltage, the lower the permittivity. Capacitance measured or applied with higher voltage can drop to values of −80% of the value measured with the standardized measuring voltage of 0.5 or 1.0 V. This behavior is a small source of nonlinearity in low-distortion filters and other analog applications. In audio applications this can be the reason for harmonic distortions.
The voltage dependence of capacitance in the two diagrams above shows curves from ceramic capacitors with NME metallization. For capacitors with BME metallization the voltage dependence of capacitance increased significantly.
Voltage proof
For most capacitors, a physically conditioned dielectric strength or a breakdown voltage usually could be specified for each dielectric material and thickness. This is not possible with ceramic capacitors. The breakdown voltage of a ceramic dielectric layer may vary depending on the electrode material and the sintering conditions of the ceramic up to a factor of 10. A high degree of precision and control of process parameters is necessary to keep the scattering of electrical properties for today's very thin ceramic layers within specified limits. | Ceramic capacitor | Wikipedia | 475 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The voltage proof of ceramic capacitors is specified as rated voltage (UR). This is the maximum DC voltage that may be continuously applied to the capacitor up to the upper temperature limit. This guaranteed voltage proof is tested according to the voltages shown in the adjacent table.
Furthermore, in periodic life time tests (endurance tests) the voltage proof of ceramic capacitors is tested with increased test voltage (120 to 150% of UR) to ensure safe construction.
Impedance
The frequency dependent AC resistance of a capacitor is called impedance and is a complex ratio of voltage to current in an AC circuit. Impedance extends the concept of Ohm's law to AC circuits, and possesses both magnitude and phase at a particular frequency, unlike resistance, which has only magnitude.
Impedance is a measure of the ability of the capacitor to pass alternating currents. In this sense impedance can be used like Ohms law
to calculate either the peak or the effective value of the current or the voltage.
As shown in the series-equivalent circuit of a capacitor, the real-world component includes an ideal capacitor , an inductance and a resistor .
To calculate the impedance the resistance and then both reactances have to be added geometrically
wherein the capacitive reactance (Capacitance) is
and an inductive reactance (Inductance) is
.
In the special case of resonance, in which both reactive resistances have the same value (), then the impedance will only be determined by .
Data sheets of ceramic capacitors only specify the impedance magnitude . The typical impedance curve shows that with increasing frequency, impedance decreases, down to a minimum. The lower the impedance, the more easily alternating currents can pass through the capacitor. At the minimum point of the curve, the point of resonance, where XC has the same value as XL, the capacitor exhibits its lowest impedance value. Here only the ohmic ESR determines the impedance. With frequencies above the resonance, impedance increases again due to the ESL. | Ceramic capacitor | Wikipedia | 431 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
ESR, dissipation factor, and quality factor
The summarized losses in ceramic capacitors are ohmic AC losses. DC losses are specified as "leakage current" or "insulating resistance" and are negligible for an AC specification. These AC losses are nonlinear and may depend on frequency, temperature, age, and for some special types, on humidity. The losses result from two physical conditions,
line losses with internal supply line resistances, the contact resistance of the electrode contact, the line resistance of the electrodes
the dielectric losses out of the dielectric polarization
The largest share of these losses in larger capacitors is usually the frequency dependent ohmic dielectric losses. Regarding the IEC 60384-1 standard, the ohmic losses of capacitors are measured at the same frequency used to measure capacitance. These are:
100 kHz, 1 MHz (preferred) or 10 MHz for ceramic capacitors with CR ≤ 1 nF:
1 kHz or 10 kHz for ceramic capacitors with 1 nF < CR ≤ 10 μF
50/60 Hz or 100/120 Hz for ceramic capacitors with CR > 10 μF
Results of the summarized resistive losses of a capacitor may be specified either as equivalent series resistance (ESR), as dissipation factor (DF, tan δ), or as quality factor (Q), depending on the application requirements.
Class 2 capacitors are mostly specified with the dissipation factor, tan δ. The dissipation factor is determined as the tangent of the reactance – and the ESR, and can be shown as the angle δ between the imaginary and impedance axes in the above vector diagram, see paragraph "Impedance".
If the inductance is small, the dissipation factor can be approximated as:
Class 1 capacitors with very low losses are specified with a dissipation factor and often with a quality factor (Q). The quality factor is defined as the reciprocal of the dissipation factor.
The Q factor represents the effect of electrical resistance, and characterizes a resonator's bandwidth relative to its center or resonant frequency . A high Q value is a mark of the quality of the resonance for resonant circuits.
In accordance with IEC 60384-8/-21/-9/-22 ceramic capacitors may not exceed the following dissipation factors: | Ceramic capacitor | Wikipedia | 509 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The ohmic losses of ceramic capacitors are frequency, temperature and voltage dependent. Additionally, class 2 capacitor measurements change because of aging. Different ceramic materials have differing losses over the temperature range and the operating frequency. The changes in class 1 capacitors are in the single-digit range while class 2 capacitors have much higher changes.
HF use, inductance (ESL) and self-resonant frequency
Electrical resonance occurs in a ceramic capacitor at a particular resonance frequency where the imaginary parts of the capacitor impedance and admittances cancel each other.
This frequency where XC is as high as XL is called the self-resonant frequency and can be calculated with:
where ω = 2πf, in which f is the resonance frequency in Hertz, L is the inductance in henries, and C is the capacitance in farads.
The smaller the capacitance C and the inductance L the higher is the resonance frequency.
The self-resonant frequency is the lowest frequency at which impedance passes through a minimum. For any AC application the self-resonant frequency is the highest frequency at which a capacitor can be used as a capacitive component. At frequencies above the resonance, the impedance increases again due to ESL: the capacitor becomes an inductor with inductance equal to capacitor's ESL, and resistance equal to ESR at the given frequency.
ESL in industrial capacitors is mainly caused by the leads and internal connections used to connect the plates to the outside world. Larger capacitors tend to higher ESL than small ones, because the distances to the plate are longer and every millimeter increases inductance.
Ceramic capacitors, which are available in the range of very small capacitance values (pF and higher) are already out of their smaller capacitance values suitable for higher frequencies up to several 100 MHz (see formula above).
Due to the absence of leads and proximity to the electrodes, MLCC chips have significantly lower parasitic inductance than f. e. leaded types, which makes them suitable for higher frequency applications. A further reduction of parasitic inductance is achieved by contacting the electrodes on the longitudinal side of the chip instead of the lateral side. | Ceramic capacitor | Wikipedia | 486 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Sample self-resonant frequencies for one set of NP0/C0G and one set of X7R ceramic capacitors are:
Note that X7Rs have better frequency response than C0Gs. It makes sense, however, since class 2 capacitors are much smaller than class 1, so they ought to have lower parasitic inductance.
Aging
In ferroelectric class 2 ceramic capacitors capacitance decreases over time. This behavior is called "aging". Aging occurs in ferroelectric dielectrics, where domains of polarization in the dielectric contribute to total polarization. Degradation of the polarized domains in the dielectric decreases permittivity over time so that the capacitance of class 2 ceramic capacitors decreases as the component ages.
The aging follows a logarithmic law. This law defines the decrease of capacitance as a percentage for a time decade after the soldering recovery time at a defined temperature, for example, in the period from 1 to 10 hours at 20 °C. As the law is logarithmic, the percentage loss of capacitance will twice between 1 h and 100 h and 3 times between 1 h and 1000 h and so on. So aging is fastest near the beginning, and the capacitance value effectively stabilizes over time.
The rate of aging of class 2 capacitors mainly depends on the materials used. A rule of thumb is, the higher the temperature dependence of the ceramic, the higher the aging percentage. The typical aging of X7R ceramic capacitors is about 2.5% per decade The aging rate of Z5U ceramic capacitors is significantly higher and can be up to 7% per decade.
The aging process of class 2 capacitors may be reversed by heating the component above the Curie point.
Class 1 capacitors do not experience ferroelectric aging like Class 2's. But environmental influences such as higher temperature, high humidity and mechanical stress can, over a longer period of time, lead to a small irreversible decline in capacitance, sometimes also called aging. The change of capacitance for P 100 and N 470 Class 1's is lower than 1%, for capacitors with N 750 to N 1500 ceramics it is ≤ 2%. | Ceramic capacitor | Wikipedia | 483 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Insulation resistance and self-discharge constant
The resistance of the dielectric is never infinite, leading to some level of DC "leakage current", which contributes to self-discharge. For ceramic capacitors this resistance, placed in parallel with the capacitor in the series-equivalent circuit of capacitors, is called "insulation resistance Rins". The insulation resistance must not be confused with the outer isolation with respect to the environment.
The rate of self-discharge with decreasing capacitor voltage follows the formula
With the stored DC voltage and the self-discharge constant
That means, after capacitor voltage dropped to 37% of the initial value.
The insulation resistance given in the unit MΩ (106 Ohm) as well as the self-discharge constant in seconds is an important parameter for the quality of the dielectric insulation. These time values are important, for example, when a capacitor is used as timing component for relays or for storing a voltage value as in a sample and hold circuits or operational amplifiers.
In accordance with the applicable standards, Class 1 ceramic capacitors have an Rins ≥ 10,000 MΩ for capacitors with CR ≤ 10 nF or τs ≥ 100 s for capacitors with CR > 10 nF. Class 2 ceramic capacitors have an Rins ≥ 4,000 MΩ for capacitors with CR ≤ 25 nF or τs ≥ 100 s for capacitors with CR > 25 nF.
Insulation resistance and thus the self-discharge time rate are temperature dependent and decrease with increasing temperature at about 1 MΩ per 60 °C.
Dielectric absorption (soakage)
Dielectric absorption is the name given to the effect by which a capacitor, which has been charged for a long time, discharges only incompletely. Although an ideal capacitor remains at zero volts after discharge, real capacitors will develop a small voltage coming from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage" or "battery action". | Ceramic capacitor | Wikipedia | 428 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
In many applications of capacitors dielectric absorption is not a problem but in some applications, such as long-time-constant integrators, sample-and-hold circuits, switched-capacitor analog-to-digital converters and very low-distortion filters, it is important that the capacitor does not recover a residual charge after full discharge, and capacitors with low absorption are specified. The voltage at the terminals generated by dielectric absorption may in some cases possibly cause problems in the function of an electronic circuit or can be a safety risk to personnel. To prevent shocks, most very large capacitors like power capacitors are shipped with shorting wires that are removed before use.
Microphony
All class 2 ceramic capacitors using ferroelectric ceramics exhibit piezoelectricity, and have a piezoelectric effect called microphonics, microphony or in audio applications squealing. Microphony describes the phenomenon wherein electronic components transform mechanical vibrations into an electrical signal which in many cases is undesired noise. Sensitive electronic preamplifiers generally use class 1 ceramic and film capacitors to avoid this effect.
In the reverse microphonic effect, the varying electric field between the capacitor plates exerts a physical force, moving them as a speaker. High current impulse loads or high ripple currents can generate audible acoustic sound coming from the capacitor, but discharges the capacitor and stresses the dielectric.
Soldering
Ceramic capacitors may experience changes to their electrical parameters due to soldering stress. The heat of the solder bath, especially for SMD styles, can cause changes of contact resistance between terminals and electrodes. For ferroelectric class 2 ceramic capacitors, the soldering temperature is above the Curie point. The polarized domains in the dielectric are going back and the aging process of class 2 ceramic capacitors is starting again.
Hence after soldering a recovery time of approximately 24 hours is necessary. After recovery some electrical parameters like capacitance value, ESR, leakage currents are changed irreversibly. The changes are in the lower percentage range depending on the style of capacitor.
Additional information
Standardization
The standardization for all electrical, electronic components and related technologies follows the rules given by the International Electrotechnical Commission (IEC), a non-profit, non-governmental international standards organization. | Ceramic capacitor | Wikipedia | 498 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
The definition of the characteristics and the procedure of the test methods for capacitors for use in electronic equipment are set out in the generic specification:
IEC 60384-1, Fixed capacitors for use in electronic equipment – Part 1: Generic specification
The tests and requirements to be met by ceramic capacitors for use in electronic equipment for approval as standardized types are set out in the following sectional specifications:
IEC 60384-8, Fixed capacitors of ceramic dielectric, Class 1
IEC 60384-9, Fixed capacitors of ceramic dielectric, Class 2
IEC 60384-21, Fixed surface mount multilayer capacitors of ceramic dielectric, Class 1
IEC 60384-22, Fixed surface mount multilayer capacitors of ceramic dielectric, Class 2
Tantalum capacitor replacement
Multilayer ceramic capacitors are increasingly used to replace tantalum and low capacitance aluminium electrolytic capacitors in applications such as bypass or high frequency switched-mode power supplies as their cost, reliability and size becomes competitive. In many applications, their low ESR allows the use of a lower nominal capacitance value.
Features and disadvantages of ceramic capacitors
For features and disadvantages of ceramic capacitors see main article Capacitor types#Comparison of types
Marking
Imprinted markings
If space permits ceramic capacitors, like most other electronic components, have imprinted markings to indicate the manufacturer, the type, their electrical and thermal characteristics and their date of manufacture. In the ideal case, if they are large enough, the capacitor will be marked with:
manufacturer's name or trademark;
manufacturer's type designation;
rated capacitance;
tolerance on rated capacitance
rated voltage and nature of supply (AC or DC)
climatic category or rated temperature;
year and month (or week) of manufacture;
certification marks of safety standards (for safety EMI/RFI suppression capacitors)
Smaller capacitors use a shorthand notation, to display all the relevant information in the limited space. The most commonly used format is: XYZ J/K/M VOLTS V, where XYZ represents the capacitance (calculated as XY × 10Z pF), the letters J, K or M indicate the tolerance (±5%, ±10% and ±20% respectively) and VOLTS V represents the working voltage. | Ceramic capacitor | Wikipedia | 500 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Examples
A capacitor with the following text on its body: 105K 330V has a capacitance of 10 × 105 pF = 1 μF (K = ±10%) with a working voltage of 330 V.
A capacitor with the following text: 473M 100V has a capacitance of 47 × 103 pF = 47 nF (M = ±20%) with a working voltage of 100 V.
Capacitance, tolerance and date of manufacture can be identified with a short code according to IEC/EN 60062. Examples of short-marking of the rated capacitance (microfarads):
μ47 = 0.47 μF
4μ7 = 4.7 μF
47μ = 47 μF
The date of manufacture is often printed in accordance with international standards.
Version 1: coding with year/week numeral code, "1208" is "2012, week number 8".
Version 2: coding with year code/month code,
Year code: "R" = 2003, "S"= 2004, "T" = 2005, "U" = 2006, "V" = 2007, "W" = 2008, "X" = 2009, "A" = 2010, "B" = 2011, "C" = 2012, "D" = 2013 etc.
Month code: "1" to "9" = Jan. to Sept., "O" = October, "N" = November, "D" = December
"X5" is then "2009, May"
For very small capacitors like MLCC chips no marking is possible. Here only the traceability of the manufacturers can ensure the identification of a type.
Colour coding
The identification of modern capacitors has no detailed color coding. | Ceramic capacitor | Wikipedia | 370 | 9221221 | https://en.wikipedia.org/wiki/Ceramic%20capacitor | Technology | Components | null |
Cuvier's dwarf caiman (Paleosuchus palpebrosus) is a small crocodilian in the alligator family from northern and central South America. It is found in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Suriname, Trinidad and Venezuela. It lives in riverine forests, flooded forests near lakes, and near fast-flowing rivers and streams. It can traverse dry land to reach temporary pools and tolerates colder water than other species of caimans. Other common names for this species include the musky caiman, the dwarf caiman, Cuvier's caiman, and the smooth-fronted caiman (the latter name is also used for P. trigonatus). It is sometimes kept in captivity as a pet and may be referred to as the wedge-head caiman by the pet trade community.
Cuvier's dwarf caiman was first described by the French zoologist Georges Cuvier in 1807 and is one of only two species in the genus Paleosuchus, the other species being P. trigonatus. Their closest relatives are the other caimans in the subfamily Caimaninae. With a total length averaging for males and up to for females, Cuvier's dwarf caiman is not only the smallest extant species in the alligator and caiman family, but also the smallest of all crocodilians (unless the Congo dwarf crocodile is considered a valid species). An adult weighs around . Its lack of size is partly made up for by its strong body armor, provided by the bony bases to its dermal scales, which provides protection against predators. Juvenile dwarf caimans mainly feed on invertebrates, but also small fish and frogs, while adults eat larger fish, amphibians, and invertebrates, such as large molluscs. This caiman sometimes uses a burrow as shelter during the day and in the Pantanal may aestivate in the burrow to stay cool in the dry season. The female buries her eggs on a mounded nest and these take about 3 months to hatch. She helps the hatchlings to escape from the nest and provides some parental care for the first few weeks of their lives. This caiman has a wide range and large total population and the IUCN lists its conservation status as being of least concern. | Cuvier's dwarf caiman | Wikipedia | 473 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
Etymology
The genus name Paleosuchus is derived from the Greek palaios meaning "ancient" and soukhos meaning "crocodile". This refers to the belief that this crocodile comes from an ancient lineage that diverged from other species of caimans some 30 million years ago. The specific name palpebrosus is derived from the Latin palpebra meaning "eyelid" and osus meaning "full of". This refers to the bony plates (palpebrals) present on the upper eyelids.
Common names include the musky caiman, the dwarf caiman, Cuvier's caiman, and the smooth-fronted caiman, although the last of these is also used to refer to the closely related P. trigonatus. In the pet trade, it is sometimes referred to as the wedge-head caiman.
Discovery and taxonomy
Cuvier's dwarf caiman was first described by Cuvier in 1807 as Crocodylus palpebrosus from a type locality described as "Cayenne". Since then, it has been given a number of names by different authorities: Crocodilus (Alligator) palpebrosus (Merrem, 1820), Jacaretinga moschifer (Spix, 1825), Champsa palpebrosa (Wagler, 1830), Alligator palpebrosus (Dumeril and Bibron, 1836), Champsa gibbiceps (Natterer, 1841), Caiman palpebrosus (Gray, 1844), Caiman (Aromosuchus) palpebrosus (Gray, 1862), and Jacaretinga palpebrosus (Vaillant, 1898). Muller, in 1924, and Schmidt, in 1928, were the first to use the currently accepted name of Paleosuchus palpebrosus. No subspecies are recognised.
At present, the genus Paleosuchus contains only two members, Paleosuchus trigonatus, commonly known as the smooth-fronted or Schneider's dwarf caiman, and P. palpebrosus, both from South America. Paleosuchus is distinguished from other caimans in the alligator subfamily Caimaninae by the absence of an interorbital ridge and the presence of four teeth in the premaxilla region of the jaw, where other species of caimans have five.
The relationships of extant (living) caimans can be shown in the cladogram below, based on molecular DNA-based phylogenetic studies: | Cuvier's dwarf caiman | Wikipedia | 512 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
A genetic study in 2012 found clear differences between various populations of Cuvier's dwarf caiman (Pantanal; Madeira River basin; Rio Negro basin), and these are apparently isolated from each other, leading to the suggestion that it may be a cryptic species complex.
Description
Cuvier's dwarf caiman is the smallest living New World crocodilian. Males grow to a maximum length around while females do not usually exceed . The largest specimen on record measured in length. This may be an underestimate of the animal's maximum size, as nearly all large adults have lost the tips of their tails and the largest specimen measured in the Pantanal region had a snout–vent length of (equivalent to a total length of with an intact tail). An adult typically weighs around , around the same weight as a 6- to 12-month-old specimen of several larger species of crocodilians. Large adults of this species can weigh up to . Cuvier's dwarf caiman has strong body armor on both its dorsal (upper) and ventral (lower) sides, which may compensate for its small body size in reducing predation. The dermal scales providing this protection have a bony base and are known as osteoderms.
The head has an unusual shape for a crocodilian, with a dome-shaped skull and a short, smooth, concave snout with an upturned tip, the shape rather resembling the head of a dog. The upper jaw extends markedly further forward than the lower jaw. Four premaxillary and 14 to 15 maxillary teeth are on either side of the upper jaw and 21 or 22 teeth on each side of the lower jaw, giving a total of about 80 teeth. The neck is relatively slender and the dorsal scutes are less prominent than in the smooth-fronted caiman. The double rows of scutes on the tail are small and project vertically. Adults are dark brownish-black with a dark brown head, while juveniles are brown with black bands. The irises of the eyes are chestnut brown at all ages and the pupils are vertical slits.The scutellation (arrangement of the scales) helps to distinguish Cuvier's dwarf caiman from Schneider's dwarf caiman. | Cuvier's dwarf caiman | Wikipedia | 459 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
Distribution and habitat
Cuvier's dwarf caiman is native to tropical northern and central South America. It is present in the drainages of the Orinoco River, the São Francisco River, and the Amazon River, and the upper reaches of the Paraná River and the Paraguay River. The countries in which it is found include Peru, Ecuador, Colombia, Venezuela, Guyana, Suriname, French Guiana, Brazil, Bolivia, Trinidad and Paraguay. The range of this species is rather larger than that of the sympatric smooth-fronted caiman, as it extends into Paraguay and includes a larger area of Brazil. They also follow seasonal fluctuations in water-level, while the smooth-fronted caiman does not, which may explain how the two species are able to live in sympatry.
Cuvier's dwarf caiman is a freshwater species and is found in forested riverine habitats and areas of flooded forest around lakes. It seems to prefer rivers and streams with fast-flowing water, but it is also found in quiet, nutrient-poor waters in Venezuela and southeastern Brazil. It is able to travel quite large distances overland at night and subadult individuals have sometimes been found in isolated, temporary pools. In the northern and southern parts of its range, it is also found in gallery forests in savanna country, but it is absent from such habitats in the Llanos and the Pantanal. Cuvier's dwarf caiman seems relatively tolerant of cool water compared to other species of caimans. During the day, individuals sometimes lie up in burrows but at other times rest on piles of rocks or sun themselves while lying, facing the sun, in shallow water with their backs exposed.
Behaviour and ecology
These caimans are mainly nocturnal. Because they occupy many different microhabitats, their diet is believed to vary regionally. Adults feed on fish, amphibians, small mammals, birds, crabs, shrimp, molluscs, insects, and other invertebrates, which they catch in the water or on land. Juveniles eat fewer fish, but also consume crustaceans, tadpoles, frogs, and snails, as well as land invertebrates, such as beetles. The prey is mostly swallowed whole and is ground up by stones in the gizzard. In the Pantanal, Cuvier's dwarf caiman estivates in burrows during the dry season and is able to maintain its temperature around for days at a time. | Cuvier's dwarf caiman | Wikipedia | 493 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
Adult Cuvier's dwarf caimans are usually found singly or in pairs. The breeding of this species has been little studied, but it does not appear to be seasonal in nature. The female builds a mound nest out of vegetation and mud somewhere in a concealed location and lays a clutch of 10 to 25 eggs, hiding them under further vegetation. Nest temperature varies between and are heated by decaying vegetation. The incubation period is around 90 days and the sex of the hatchlings depends on the temperature of the nest during that time. When the eggs begin to hatch, the female opens the nest in response to the calls made by the young. Newly emerged juveniles have a coating of mucus and may delay entering the water for a few days until this has dried. Its continuing presence on their skin is believed to reduce algal growth. The female stays with the young for around a year, with the longest recorded care extending to 21 months. After this the hatchlings disperse. The young grow at a rate around per year. Females reach sexual maturity around 8 years old and males around 6 years old.
Cuvier's dwarf caiman is considered to be a keystone species whose presence in the ecosystem maintains a healthy balance of organisms. In its absence, fish, such as piranhas, might dominate the environment. The eggs and newly hatched young are most at risk and are preyed on by birds, snakes, rats, raccoons, and other mammals. Adults are protected by the bony osteoderms under the scales and their main predators are jaguars, green anacondas (Eunectes murinus), and large boa constrictors (Boa constrictor).
The Cuvier's dwarf caiman is the only crocodilian species that seemingly does not perform the near-universal "death roll" technique used by other extant crocodilians for feeding or intra-specific combat. However, this may only be circumstantial, as specimens tested for the behavior may have been acting uncooperatively with the researchers. | Cuvier's dwarf caiman | Wikipedia | 424 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
Status and conservation
Many crocodilians are hunted for their skins, but this is not the case with the Cuvier's dwarf caiman. This may be because the ventral skin in this species is too heavily armored to make it easy to tan. Some individuals are killed by indigenous peoples for food and some traditional South American cultures believe dwarf caiman teeth protect from snake bites. Others, particularly in Guyana, are collected for the pet trade; but no evidence shows that populations are dwindling as a result. Some threats to this species are from habitat destruction, including the mining of gold, but these are not thought to be of great significance. The estimated total population is over a million individuals.
In its Red List of Threatened Species, the IUCN lists Cuvier's dwarf caiman as being of least concern, which is because its range is extensive, covering much of northern and central South America, and although its population trend is unknown, it appears to be abundant in many of the localities in which it is found. It is listed in Appendix II of CITES.
Captive care
Cuvier's dwarf caiman can be kept as a pet, though providing suitable care is expensive and requires extremely large enclosures. In many countries, permits or licenses are necessary and most veterinarians have little experience with these exotic animals. | Cuvier's dwarf caiman | Wikipedia | 266 | 6935153 | https://en.wikipedia.org/wiki/Cuvier%27s%20dwarf%20caiman | Biology and health sciences | Crocodilia | Animals |
A gunshot wound (GSW) is a penetrating injury caused by a projectile (e.g. a bullet) shot from a gun (typically a firearm). Damage may include bleeding, bone fractures, organ damage, wound infection, and loss of the ability to move part of the body. Damage depends on the part of the body hit, the path the bullet follows through (or into) the body, and the type and speed of the bullet. In severe cases, although not uncommon, the injury is fatal. Long-term complications can include bowel obstruction, failure to thrive, neurogenic bladder and paralysis, recurrent cardiorespiratory distress and pneumothorax, hypoxic brain injury leading to early dementia, amputations, chronic pain and pain with light touch (hyperalgesia), deep venous thrombosis with pulmonary embolus, limb swelling and debility, and lead poisoning.
Factors that determine rates of gun violence vary by country. These factors may include the illegal drug trade, easy access to firearms, substance misuse including alcohol, mental health problems, firearm laws, social attitudes, economic differences, and occupations such as being a police officer. Where guns are more common, altercations more often end in death.
Before management begins, the area must be verified as safe. This is followed by stopping major bleeding, then assessing and supporting the airway, breathing, and circulation. Firearm laws, particularly background checks and permit to purchase, decrease the risk of death from firearms. Safer firearm storage may decrease the risk of firearm-related deaths in children.
In 2015, about a million gunshot wounds occurred from interpersonal violence. In 2016, firearms resulted in 251,000 deaths globally, up from 209,000 in 1990. Of these deaths, 161,000 (64%) were the result of assault, 67,500 (27%) were the result of suicide, and 23,000 (9%) were accidents. In the United States, guns resulted in about 40,000 deaths in 2017. Firearm-related deaths are most common in males between the ages of 20 and 24 years. Economic costs due to gunshot wounds have been estimated at $140 billion a year in the United States. | Gunshot wound | Wikipedia | 458 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Signs and symptoms
Trauma from a gunshot wound varies widely based on the bullet, velocity, mass, entry point, trajectory, affected anatomy, and exit point. Gunshot wounds can be particularly devastating compared to other penetrating injuries because the trajectory and fragmentation of bullets can be unpredictable after entry. Moreover, gunshot wounds typically involve a large degree of nearby tissue disruption and destruction caused by the physical effects of the projectile correlated with the bullet velocity classification.
The immediate damaging effect of a gunshot wound is typically severe bleeding with the potential for a type of shock known as hypovolemic shock, a condition characterized by inadequate delivery of oxygen to vital organs. In the case of traumatic hypovolemic shock, this failure of adequate oxygen delivery is due to blood loss, as blood is the means of delivering oxygen to the body's constituent parts. Besides blood loss, internal bleeding can lead to complications.
Devastating effects can result when a bullet strikes a vital organ such as the heart, lungs, or liver, or damages a component of the central nervous system such as the spinal cord or brain. It can lead to organ failure and death.
Common causes of death following gunshot injury include bleeding, low oxygen caused by pneumothorax, catastrophic injury to the heart and major blood vessels, and damage to the brain or central nervous system. Non-fatal gunshot wounds frequently have mild to severe long-lasting effects, typically some form of major disfigurement such as amputation because of a severe bone fracture and may cause permanent disability. A sudden blood gush may take effect immediately from a gunshot wound if a bullet directly damages larger blood vessels, especially arteries.
Pathophysiology | Gunshot wound | Wikipedia | 336 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
The degree of tissue disruption caused by a projectile is related to the cavitation the projectile creates as it passes through tissue. A bullet with sufficient energy will have a cavitation effect in addition to the penetrating track injury. As the bullet passes through the tissue, initially crushing then lacerating, the space left forms a cavity; this is called the permanent cavity. Higher-velocity bullets create a pressure wave that forces the tissues away, creating not only a permanent cavity the size of the caliber of the bullet but a temporary cavity or secondary cavity, which is often many times larger than the bullet itself. The temporary cavity is the radial stretching of tissue around the bullet's wound track, which momentarily leaves an empty space caused by high pressures surrounding the projectile that accelerate material away from its path. The extent of cavitation, in turn, is related to the following characteristics of the projectile:
Kinetic energy: KE = 1/2mv2 (where m is mass and v is velocity). This helps to explain why wounds produced by projectiles of higher mass and/or higher velocity produce greater tissue disruption than projectiles of lower mass and velocity. The velocity of the bullet is a more important determinant of tissue injury. Although both mass and velocity contribute to the overall energy of the projectile, the energy is proportional to the mass while proportional to the square of its velocity. As a result, for constant velocity, if the mass is doubled, the energy is doubled; however, if the velocity of the bullet is doubled, the energy increases four times. The initial velocity of a bullet is largely dependent on the firearm. The US military commonly uses 5.56-mm bullets, which have a relatively low mass as compared with other bullets; however, the speed of these bullets is relatively fast. As a result, they produce a larger amount of kinetic energy, which is transmitted to the tissues of the target. The size of the temporary cavity is approximately proportional to the kinetic energy of the bullet and depends on the resistance of the tissue to stress. Muzzle energy, which is based on muzzle velocity, is often used for ease of comparison.
Yaw: Handgun bullets will generally travel in a relatively straight line or make one turn if a bone is hit. Upon travel through deeper tissue, high-energy rounds may become unstable as they decelerate, and may tumble (pitch and yaw) as the energy of the projectile is absorbed, causing stretching and tearing of the surrounding tissue. | Gunshot wound | Wikipedia | 498 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Fragmentation: Most commonly, bullets do not fragment, and secondary damage from fragments of shattered bone is a more common complication than bullet fragments. | Gunshot wound | Wikipedia | 28 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Diagnosis
Classification
Gunshot wounds are classified according to the speed of the projectile using the Gustilo open fracture classification:
Low-velocity: Less than 335 m/s (1,100 ft/s)
Low velocity wounds are typical of small caliber handguns. They do not usually cause extensive soft tissue damage, and in the Gustilo open fracture classification are classified as Type 1 or 2 wounds.
Medium-velocity: Between 360 m/s (1,200 ft/s) and 600 m/s (2,000 ft/s)
These are more typical of shotgun blasts or higher caliber handguns like magnums. The risk of infection from these types of wounds can vary depending on the type and pattern of bullets fired as well as the distance from the firearm.
High-velocity: Between 600 m/s (2,000 ft/s) and 1,000 m/s (3,500 ft/s)
Usually caused by powerful assault or hunting rifles and usually cause Gustilo Type 3 wounds. The risk of infection is especially high due to the large area of injury and destroyed tissue.
Bullets from handguns are sometimes less than but with modern pistol loads, they usually are slightly above , while bullets from most modern rifles exceed . One recently developed class of firearm projectiles is the hyper-velocity bullet, such cartridges are usually made for achieving such high speed, purpose-built in factories or made by amateurs. Examples of hyper velocity cartridges include the .220 Swift, .17 Remington and .17 Mach IV cartridges. The US military commonly uses 5.56mm bullets, which have a relatively low mass as compared with other bullets (2,6-4,0 grams); however, the speed of these bullets is relatively fast (approximately , placing them in the high velocity category). As a result, they produce a larger amount of kinetic energy, which is transmitted to the tissues of the target. High energy transfer results in more tissue disruption, which plays a role in incapacitation, but other factors such as wound size and shot placement are also important. | Gunshot wound | Wikipedia | 414 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Kronlein shot
The "Kronlein shot" (German: Krönleinschuss) is a distinctive type of headshot wound that can only be created by a high velocity rifle bullet or shotgun slug. In a Kronlein shot, the intact brain is ejected from the skull and deposited some distance from the victim's body. This type of wound is believed to be caused by a hydrodynamic effect. Hydraulic pressure generated within the skull by a high velocity bullet leads to the explosive ejection of the brain from the fractured skull.
Prevention
Interventions have been recommended to reduce the risk of firearm related injury or death. Medical organizations in the United States recommend a criminal background check being held before a person buys a gun and that a person who has convictions for crimes of violence should not be permitted to buy a gun. Safe storage of guns is recommended, as well as better mental health care and removal of guns from those at risk of suicide. Experts recommend that physicians counsel patients regarding safe storage of guns and other injury prevention strategies related to guns as part of routine medical care. Having guns locked and unloaded is associated with a lower risk of gun related injury or death (including a lower risk of suicide) for all household members as compared to guns that are stored loaded and unlocked.
Temporarily removing guns from the home, either voluntarily or by court order (such as with extreme risk protection orders [so called "red flag laws"] in the United States) is recommended for those who are at risk of suicide or violence towards others. Such laws have been associated with a lower risk of suicide using guns in population based studies.
In an effort to prevent mass shootings, greater regulations on guns that can rapidly fire many bullets is recommended. | Gunshot wound | Wikipedia | 349 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Management
Initial assessment for a gunshot wound is approached in the same way as other acute trauma using the advanced trauma life support (ATLS) protocol. These include:
A) Airway - Assess and protect airway and potentially the cervical spine
B) Breathing - Maintain adequate ventilation and oxygenation
C) Circulation - Assess for and control bleeding to maintain organ perfusion including focused assessment with sonography for trauma (FAST)
D) Disability - Perform basic neurological exam including Glasgow Coma Scale (GCS)
E) Exposure - Expose entire body and search for any missed injuries, entry points, and exit points while maintaining body temperature
Depending on the extent of injury, management can range from urgent surgical intervention to observation. As such, any history from the scene such as gun type, shots fired, shot direction and distance, blood loss on scene, and pre-hospital vitals signs can be very helpful in directing management. Unstable people with signs of bleeding that cannot be controlled during the initial evaluation require immediate surgical exploration in the operating room. Otherwise, management protocols are generally dictated by anatomic entry point and anticipated trajectory.
Neck
A gunshot wound to the neck can be particularly dangerous because of the high number of vital anatomical structures contained within a small space. The neck contains the larynx, trachea, pharynx, esophagus, vasculature (carotid, subclavian, and vertebral arteries; jugular, brachiocephalic, and vertebral veins; thyroid vessels), and nervous system anatomy (spinal cord, cranial nerves, peripheral nerves, sympathetic chain, brachial plexus). Gunshots to the neck can thus cause severe bleeding, airway compromise, and nervous system injury.
Initial assessment of a gunshot wound to the neck involves non-probing inspection of whether the injury is a penetrating neck injury (PNI), classified by violation of the platysma muscle. If the platysma is intact, the wound is considered superficial and only requires local wound care. If the injury is a PNI, surgery should be consulted immediately while the case is being managed. Of note, wounds should not be explored on the field or in the emergency department given the risk of exacerbating the wound. | Gunshot wound | Wikipedia | 460 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Due to the advances in diagnostic imaging, management of PNI has been shifting from a "zone-based" approach, which uses anatomical site of injury to guide decisions, to a "no-zone" approach which uses a symptom-based algorithm. The no-zone approach uses a hard signs and imaging system to guide next steps. Hard signs include airway compromise, unresponsive shock, diminished pulses, uncontrolled bleeding, expanding hematoma, bruits/thrill, air bubbling from wound or extensive subcutaneous air, stridor/hoarseness, neurological deficits. If any hard signs are present, immediate surgical exploration and repair is pursued alongside airway and bleeding control. If there are no hard signs, the person receives a multi-detector CT angiography for better diagnosis. A directed angiography or endoscopy may be warranted in a high-risk trajectory for the gunshot. A positive finding on CT leads to operative exploration. If negative, the person may be observed with local wound care.
Chest
Important anatomy in the chest includes the chest wall, ribs, spine, spinal cord, intercostal neurovascular bundles, lungs, bronchi, heart, aorta, major vessels, esophagus, thoracic duct, and diaphragm. Gunshots to the chest can thus cause severe bleeding (hemothorax), respiratory compromise (pneumothorax, hemothorax, pulmonary contusion, tracheobronchial injury), cardiac injury (pericardial tamponade), esophageal injury, and nervous system injury. | Gunshot wound | Wikipedia | 338 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Initial workup as outlined in the Workup section is particularly important with gunshot wounds to the chest because of the high risk for direct injury to the lungs, heart, and major vessels. Important notes for the initial workup specific for chest injuries are as follows. In people with pericardial tamponade or tension pneumothorax, the chest should be evacuated or decompressed if possible prior to attempting tracheal intubation because the positive pressure ventilation can cause hypotention or cardiovascular collapse. Those with signs of a tension pneumothorax (asymmetric breathing, unstable blood flow, respiratory distress) should immediately receive a chest tube (> French 36) or needle decompression if chest tube placement is delayed. FAST exam should include extended views into the chest to evaluate for hemopericardium, pneumothorax, hemothorax, and peritoneal fluid.
Those with cardiac tamponade, uncontrolled bleeding, or a persistent air leak from a chest tube all require surgery. Cardiac tamponade can be identified on FAST exam. Blood loss warranting surgery is 1–1.5 L of immediate chest tube drainage or ongoing bleeding of 200-300 mL/hr. Persistent air leak is suggestive of tracheobronchial injury which will not heal without surgical intervention. Depending on the severity of the person's condition and if cardiac arrest is recent or imminent, the person may require surgical intervention in the emergency department, otherwise known as an emergency department thoracotomy (EDT).
However, not all gunshot to the chest require surgery. Asymptomatic people with a normal chest X-ray can be observed with a repeat exam and imaging after 6 hours to ensure no delayed development of pneumothorax or hemothorax. If a person only has a pneumothorax or hemothorax, a chest tube is usually sufficient for management unless there is large volume bleeding or persistent air leak as noted above. Additional imaging after initial chest X-ray and ultrasound can be useful in guiding next steps for stable people. Common imaging modalities include chest CT, formal echocardiography, angiography, esophagoscopy, esophagography, and bronchoscopy depending on the signs and symptoms.
Abdomen | Gunshot wound | Wikipedia | 483 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Important anatomy in the abdomen includes the stomach, small bowel, colon, liver, spleen, pancreas, kidneys, spine, diaphragm, descending aorta, and other abdominal vessels and nerves. Gunshots to the abdomen can thus cause severe bleeding, release of bowel contents, peritonitis, organ rupture, respiratory compromise, and neurological deficits.
The most important initial evaluation of a gunshot wound to the abdomen is whether there is uncontrolled bleeding, inflammation of the peritoneum, or spillage of bowel contents. If any of these are present, the person should be transferred immediately to the operating room for laparotomy. If it is difficult to evaluate for those indications because the person is unresponsive or incomprehensible, it is up to the surgeon's discretion whether to pursue laparotomy, exploratory laparoscopy, or alternative investigative tools.
Although all people with abdominal gunshot wounds were taken to the operating room in the past, practice has shifted in recent years with the advances in imaging to non-operative approaches in more stable people. If the person's vital signs are stable without indication for immediate surgery, imaging is done to determine the extent of injury. Ultrasound (FAST) and help identify intra-abdominal bleeding and X-rays can help determine bullet trajectory and fragmentation. However, the best and preferred mode of imaging is high-resolution multi-detector CT (MDCT) with IV, oral, and sometimes rectal contrast. Severity of injury found on imaging will determine whether the surgeon takes an operative or close observational approach.
Diagnostic peritoneal lavage (DPL) has become largely obsolete with the advances in MDCT, with use limited to centers without access to CT to guide requirement for urgent transfer for operation.
Extremities
The four main components of extremities are bones, vessels, nerves, and soft tissues. Gunshot wounds can thus cause severe bleeding, fractures, nerve deficits, and soft tissue damage. The Mangled Extremity Severity Score (MESS) is used to classify the severity of injury and evaluates for severity of skeletal and/or soft tissue injury, limb ischemia, shock, and age. Depending on the extent of injury, management can range from superficial wound care to limb amputation. | Gunshot wound | Wikipedia | 479 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
Vital sign stability and vascular assessment are the most important determinants of management in extremity injuries. As with other traumatic cases, those with uncontrolled bleeding require immediate surgical intervention. If surgical intervention is not readily available and direct pressure is insufficient to control bleeding, tourniquets or direct clamping of visible vessels may be used temporarily to slow active bleeding. People with hard signs of vascular injury also require immediate surgical intervention. Hard signs include active bleeding, expanding or pulsatile hematoma, bruit/thrill, absent distal pulses and signs of extremity ischemia.
For stable people without hard signs of vascular injury, an injured extremity index (IEI) should be calculated by comparing the blood pressure in the injured limb compared to an uninjured limb in order to further evaluate for potential vascular injury. If the IEI or clinical signs are suggestive of vascular injury, the person may undergo surgery or receive further imaging including CT angiography or conventional arteriography.
In addition to vascular management, people must be evaluated for bone, soft tissue, and nerve injury. Plain films can be used for fractures alongside CTs for soft tissue assessment. Fractures must be debrided and stabilized, nerves repaired when possible, and soft tissue debrided and covered. This process can often require multiple procedures over time depending on the severity of injury.
Epidemiology
In 2015, about a million gunshot wounds occurred from interpersonal violence. Firearms, globally in 2016, resulted in 251,000 deaths up from 209,000 in 1990. Of these deaths 161,000 (64%) were the result of assault, 67,500 (27%) were the result of suicide, and 23,000 were accidents. Firearm related deaths are most common in males between the ages of 20 and 24 years.
The countries with the greatest number of deaths from firearms are Brazil, United States, Mexico, Colombia, Venezuela, Guatemala, Bahamas and South Africa which make up just over half the total. In the United States in 2015, about half of the 44,000 people who died by suicide did so with a gun. | Gunshot wound | Wikipedia | 432 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
As of 2016, the countries with the highest rates of gun violence per capita were El Salvador, Venezuela, and Guatemala with 40.3, 34.8, and 26.8 violent gun deaths per 100,000 people respectively. The countries with the lowest rates of were Singapore, Japan, and South Korea with 0.03, 0.04, and 0.05 violent gun deaths per 100,000 people respectively.
Canada
In 2016, about 893 people died due to gunshot wounds in Canada (2.1 per 100,000). About 80% were suicides, 12% were assaults, and 4% were accidents.
United States
In 2017, there were 39,773 deaths in the United States as a result gunshot wounds. Of these 60% were suicides, 37% were homicides, 1.4% were by law enforcement, 1.2% were accidents, and 0.9% were from an unknown cause. This is up from 37,200 deaths in 2016 due to a gunshot wound (10.6 per 100,000). With respect to those that pertain to interpersonal violence, it had the 31st highest rate in the world with 3.85 deaths per 100,000 people in 2016. The majority of all homicides and suicides are firearm-related, and the majority of firearm-related deaths are the result of murder and suicide. When sorted by GDP, however, the United States has a much higher violent gun death rate compared to other developed countries, with over 10 times the number of firearms assault deaths than the next four highest GDP countries combined. Gunshot violence is the third most costly cause of injury and the fourth most expensive form of hospitalization in the United States.
History
Until the 1880s, the standard practice for treating a gunshot wound called for physicians to insert their unsterilized fingers into the wound to probe and locate the path of the bullet. Standard surgical theory such as opening abdominal cavities to repair gunshot wounds, germ theory, and Joseph Lister's technique for antiseptic surgery using diluted carbolic acid, had not yet been accepted as standard practice. For example, sixteen doctors attended to President James A. Garfield after he was shot in 1881, and most probed the wound with their fingers or dirty instruments. Historians agree that massive infection was a significant factor in Garfield's death. | Gunshot wound | Wikipedia | 476 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
At almost the same time, in Tombstone, Arizona Territory, on 13 July 1881, George E. Goodfellow performed the first laparotomy to treat an abdominal gunshot wound. Goodfellow pioneered the use of sterile techniques in treating gunshot wounds, washing the person's wound and his hands with lye soap or whisky, and his patient, unlike the President, recovered. He became America's leading authority on gunshot wounds and is credited as the United States' first civilian trauma surgeon.
Mid-nineteenth-century handguns such as the Colt revolvers used during the American Civil War had muzzle velocities of just 230– /s and their powder and ball predecessors had velocities of 167 m/s or less. Unlike today's high-velocity bullets, nineteenth-century balls produced almost little or no cavitation and, being slower moving, they were liable to lodge in unusual locations at odds with their trajectory.
Wilhelm Röntgen's discovery of X-rays in 1895 led to the use of radiographs to locate bullets in wounded soldiers.
Survival rates for gunshot wounds improved among US military personnel during the Korean and Vietnam Wars, due in part to helicopter evacuation, along with improvements in resuscitation and battlefield medicine. Similar improvements were seen in US trauma practices during the Iraq War. Military health care providers who return to civilian practice sometimes disseminate military trauma care practices. One such practice is to transfer major trauma cases to an operating theater as soon as possible, to stop internal bleeding. Within the United States, the survival rate for gunshot wounds has increased, leading to declines in the gun death rate in states that have stable rates of gunshot hospitalizations. | Gunshot wound | Wikipedia | 341 | 10725984 | https://en.wikipedia.org/wiki/Gunshot%20wound | Biology and health sciences | Types | Health |
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.
The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago. The only living synapsids are mammals, while the sauropsids gave rise to the dinosaurs, and today's reptiles and birds along with all the extinct amniotes more closely related to them than to mammals. Primitive synapsids were traditionally called "mammal-like reptiles" or "pelycosaurs", but both are now seen as outdated and disfavored paraphyletic terms, since they were not reptiles, nor part of reptile lineage. The modern term for these is "stem mammals", and sometimes "protomammals" or "paramammals". | Evolution of mammals | Wikipedia | 415 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores. In the subsequent Triassic period, however, a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. (Conversely, mammaliaforms' success in these niches may have prevented archosaurs from becoming smaller or nocturnal themselves.) The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.
Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; but, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and warm-bloodedness.
Definition of "mammal"
While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils. | Evolution of mammals | Wikipedia | 441 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
One such feature available for paleontology, shared by all living mammals (including monotremes), but not present in any of the early Triassic therapsids, is shown in Figure 1 (on the right), namely: mammals use two bones for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian tetrapods use this system including amphibians, turtles, lizards, snakes, crocodilians, dinosaurs (including the birds), ichthyosaurs, pterosaurs and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal (another small skull bone). In the Jurassic, their quadrate and articular bones evolved into the incus and malleus bones in the middle ear. Mammals also have a double occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.
In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molar occlusion was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved." Today, most paleontologists consider that animals are mammals if they satisfy this criterion.
The ancestry of mammals
Amniotes
The first fully terrestrial vertebrates were reptilian amniotes — their eggs had internal membranes that allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the common Suriname toad, have evolved other ways of getting around this limitation). The first amniotes apparently arose in the middle Carboniferous from the ancestral reptiliomorphs. | Evolution of mammals | Wikipedia | 499 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Within a few million years, two important amniote lineages became distinct: synapsids, from which mammals are descended, and sauropsids, from which lizards, snakes, turtles/tortoises, crocodilians, dinosaurs, and birds are descended. The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus, respectively) date from about 320 to 315 million years ago. The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier than the first fossil.
Synapsids
Synapsid skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:
made the skull lighter without sacrificing strength.
saved energy by using less bone.
probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.
A number of creatures often – and incorrectly – believed to be dinosaurs, hence part of the reptile lineage and sauropsids, were in fact synapsids. This includes the well-known Dimetrodon.
Terms used for discussing non-mammalian synapsids
When referring to the ancestors and close relatives of mammals, paleontologists also use the following terms of convenience: | Evolution of mammals | Wikipedia | 319 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Pelycosaurs — all synapsids, and all of their descendants, except for therapsids – the eventual ancestor of mammals. The pelycosaurs included the largest land vertebrates of the Early Permian, such as the 6 metre (20 foot)-long Cotylorhynchus hancocki. Among the other large pelycosaurs were Dimetrodon grandis and Edaphosaurus cruciger.
Stem mammals (sometimes called protomammals or paramammals, and previously called mammal-like reptiles) — all synapsids, and all of their descendants, except for mammals themselves. Stem mammals therefore include all pelycosaurs, and also all non-mammalian therapsids. Traditionally these were known as "mammal-like reptiles", but this is incorrect; terms such as "stem mammal" are preferred instead, because these synapsids were neither reptiles nor even part of reptile lineage.
Therapsids
Therapsids descended from sphenacodonts, a primitive synapsid, in the middle Permian, and took over from them as the dominant land vertebrates. They differ from earlier synapsids in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.
The therapsid lineage then went through several stages, leading to the evolution of cynodonts in the late Permian, some of which had begun to resemble early mammals:
gradual development of a bony secondary palate. Most books and articles interpret this as a prerequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectotherms use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing. The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, because chewing reduces the size of food particles delivered to the stomach and can therefore speed their digestion. In mammals, the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
the dentary gradually becomes the main bone of the lower jaw. | Evolution of mammals | Wikipedia | 501 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Therapsid family tree
A simplified phylogenetic tree showing only what is most relevant to the evolution of mammals is shown below:
Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.
Biarmosuchia
The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.
Dinocephalians
Dinocephalians ("terrible heads") included both carnivores and herbivores. They were large; Anteosaurus was up to long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather "reptilian".
Anomodonts
The anomodonts ("anomalous teeth") were among the most successful of the herbivorous therapsids — one sub-group, the dicynodonts, survived to the end of the Triassic. But dicynodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw (lost in some derived kannemeyeriiformes) and it is generally agreed that they had beaks like those of birds or ceratopsians.
Theriodonts
The theriodonts ("beast teeth") and their descendants had jaw joints in which the articular bone of the lower jaw tightly gripped the very small quadrate bone of the skull. This allowed a much wider gape and allowed one group, the carnivorous gorgonopsians ("gorgon faces"), to develop "sabre teeth". However, the jaw hinge of the theriodont had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear. | Evolution of mammals | Wikipedia | 413 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The gorgonopsians still had some primitive features: no bony secondary palate (other bones in the right places perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. The therocephalians ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans). Numerous Changhsingian coprolites that possibly belong to therocephalians and indeterminate basal archosaurs (proterosuchids) contain elongated hollow structures that could be remains of hair. That means therapsids were covered in hair as early as 252 million years ago.
Cynodonts
The cynodonts, a theriodont group that also arose in the late Permian, include the ancestors of all mammals. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.
Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon; the animals are thought to have been drowned by a flash flood. The extensive shared burrows indicate that these animals were capable of complex social behaviors. | Evolution of mammals | Wikipedia | 328 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Their primitive synapsid and therapsid ancestors were very large (between ) but cynodonts gradually decreased in size (to ) even before the Permian-Triassic extinction event, probably due to competition with other therapsids. After the extinction event, the probainognathian cynodont group rapidly decreased in size (to ) due to new competition with archosaurs and transitioned to nocturnality, evolving nocturnal features, pulmonary alveoli, bronchioles and a developed diaphragm for a larger surface area for breathing, enucleated erythrocytes, a large intestine which bears a true colon after the cecum, endothermy, a hairy, glandular and thermoregulatory skin (which releases sebum and sweat), and a 4-chambered heart to maintain their high metabolism, larger brains, and fully upright hindlimb (forelimbs remained semi sprawling, and became like that only later, in therians). Some skin glands may have evolved into mammary glands in females for fulfilling the metabolic demands of their offspring (which increased 10 times). Many skeletal changes occurred also: the dentary bone became stronger and held differentiated teeth, for example, and the pair of nasal openings in the skull became fused.
These evolutionary changes led to the first mammals (size around ). They appear to have evolved rapid growth and short lifespan, a life history trait also found in numerous modern small-bodied mammals. They also adapted to a burrowing lifestyle, losing their large tail-based leg muscles which allowed dinosaurs to become bipedal, which may explain why bipedal mammals are so rare.
Triassic takeover
The catastrophic mass extinction at the end of the Permian, around 252 million years ago, killed off about 70% of terrestrial vertebrate species and the majority of land plants.
As a result, ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, which were dominant predators in the late Permian, the cynodonts' principal competitors for dominance of the carnivorous niches were a previously obscure sauropsid group, the archosaurs, which includes the ancestors of crocodilians and dinosaurs. | Evolution of mammals | Wikipedia | 471 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The archosaurs quickly became the dominant carnivores, a development often called the "Triassic takeover". Their success may have been due to the fact that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea, as mammals do today; considerable water is required to keep urea dissolved.
However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which has been tested) and the fact that desert-dwelling mammals are as well adapted in this department as archosaurs, and some cynodonts like Trucidocynodon were large-sized predators.
The Triassic takeover was probably a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches that had minimal competition from the archosaurs: the tritylodonts, which were herbivores, and the mammals, most of which were small nocturnal insectivores (although some, like Sinoconodon, were carnivores that fed on vertebrate prey, while still others were herbivores or omnivores). As a result: | Evolution of mammals | Wikipedia | 306 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
As the body length of the mammals' ancestors fell below , advances in thermal insulation and temperature regulation would have become necessary for nocturnal life.
Acute senses of hearing and smell became vital.
This accelerated the development of the mammalian middle ear (though the complete detachment of the middle ear bones from the jaw happened independently in monotremes).
The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight. Brain tissue requires a disproportionate amount of energy. The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
Probably as a side-effect of the nocturnal life, mammals lost two of the four cone opsins, photoreceptors in the retina, present in the eyes of the earliest amniotes. Paradoxically, this might have improved their ability to discriminate colors in dim light.
This retreat to a nocturnal role is called a nocturnal bottleneck, and is thought to explain many of the features of mammals.
From cynodonts to crown mammals
Fossil record
Mesozoic synapsids that had evolved to the point of having a jaw joint composed of the dentary and squamosal bones are preserved in few good fossils, mainly because they were mostly smaller than rats:
They were largely restricted to environments that are less likely to provide good fossils. Floodplains as the best terrestrial environments for fossilization provide few mammal fossils, because they are dominated by medium to large animals, and the mammals could not compete with archosaurs in the medium to large size range.
Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including fungi and bacteria) and by being trodden on.
Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
In the past years, however, the number of Mesozoic fossil mammals has increased decisively; only 116 genera were known in 1979, for example, but about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons". | Evolution of mammals | Wikipedia | 483 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Mammals or mammaliaforms
Some writers restrict the term "mammal" to the crown group mammals, the group consisting of the most recent common ancestor of the monotremes, marsupials, and placentals, together with all the descendants of that ancestor. In an influential 1988 paper, Timothy Rowe advocated this restriction, arguing that "ancestry... provides the only means of properly defining taxa" and, in particular, that the divergence of the monotremes from the animals more closely related to marsupials and placentals "is of central interest to any study of Mammalia as a whole." To accommodate some related taxa falling outside the crown group, he defined the Mammaliaformes as comprising "the last common ancestor of Morganucodontidae and Mammalia [as he had defined the latter term] and all its descendants." Besides Morganucodontidae, the newly defined taxon includes Docodonta and Kuehneotheriidae. Though haramiyids have been referred to the mammals since the 1860s, Rowe excluded them from the Mammaliaformes as falling outside his definition, putting them in a larger clade, the Mammaliamorpha.
Some writers have adopted this terminology noting, to avoid misunderstanding, that they have done so. Most paleontologists, however, still think that animals with the dentary-squamosal jaw joint and the sort of molars characteristic of modern mammals should formally be members of Mammalia.
Where the ambiguity in the term "mammal" may be confusing, this article uses "mammaliaform" and "crown mammal".
Family tree – cynodonts to crown group mammals
(based on Cynodontia:Dendrogram – Palaeos)
Morganucodontidae
The Morganucodontidae first appeared in the late Triassic, about 205 million years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints. They were also one of the first discovered and most thoroughly studied of the mammaliaforms outside of the crown-group mammals, since an unusually large number of morganucodont fossils have been found.
Docodonts | Evolution of mammals | Wikipedia | 443 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Docodonts, among the most common Jurassic mammaliaforms, are noted for the sophistication of their molars. They are thought to have had general semi-aquatic tendencies, with the fish-eating Castorocauda ("beaver tail"), which lived in the mid-Jurassic about 164M years ago and was first discovered in 2004 and described in 2006, being the most well-understood example. Castorocauda was not a crown group mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the "small nocturnal insectivore" stereotype:
It was noticeably larger than most Mesozoic mammaliaform fossils — about from its nose to the tip of its tail, and may have weighed .
It provides the earliest absolutely certain evidence of hair and fur. Previously the earliest was Eomaia, a crown group mammal from about 125M years ago.
It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammaliaforms were from the Eocene, about 110M years later.
Castorocauda'''s powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the platypus, which also swims and digs.
Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey.
Hadrocodium
The family tree above shows Hadrocodium as a close relative of crown-group mammals. This mammaliaform, dated about 195 million years ago in the very early Jurassic, exhibits some important features: | Evolution of mammals | Wikipedia | 390 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
In therapsids and early mammaliaforms, the eardrum may have stretched over a trough at the rear of the lower jaw. But Hadrocodium had no such trough, which suggests its ear was part of the cranium, as it is in crown-group mammals — and hence that the former articular and quadrate had migrated to the middle ear and become the malleus and incus. On the other hand, the dentary has a "bay" at the rear that mammals lack. This suggests that Hadrocodium's dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that Hadrocodium or a very close ancestor may have been the first to have a fully mammalian middle ear.
Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. Hadrocodium's braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain — in other words it now became possible for mammaliaforms both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.
Kuehneotheriidae
The family Kuehneotheriidae, known from the Late Triassic and Early Jurassic, was originally classified as part of either 'Symmetrodonta' or 'Pantotheria' based on their tooth structure, with Kuehneotherium once being considered the oldest known representative of Theria. They have since been recovered as among the closest relatives of crown-group mammals. As only tooth fossils have been discovered, they nevertheless remain poorly known, and have rarely been included in phylogenetic studies. | Evolution of mammals | Wikipedia | 485 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Earliest crown mammals
The crown group mammals, sometimes called 'true mammals', are the extant mammals and their close relatives back to their last common ancestor. Since this group has living members, DNA analysis can be applied in an attempt to explain the evolution of features that do not appear in fossils. This endeavor often involves molecular phylogenetics, a technique that has become popular since the mid-1980s.
Family tree of early crown mammals
Cladogram after Z.-X Luo († marks extinct groups) and Hackländer.
Color vision
Early amniotes had four opsins in the cones of their retinas to use for distinguishing colours: one sensitive to red, one to green, and two corresponding to different shades of blue. The green opsin was not inherited by any crown mammals, but all normal individuals did inherit the red one. Early crown mammals thus had three cone opsins, the red one and both of the blues. All their extant descendants have lost one of the blue-sensitive opsins but not always the same one: monotremes retain one blue-sensitive opsin, while marsupials and placentals retain the other (except cetaceans, which later lost the other blue opsin as well). Some placentals and marsupials, including higher primates, subsequently evolved green-sensitive opsins; like early crown mammals, therefore, their vision is trichromatic. | Evolution of mammals | Wikipedia | 285 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Australosphenida and Ausktribosphenidae
Ausktribosphenidae is a group name that has been given to some rather puzzling finds that:
appear to have tribosphenic molars, a type of tooth that is otherwise known only in placentals and marsupials.
come from mid-Cretaceous deposits in Australia — but Australia was connected only to Antarctica, and placentals originated in the Northern Hemisphere and were confined to it until continental drift formed land connections from North America to South America, from Asia to Africa and from Asia to India.
are represented only by teeth and jaw fragments, which is not very helpful.
Australosphenida is a group that has been defined in order to include the Ausktribosphenidae and monotremes. Asfaltomylos (mid- to late Jurassic, from Patagonia) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout Gondwanaland (the old Southern Hemisphere super-continent).
Recent analysis of Teinolophos, which lived somewhere between 121 and 112.5 million years ago, suggests that it was a "crown group" (advanced and relatively specialised) monotreme. This was taken as evidence that the basal (most primitive) monotremes must have appeared considerably earlier, but this has been disputed (see the following section). The study also indicated that some alleged Australosphenids were also "crown group" monotremes (e.g. Steropodon) and that other alleged Australosphenids (e.g. Ausktribosphenos, Bishops, Ambondro, Asfaltomylos) are more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below). | Evolution of mammals | Wikipedia | 447 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
MonotremesTeinolophos, from Australia, is the earliest known monotreme. A 2007 study (published 2008) suggests that it was not a basal (primitive, ancestral) monotreme but a full-fledged platypus, and therefore that the platypus and echidna lineages diverged considerably earlier. A more recent study (2009), however, has suggested that, while Teinolophos was a type of platypus, it was also a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to remain attached to their mothers and would drown if their mothers ventured into water (though there are exceptions like the water opossum and the lutrine opossum; however, they both live in South America and thus do not come into contact with monotremes). Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M, when they made their transition from semi-aquatic to terrestrial lifestyle.
Monotremes have some features that may be inherited from the cynodont ancestors:
like lizards and birds, they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole").
they lay eggs that are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
These features are not visible in fossils, and the main characteristics from paleontologists' point of view are:
a slender dentary bone in which the coronoid process is small or non-existent.
the external opening of the ear lies at the posterior base of the jaw.
the jugal bone is small or non-existent.
a primitive pectoral girdle with strong ventral elements: coracoids, clavicles and interclavicle. Note: therian mammals have no interclavicle.
sprawling or semi-sprawling forelimbs.
Multituberculates | Evolution of mammals | Wikipedia | 473 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Multituberculates (named for the multiple tubercles on their "molars") are often called the "rodents of the Mesozoic", but this is an example of convergent evolution rather than meaning that they are closely related to the Rodentia. They existed for approximately 120 million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene.
Some authors have challenged the phylogeny represented by the cladogram above. They exclude the multituberculates from the mammalian crown group, holding that multituberculates are more distantly related to extant mammals than even the Morganucodontidae. Multituberculates are like undisputed crown mammals in that their jaw joints consist of only the dentary and squamosal bones-whereas the quadrate and articular bones are part of the middle ear; their teeth are differentiated, occlude, and have mammal-like cusps; they have a zygomatic arch; and the structure of the pelvis suggests that they gave birth to tiny helpless young, like modern marsupials. On the other hand, they differ from modern mammals:
Their "molars" have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molars of uncontested early crown mammals.
The chewing action differs in that undisputed crown mammals chew with a side-to-side grinding action, which means that the molars usually occlude on only one side at a time, while multituberculates' jaws were incapable of side-to-side movement—they chewed, rather, by dragging the lower teeth backwards against the upper ones as the jaw closed.
The anterior (forward) part of the zygomatic arch mostly consists of the maxilla (upper jawbone) rather than the jugal, a small bone in a little slot in the maxillary process (extension).
The squamosal does not form part of the braincase.
The rostrum (snout) is unlike that of undisputed crown mammals; in fact it looks more like that of a pelycosaur, such as Dimetrodon. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the nasal the top, and the tall premaxilla at the front.
Theria | Evolution of mammals | Wikipedia | 509 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Theria ("beasts") is the clade originating with the last common ancestor of the Eutheria (including placentals) and Metatheria (including marsupials). Common features include:
no interclavicle.
coracoid bones non-existent or fused with the shoulder blades to form coracoid processes.
a type of crurotarsal ankle joint in which: the main joint is between the tibia and astragalus; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach. (The other well-known type of crurotarsal ankle is seen in crocodilians and works differently — most of the bending at the ankle is between the calcaneum and astragalus).
tribosphenic molars.
Metatheria
The living Metatheria are all marsupials (animals with pouches). A few fossil genera, such as the Mongolian late Cretaceous Asiatherium, may be marsupials or members of some other metatherian group(s).
The oldest known metatherian is Sinodelphys, found in 125M-year-old early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
Didelphimorphia (common opossums of the Western Hemisphere) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-arboreal unspecialized omnivores.
Tracks from the Early Cretaceous of Angola show the existence of raccoon-size mammals 118 million years ago. | Evolution of mammals | Wikipedia | 339 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The best-known feature of marsupials is their method of reproduction:
The mother develops a kind of yolk sack in her womb that delivers nutrients to the embryo. Embryos of bandicoots, koalas and wombats additionally form placenta-like organs that connect them to the uterine wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.
Pregnancy is very short, typically four to five weeks. The embryo is born at a very early stage of development, and is usually less than long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune system will attack the embryo.
The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a nipple, which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her mammary glands, as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple was historically thought to have restricted metatherian evolution, as it was assumed that the forelimb could not become specialised intro structures like wings, hooves or flippers. However, several bandicoots, most notably the pig-footed bandicoot, have true hooves similar to those of placental ungulates, and several marsupial gliders have evolved.
Although some marsupials look very like some placentals (the thylacine, "marsupial tiger" or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals:
Some, including the thylacine, have four molars; whereas no known placental has more than three.
All have a pair of palatal fenestrae, window-like openings on the bottom of the skull (in addition to the smaller nostril openings). | Evolution of mammals | Wikipedia | 411 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Marsupials also have a pair of marsupial bones (sometimes called "epipubic bones"), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.
Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.
Eutheria
The time of appearance of the earliest eutherians has been a matter of controversy. On one hand, recently discovered fossils of Juramaia have been dated to 160 million years ago and classified as eutherian. Fossils of Eomaia from 125 million years ago in the Early Cretaceous have also been classified as eutherian. A recent analysis of phenomic characters, however, classified Eomaia as pre-eutherian and reported that the earliest clearly eutherian specimens came from Maelestes, dated to 91 million years ago. That study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the Cretaceous–Paleogene boundary, about 66 million years ago.Eomaia was found to have some features that are more like those of marsupials and earlier metatherians: | Evolution of mammals | Wikipedia | 295 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals — early mammaliaforms, non-placental eutherians, marsupials, and monotremes — as well as in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.
A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
Five incisors in each side of the upper jaw. This number is typical of metatherians, and the maximum number in modern placentals is three, except for homodonts, such as the armadillo. But Eomaia's molar to premolar ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials.Eomaia also has a Meckelian groove, a primitive feature of the lower jaw that is not found in modern placental mammals.
These intermediate features are consistent with molecular phylogenetics estimates that the placentals diversified about 110M years ago, 15M years after the date of the Eomaia fossil.Eomaia also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.
Placentals' best-known feature is their method of reproduction:
The embryo attaches itself to the uterus via a large placenta via which the mother supplies food and oxygen and removes waste products.
Pregnancy is relatively long and the young are fairly well developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.
It has been suggested that the evolution of placental reproduction was made possible by retroviruses that: | Evolution of mammals | Wikipedia | 452 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
make the interface between the placenta and uterus into a syncytium, i.e. a thin layer of cells with a shared external membrane. This allows the passage of oxygen, nutrients and waste products, but prevents the passage of blood and other cells that would cause the mother's immune system to attack the fetus.
reduce the aggressiveness of the mother's immune system, which is good for the foetus but makes the mother more vulnerable to infections.
From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth, ankles and feet.
Expansion of ecological niches in the Mesozoic
thumb|Skull cast of Late Cretaceous Didelphodon, showing its robust teeth adapted to a durophagous diet.
Generally speaking, most species of mammaliaforms did occupy the niche of small, nocturnal insectivores, but recent finds, mainly in China, show that some species and especially crown group mammals were larger and that there was a larger variety of lifestyles than previously thought. For example: | Evolution of mammals | Wikipedia | 217 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
The therian Patagomaia, found in the Late Cretaceous Chorrillo Formation (Argentina) is the largest known Mesozoic mammal, weighing an estimated .
Adalatherium hui is a large sized, erect limbed herbivore from the Cretaceous of Madagascar.Castorocauda, a member of Docodonta which lived in the middle Jurassic about 164 million years, was about long, weighed , had a beaver-like tail that was adapted for swimming, limbs adapted for swimming and digging, and teeth adapted for eating fish. Another docodont, Haldanodon, also had semi-aquatic habits, and indeed aquatic tendencies were probably common among docodonts based on their prevalence in wetland environments. The eutriconodonts Liaoconodon and Yanoconodon have more recently also have been suggested to be freshwater swimmers, lacking Castorocaudas powerful tail but possessing paddle-like limbs; the eutriconodont Astroconodon has similarly been suggested as being semi-aquatic in the past, albeit to less convincing evidence.
Multituberculates are allotherians that survived for over 125 million years (from mid-Jurassic, about 160M years ago, to late Eocene, about 35M years ago) are often called the "rodents of the Mesozoic". As noted above, they may have given birth to tiny live neonates rather than laying eggs.
Fruitafossor, from the late Jurassic period about 150 million years ago, was about the size of a chipmunk and its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably termites, as ants had not yet appeared).
Similarly, the gobiconodontid Spinolestes possessed adaptations for fossoriality and convergent traits with placental xenarthrans like scutes and xenarthrous vertebrae, so it too might have had anteater like habits. It is also notable for the presence of quills akin to those of modern spiny mice.
Volaticotherium, from the boundary the early Cretaceous about 125M years ago, is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like that of a modern flying squirrel. This also suggests it was active mainly during the day. The closely related Argentoconodon also shows similar adaptations that may also suggest aerial locomotion. | Evolution of mammals | Wikipedia | 499 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Repenomamus, a eutriconodont from the early Cretaceous 130 million years ago, was a stocky, badger-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, one more than long and weighing about , the other less than long and weighing .
Schowalteria is a Late Cretaceous species almost as large if not larger than R. giganticus that shows speciations towards herbivory, comparable to those of modern ungulates.
Zhelestidae is a lineage of Late Cretaceous herbivorous eutherians, to the point of being mistaken for stem-ungulates.
Similarly, mesungulatids are also fairly large sized herbivorous mammals from the Late Cretaceous
Deltatheroidans were metatherians that were specialised towards carnivorous habits, and possible forms like Oxlestes and Khudulestes might have been among the largest Mesozoic mammals, though their status as deltatheroidans is questionable.
Ichthyoconodon, a eutriconodont from the Berriasian of Morocco, is currently known from molariforms found in marine deposits. These teeth are sharp-cusped and similar in shape to those of piscivorous mammals, and unlike the teeth of contemporary mammals they do not show degradation, so rather than being carried down by river deposits the animal died in situ or close. This has been taken to mean that it was a marine mammal, likely one of the few examples known from the Mesozoic.
Didelphodon is a Late Cretaceous riverine species of stagodontid marsupialiform with a durophagous dentition, robust jaws similar to a modern Tasmanian devil, and a postcranial skeleton very similar in size and shape to an otter. This animal has been lauded as the strongest bite of all Mesozoic mammals. It possibly specialized on eating freshwater crabs and molluscs.
Tracks of a raccoon-sized mammaliaform representing the morphofamily Ameghinichnidae are described from the Early Cretaceous (late Aptian) Calonda Formation (Angola) by Mateus et al. (2017), who name a new ichnotaxon Catocapes angolanus.
A gobiconodontid was preserved attacking a substantially larger dinosaur. | Evolution of mammals | Wikipedia | 476 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
A study on Mesozoic mammaliaforms suggests that they were a primary factor in constraining mammalian body size, rather than solely competition from dinosaurs. In general, it appears mammal faunas on southern continents had attained larger body sizes than those of northern continents.
Evolution of major groups of living mammals
There are currently vigorous debates between traditional paleontologists and molecular phylogeneticists about how and when the modern groups of mammals diversified, especially the placentals. Generally, the traditional paleontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the Cretaceous) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genetics. These debates extend to the definition of and relationships between the major groups of placentals.
Molecular phylogenetics-based family tree of placental mammals
Molecular phylogenetics uses features of organisms' genes to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.
Molecular phylogeneticists have proposed a family tree that is both broadly similar to but has notable differences from that of the paleontologists. Like paleontologists, molecular phylogeneticists have differing ideas about various details, but here is a typical family tree according to molecular phylogenetics: Note that the diagram shown here omits extinct groups, as one cannot extract DNA from most fossils. Some finer-level subdivisions are glossed-over. | Evolution of mammals | Wikipedia | 345 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Here are the most significant of the differences between this family tree and the one familiar to paleontologists:
The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. However, analysis of transposable element insertions supports a three-way top-level split between Xenarthra, Afrotheria and Boreoeutheria and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.
Afrotheria contains several groups that are only distantly related according to the paleontologists' version: Afroinsectiphilia ("African insectivores"), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria that paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
Members of the defunct order of Insectivores are divided among three clades: One clade is part of Afrotheria, and the other two clades are distinct sub-groups within Boreoeutheria.
Bats are closer to Carnivora and odd-toed ungulates than to Primates and Dermoptera (colugos).
Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).
The grouping together of the Afrotheria has some geological justification: All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the Indian elephant, which diverged from an African lineage about . As Pangaea broke up, Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago. So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated into all the available ecological niches. | Evolution of mammals | Wikipedia | 471 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Nevertheless, these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:
Mitochondrial DNA's mutation rate in mammals varies from region to region — some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species.
Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.
Timing of placental evolution
Recent molecular phylogenetic studies suggest that most placental orders diverged late in the Cretaceous period, about 100 to 85 million years ago, but that modern families first appeared later, in the late Eocene and early Miocene epochs of the Cenozoic period. Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic. Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is possible that they simply did not exist, and that the molecular clock runs fast during major evolutionary radiations. On the other hand, there is fossil evidence from of hoofed mammals that may be ancestors of modern ungulates.
Fossils of the earliest members of most modern groups date from the Paleocene, a few date from later and very few from the Cretaceous, before the extinction of the dinosaurs. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolate backwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous. However, statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene.
Evolution of mammalian features
Jaws and middle ears
Hadrocodium, whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the dentary and squamosal bones while the articular and quadrate move to the middle ear, where they are known as the incus and malleus. | Evolution of mammals | Wikipedia | 501 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
One analysis of the monotreme Teinolophos suggested that this animal had a pre-mammalian jaw joint formed by the angular and quadrate bones and that the definitive mammalian middle ear evolved twice independently, in monotremes and in therian mammals, but this idea has been disputed. In fact, two of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that Teinolophos was a full-fledged platypus, which means it would have had a mammalian jaw joint and middle ear.
Lactation
It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):
While the amniote egg is usually described as able to evolve away from water, most reptile eggs actually need moisture if they are not to dry out.
Monotremes do not have nipples, but secrete milk from a hairy patch on their bellies.
During incubation, monotreme eggs are covered in a sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards, a fourth layer appears with a composition different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
If so, that may explain why the patches from which monotremes secrete milk are hairy. It is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.
Later research demonstrated that caseins already appeared in the common mammalian ancestor approximately 200–310 million years ago. The question of whether secretions of a substance to keep eggs moist translated into actual lactation in therapsids is open. A small mammaliomorph called Sinocodon, generally assumed to be the sister group of all later mammals, had front teeth in even the smallest individuals. Combined with a poorly ossified jaw, they very probably did not suckle. Thus suckling may have evolved right at the pre-mammal/mammal transition. However, tritylodontids, generally assumed to be more basal, show evidence of suckling. Morganucodontans, also assumed to be basal Mammaliaformes, also show evidence of lactation. | Evolution of mammals | Wikipedia | 475 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Digestive system
The evolution of the digestive system has formed a significant influence in mammal evolution. With the emergence of mammals, the digestive system was modified in a variety of ways depending on the animal's diet. For example, cats and most carnivores have simple large intestines, while the horse as a herbivore has a voluminous large intestine. An ancestral feature of ruminants is their multi-chambered (usually four-chambered) stomach, which evolved about 50 million years ago. Along with morphology of the gut, gastric acidity has been proposed as a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. Comparisons of stomach acidity across trophic groups in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish.
Despite the lack of fossilization of the gut, microbial evolution of the gut can be inferred from the interrelationships of existing animals, microbes and probable foodstuffs. Mammals are metagenomic, in that they are composed of not only their own genes, but also those of all of their associated microbes. Gut microbiota has co-diversified as mammalian species have evolved. Recent studies indicate that adaptive divergence between mammalian species is shaped in part by changes in the gut microbiota. The house mouse may have evolved not only with, but also in response to, the unique bacteria inhabiting its gut.
Hair and fur
The first clear evidence of hair or fur is in fossils of Castorocauda and Megaconus, from 164M years ago in the mid-Jurassic. As both mammals Megaconus and Castorocauda have a double coat of hair, with both guard hairs and an undercoat, it may be assumed that their last common ancestor did as well. More recently, the discovery of hair remnants in Permian coprolites pushes back the origin of mammalian hair much further back in the synapsid line to Paleozoic therapsids. | Evolution of mammals | Wikipedia | 442 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
In the mid-1950s, some scientists interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels that supplied blood vessels and nerves to vibrissae (whiskers) and suggested that this was evidence of hair or fur. It was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae; the modern lizard Tupinambis has foramina that are almost identical to those found in the non-mammalian cynodont Thrinaxodon. Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon. A trace fossil from the Lower Triassic had been erroneously regarded as a cynodont footprint showing hair, but this interpretation has been refuted. A study of cranial openings for facial nerves connected whiskers in extant mammals indicate the Prozostrodontia, small immediate ancestors of mammals, presented whiskers similar to mammals, but that less advanced therapsids would either have immobile whiskers or no whisker at all. Fur may have evolved from whiskers. Whiskers themselves may have evolved as a response to nocturnal and/or burrowing lifestyle.
Ruben & Jones (2000) note that the Harderian glands, which secrete lipids for coating the fur, were present in the earliest mammals like Morganucodon, but were absent in near-mammalian therapsids like Thrinaxodon. The Msx2 gene associated with hair follicle maintenance is also linked to the closure of the parietal eye in mammals, indicating that fur and lack of pineal eye is linked. The pineal eye is present in Thrinaxodon, but absent in more advanced cynognaths (the Probainognathia).
Insulation is the "cheapest" way to maintain a fairly constant body temperature, without consuming energy to produce more body heat. Therefore, the possession of hair or fur would be good evidence of homeothermy, but would not be such strong evidence of a high metabolic rate. | Evolution of mammals | Wikipedia | 441 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Erect limbs
Understanding of the evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. Some scientists think that the parasagittal (non-sprawling) limb posture is limited to the Boreosphenida, a group that contains the therians but not, for example, the multituberculates. In particular, they attribute a parasagittal stance to the therians Sinodelphys and Eomaia, which means that the stance had arisen by 125 million years ago, in the Early Cretaceous. However, they also discuss that earlier mammals had more erect forelimbs as opposed to the more sprawling hindlimbs, a trend still continued to some extent in modern placentals and marsupials.
Warm-bloodedness
"Warm-bloodedness" is a complex and rather ambiguous term, because it includes some or all of the following:Endothermy, the ability to generate heat internally rather than via behaviors such as basking or muscular activity.Homeothermy, maintaining a fairly constant body temperature. Most enzymes have an optimum operating temperature; efficiency drops rapidly outside the preferred range. A homeothermic organism needs only to possess enzymes that function well in a small range of temperatures.Tachymetabolism', maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature because of the Q10 effect: biochemical processes run about half as fast if an animal's temperature drops by 10 °C.
Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism. However, it is generally agreed that endothermy first evolved in non-mammalian synapsids such as dicynodonts, which possess body proportions associated with heat retention, high vascularised bones with Haversian canals, and possibly hair. More recently, it has been suggested that endothermy evolved as far back as Ophiacodon.
Modern monotremes have a low body temperature compared to marsupials and placental mammals, around . Phylogenetic bracketing suggests that the body temperatures of early crown-group mammals were not less than that of extant monotremes. There is cytological evidence that the low metabolism of monotremes is a secondarily evolved trait. | Evolution of mammals | Wikipedia | 480 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Respiratory turbinates
Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with mucous membranes that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in advanced Triassic cynodonts, such as Thrinaxodon and Diademodon, which suggests that they may have had fairly high metabolic rates.
Bony secondary palate
Mammals have a secondary bony palate, which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates. But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not.
Diaphragm
A muscular diaphragm helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. Diaphragms are known in caseid pelycosaurs, indicating an early origin within synapsids, though they were still fairly inefficient and likely required support from other muscle groups and limb motion.
The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had more developed diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates. On the other hand, these mammal-like rib cages may have evolved to increase agility. However, the movement of even advanced therapsids was "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaurs. So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful. | Evolution of mammals | Wikipedia | 499 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Limb posture
The therapsids had sprawling forelimbs and semi-erect hindlimbs. This suggests that Carrier's constraint would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs. Advanced therapsids may therefore have been significantly less active than modern mammals of similar size and so may have had slower metabolisms overall or else been bradymetabolic (lower metabolism when at rest).
Brain
Mammals are noted for their large brain size relative to body size, compared to other animal groups. Recent findings suggest that the first brain area to expand was that involved in smell. Scientists scanned the skulls of early mammal species dating back to 190–200 million years ago and compared the brain case shapes to earlier pre-mammal species; they found that the brain area involved in the sense of smell was the first to enlarge. This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.
After the extinction of the dinosaurs 66 million years ago, mammals began to increase in body size as new niches became available, but their brain lagged behind their bodies for the first ten million years. Relative to body size the brain of Paleocene mammal was relatively smaller than that of Mesozoic mammals. It was not until the Eocene that the mammalian brains began to catch up with their bodies, particularly in certain areas associated with their senses.
Testicular descent
Sexual selection | Evolution of mammals | Wikipedia | 305 | 10727548 | https://en.wikipedia.org/wiki/Evolution%20of%20mammals | Biology and health sciences | Basics_4 | Biology |
Construction aggregate, or simply aggregate, is a broad category of coarse- to medium-grained particulate material used in construction. Traditionally, it includes natural materials such as sand, gravel, crushed stone. As with other types of aggregates, it is a component of composite materials, particularly concrete and asphalt.
Aggregates are the most mined materials in the world, being a significant part of 6 billion tons of concrete produced per year.
Aggregate serves as reinforcement to add strength to the resulting material.
Due to the relatively high hydraulic conductivity as compared to most soil types, aggregates are widely used in drainage applications such as foundation and French drains, septic drain fields, retaining wall drains, and roadside edge drains. Aggregates are also used as base material under building foundations, roads, and railroads (aggregate base). It has predictable, uniform properties, preventing differential settling under the road or building.
Aggregates are also used as a low-cost extender that binds with more expensive cement or asphalt to form concrete. Although most kinds of aggregate require a form of binding agent, there are types of self-binding aggregate which require no form of binding agent.
More recently, recycled concrete and geosynthetic materials have also been used as aggregates.
Sources
Sources for these basic materials can be grouped into three main areas: mining of mineral aggregate deposits, including sand, gravel, and stone; use of waste slag from the manufacture of iron and steel; and recycling of concrete, which is itself chiefly manufactured from mineral aggregates. In addition, there are some (minor) materials that are used as specialty lightweight aggregates: clay, pumice, perlite, and vermiculite. Other minerals include:
basalt
dolomite
granite
gravel
limestone
sand
sandstone
Specifications
In Europe, sizing ranges are specified as d/D, where the d shows the smallest and D shows the largest square mesh grating that the particles can pass. Application-specific preferred sizings are covered in European Standard EN 13043 for road construction, EN 13383 for larger armour stone, EN 12620 for concrete aggregate, EN 13242 for base layers of road construction, and EN 13450 for railway ballast. | Construction aggregate | Wikipedia | 442 | 7096085 | https://en.wikipedia.org/wiki/Construction%20aggregate | Technology | Building materials | null |
The American Society for Testing and Materials publishes an exhaustive listing of specifications including ASTM D 692 and ASTM D 1073 for various construction aggregate products, which, by their individual design, are suitable for specific construction purposes. These products include specific types of coarse and fine aggregate designed for such uses as additives to asphalt and concrete mixes, as well as other construction uses. State transportation departments further refine aggregate material specifications in order to tailor aggregate use to the needs and available supply in their particular locations.
History
People have used sand and stone for foundations for thousands of years. Significant refinement of the production and use of aggregate occurred during the Roman Empire, which used aggregate to build its vast network of roads and aqueducts. The invention of concrete, which was essential to architecture utilizing arches, created an immediate, permanent demand for construction aggregates.
Vitruvius writes in De architectura:
Economy denotes the proper management of materials and of site, as well as a thrifty balancing of cost and common sense in the construction of works. This will be observed if, in the first place, the architect does not demand things which cannot be found or made ready without great expense. For example: it is not everywhere that there is plenty of pit-sand, rubble, fir, clear fir, and marble... Where there is no pit sand, we must use the kinds washed up by rivers or by the sea... and other problems we must solve in similar ways.
Modern production
The advent of modern blasting methods enabled the development of quarries, which are now used throughout the world, wherever competent bedrock deposits of aggregate quality exist. In many places, good limestone, granite, marble or other quality stone bedrock deposits do not exist. In these areas, natural sand and gravel are mined for use as aggregate. Where neither stone, nor sand and gravel, are available, construction demand is usually satisfied by shipping in aggregate by rail, barge or truck. Additionally, demand for aggregates can be partially satisfied through the use of slag and recycled concrete. However, the available tonnages and lesser quality of these materials prevent them from being a viable replacement for mined aggregates on a large scale. | Construction aggregate | Wikipedia | 443 | 7096085 | https://en.wikipedia.org/wiki/Construction%20aggregate | Technology | Building materials | null |
Large stone quarry and sand and gravel operations exist near virtually all population centers due to the high cost of transportation relative to the low value of the product. Trucking aggregate more than 40 kilometers is typically uneconomical. These are capital-intensive operations, utilizing large earth-moving equipment, belt conveyors, and machines specifically designed for crushing and separating various sizes of aggregate, to create distinct product stockpiles.
According to the USGS, 2006 U.S. crushed stone production was 1.72 billion tonnes valued at $13.8 billion (compared to 1.69 billion tonnes valued at $12.1 billion in 2005), of which limestone was 1,080 million tonnes valued at $8.19 billion from 1,896 quarries, granite was 268 million tonnes valued at $2.59 billion from 378 quarries, trap rock was 148 million tonnes valued at $1.04 billion from 355 quarries, and the balance other kinds of stone from 729 quarries. Limestone and granite are also produced in large amounts as dimension stone. The great majority of crushed stone is moved by heavy truck from the quarry/plant to the first point of sale or use. According to the USGS, 2006 U.S. sand and gravel production was 1.32 billion tonnes valued at $8.54 billion (compared to 1.27 billion tonnes valued at $7.46 billion in 2005), of which 264 million tonnes valued at $1.92 billion was used as concrete aggregates. The great majority of this was again moved by truck, instead of by electric train.
Currently, total U.S. aggregate demand by final market sector was 30%–35% for non-residential building (offices, hotels, stores, manufacturing plants, government and institutional buildings, and others), 25% for highways, and 25% for housing.
Recycled materials
Recycled material such as blast furnace and steel furnace slag can be used as aggregate or partly substitute for portland cement. Blast furnace and steel slag is either air-cooled or water-cooled. Air-cooled slag can be used as aggregate. Water-cooled slag produces sand-sized glass-like particles (granulated). Adding free lime to the water during cooling gives granulated slag hydraulic cementitious properties. | Construction aggregate | Wikipedia | 463 | 7096085 | https://en.wikipedia.org/wiki/Construction%20aggregate | Technology | Building materials | null |
In 2006, according to the USGS, air-cooled blast furnace slag sold or used in the U.S. was 7.3 million tonnes valued at $49 million, granulated blast furnace slag sold or used in the U.S. was 4.2 million tonnes valued at $318 million, and steel furnace slag sold or used in the U.S. was 8.7 million tonnes valued at $40 million. Air-cooled blast furnace slag sales in 2006 were for use in road bases and surfaces (41%), asphaltic concrete (13%), ready-mixed concrete (16%), and the balance for other uses. Granulated blast furnace slag sales in 2006 were for use in cementitious materials (94%), and the balance for other uses. Steel furnace slag sales in 2006 were for use in road bases and surfaces (51%), asphaltic concrete (12%), for fill (18%), and the balance for other uses.
Recycled glass aggregate crushed to a small size is substituted for many construction and utility projects in place of pea gravel or crushed rock. Glass aggregate is not dangerous to handle. It can be used as pipe bedding—placed around sewer, storm water or drinking water pipes to transfer weight from the surface and protect the pipe. Another common use is as fill to bring the level of a concrete floor even with a foundation. Use of glass aggregate helps close the loop in glass recycling in many places where glass cannot be smelted into new glass.
Aggregates themselves can be recycled as aggregates. Recyclable aggregate tends to be concentrated in urban areas. The supply of recycled aggregate depends on physical decay and demolition of structures. Mobile recycling plants eliminate the cost of transporting the material to a central site. The recycled material is typically of variable quality.
Many aggregate products are recycled for other industrial purposes. Contractors save on disposal costs and less aggregate is buried or piled and abandoned. In Bay City, Michigan, for example, a recycle program exists for unused products such as mixed concrete, block, brick, gravel, pea stone, and other used materials. The material is crushed to provide subbase for roads and driveways, among other purposes. | Construction aggregate | Wikipedia | 456 | 7096085 | https://en.wikipedia.org/wiki/Construction%20aggregate | Technology | Building materials | null |
According to the USGS in 2006, 2.9 million tonnes of Portland cement concrete (including aggregate) worth $21.9 million was recycled, and 1.6 million tonnes of asphalt concrete (including aggregate) worth $11.8 million was recycled, both by crushed stone operations. Much more of both materials are recycled by construction and demolition firms not included in the USGS survey. For sand and gravel, the survey showed that 4.7 million tonnes of cement concrete valued at $32.0 million was recycled, and 6.17 million tonnes of asphalt concrete valued at $45.1 million was recycled. Again, more of both materials are recycled by construction and demolition firms not in this USGS survey. The Construction Materials Recycling Association indicates that there are 325 million tonnes of recoverable construction and demolition materials produced annually.
Organic materials
Many geosynthetic aggregates are made from recycled materials. Recyclable plastics can be reused in aggregates. For example, Ring Industrial Group's EZflow product lines are produced with geosynthetic aggregate pieces that are more than 99.9% recycled polystyrene. This polystyrene, otherwise destined for a landfill, is gathered, melted, mixed, reformulated and expanded to create low density aggregates that maintain high strength properties under compressive loads. Such geosynthetic aggregates replace conventional gravel while simultaneously increasing porosity, increasing hydraulic conductivity and eliminating the fine dust "fines" inherent to gravel aggregates which otherwise serve to clog and disrupt the operation of many drainage applications.
Several groups have attempted to use minced tires as part of concrete aggregate. The result is tougher than regular concrete, because it can bend instead of breaking under pressure. However, tires reduce compressive strength partially because the cement bonds poorly with the rubber. Pores in the rubber fill with water when the concrete is mixed, but become voids as the concrete sets. One group put the concrete under pressure as it sets, reducing pore volumes.
Recycled aggregates in the UK
Recycled aggregate in the UK results from the processing of construction material. To ensure the aggregate is inert, it is manufactured from material tested and characterised under European Waste Codes.
In 2008, 210 million tonnes of aggregate were produced including 67 million tonnes of recycled product, according to the Quarry Products Association. The Waste and Resource Action Programme has produced a Quality Protocol for the regulated production of recycled aggregates. | Construction aggregate | Wikipedia | 491 | 7096085 | https://en.wikipedia.org/wiki/Construction%20aggregate | Technology | Building materials | null |
Archaeopteris is an extinct genus of progymnosperm tree with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous (), the oldest fossils being 385 million years old, and had global distribution.
Until the 2007 discovery of Wattieza, many scientists considered Archaeopteris to be the earliest known tree. Bearing buds, reinforced branch joints, and branched trunks similar to today's woody plants, it is more reminiscent of modern seed-bearing trees than other spore-bearing taxa. It combines characteristics of woody trees and herbaceous ferns, and belongs to the progymnosperms, a group of extinct plants more closely related to seed plants than to ferns, but unlike seed plants, reproducing using spores like ferns.
Taxonomy
John William Dawson described the genus in 1871. The name derives from the ancient Greek (archaīos, "ancient"), and (ptéris, "fern"). Archaeopteris was originally classified as a fern, and it remained classified so for over 100 years. In 1911, Russian paleontologist Mikhail Dimitrievich Zalessky described a new type of petrified wood from the Donets Basin in modern Ukraine. He called the wood Callixylon, though he did not find any structures other than the trunk. The similarity to conifer wood was recognized. It was also noted that ferns of the genus Archaeopteris were often found associated with fossils of Callixylon.
In the 1960s, paleontologist Charles B. Beck was able to demonstrate that the fossil wood known as Callixylon and the leaves known as Archaeopteris were actually part of the same plant. It was a plant with a mixture of characteristics not seen in any living plant, a link between true gymnosperms and ferns.
The genus Archaeopteris is placed in the order Archaeopteridales and family Archaeopteridaceae. The name is similar to that of the first known feathered bird, Archaeopteryx, but in this case refers to the fern-like nature of the plant's fronds. | Archaeopteris | Wikipedia | 455 | 1495085 | https://en.wikipedia.org/wiki/Archaeopteris | Biology and health sciences | Pteridophytes | Plants |
Relationship to spermatophytes
Archaeopteris is a member of a group of free-sporing woody plants called the progymnosperms that are interpreted as distant ancestors of the gymnosperms. Archaeopteris reproduced by releasing spores rather than by producing seeds, but some of the species, such as Archaeopteris halliana were heterosporous, producing two types of spores. This is thought to represent an early step in the evolution of vascular plants towards reproduction by seeds, which first appeared in the earliest, long-extinct gymnosperm group, the seed ferns (Pteridospermatophyta). The conifers or Pinophyta are one of four divisions of extant gymnosperms that arose from the seed ferns during the Carboniferous period.
Description
The trees of this genus typically grew to in height with leafy foliage reminiscent of some conifers. The large fern-like fronds were thickly set with fan-shaped leaflets or pinnae. The trunks of some species exceeded in diameter. The branches were borne in spiral arrangement, and a forked stipule was present at the base of each branch. Within a branch, leafy shoots were in opposite arrangement in a single plane. On fertile branches, some of the leaves were replaced by sporangia (spore capsules).
Other modern adaptations
Aside from its woody trunk, Archaeopteris possessed other modern adaptations to light interception and perhaps to seasonality as well. The large umbrella of fronds seems to have been quite optimized for light interception at the canopy level. In some species, the pinnules were shaped and oriented to avoid shading one another. There is evidence that whole fronds were shed together as single units, perhaps seasonally like modern deciduous foliage or like trees in the cypress family Cupressaceae.
The plant had nodal zones that would have been important sites for the subsequent development of lateral roots and branches. Some branches were latent and adventitious, similar to those produced by living trees that eventually develop into roots. Before this time, shallow, rhizomatous roots had been the norm, but with Archaeopteris, deeper root systems were being developed that could support ever higher growth. | Archaeopteris | Wikipedia | 469 | 1495085 | https://en.wikipedia.org/wiki/Archaeopteris | Biology and health sciences | Pteridophytes | Plants |
Habitat
Evidence indicates that Archaeopteris preferred wet soils, growing close to river systems and in floodplain woodlands. It would have formed a significant part of the canopy vegetation of early forests. Speaking of the first appearance of Archaeopteris on the world-scene, Stephen Scheckler, a professor of biology and geological sciences at Virginia Polytechnic Institute, says, "When [Archaeopteris] appears, it very quickly became the dominant tree all over the Earth. On all of the land areas that were habitable, they all had this tree". One species, Archaeopteris notosaria, has even been reported from within what was then the Antarctic Circle: leaves and fertile structures were identified from the Waterloo Farm lagerstätte in what is now South Africa.
Scheckler believes that Archaeopteris had a major role in transforming its environment. "Its litter fed the streams and was a major factor in the evolution of freshwater fishes, whose numbers and varieties exploded in that time, and influenced the evolution of other marine ecosystems. It was the first plant to produce an extensive root system, so had a profound impact on soil chemistry. And once these ecosystem changes happened, they were changed for all time. It was a one-time thing."
Looking roughly like a top-heavy Christmas tree, Archaeopteris may have played a part in the transformation of Earth's climate during the Devonian before becoming extinct within a short period of time at the beginning of the Carboniferous period. | Archaeopteris | Wikipedia | 314 | 1495085 | https://en.wikipedia.org/wiki/Archaeopteris | Biology and health sciences | Pteridophytes | Plants |
Li or ri (, lǐ, or , shìlǐ), also known as the Chinese mile, is a traditional Chinese unit of distance. The li has varied considerably over time but was usually about one third of an English mile and now has a standardized length of a half-kilometer (). This is then divided into 1,500 chi or "Chinese feet".
The character 里 combines the characters for "field" (田, tián) and "earth" (土, tǔ), since it was considered to be about the length of a single village. As late as the 1940s, a "li" did not represent a fixed measure but could be longer or shorter depending on the effort required to cover the distance.
There is also another li (Traditional: 釐, Simplified: 厘, lí) that indicates a unit of length of a chi, but it is used much less commonly. This li is used in the People's Republic of China as the equivalent of the centi- prefix in metric units, thus limi (厘米, límǐ) for centimeter. The tonal difference makes it distinguishable to speakers of Chinese, but unless specifically noted otherwise, any reference to li will always refer to the longer traditional unit and not to either the shorter unit or the kilometer. This traditional unit, in terms of historical usage and distance proportion, can be considered the East Asian counterpart to the Western league unit. However, in English league commonly means "3 miles."
Changing values
Like most traditional Chinese measurements, the li was reputed to have been established by the Yellow Emperor at the founding of Chinese civilization around 2600 BC and standardized by Yu the Great of the Xia dynasty six hundred years later. Although the value varied from state to state during the Spring and Autumn period and Warring States periods, historians give a general value to the li of 405 meters prior to the Qin dynasty imposition of its standard in the 3rd century BC.
The basic Chinese traditional unit of distance was the chi. As its value changed over time, so did the lis. In addition, the number of chi per li was sometimes altered. To add further complexity, under the Qin dynasty, the li was set at 360 "paces" (, bù) but the number of chi per bu was subsequently changed from 6 to 5, shortening the li by . Thus, the Qin li of about 576 meters became (with other changes) the Han li, which was standardized at 415.8 meters. | Li (unit) | Wikipedia | 500 | 1495421 | https://en.wikipedia.org/wiki/Li%20%28unit%29 | Physical sciences | East Asian | Basics and measurement |
The basic units of measurement remained stable over the Qin and Han periods. A bronze imperial standard measure, dated AD 9, had been preserved at the Imperial Palace in Beijing and came to light in 1924. This has allowed very accurate conversions to modern measurements, which has provided a new and extremely useful additional tool in the identification of place names and routes. These measurements have been confirmed in many ways including the discovery of a number of rulers found at archaeological sites, and careful measurements of distances between known points. The Han li was calculated by Dubs to be 415.8 metres and all indications are that this is a precise and reliable determination.
Under the Tang dynasty (AD 618–907), the li was approximately 323 meters.
In the late Manchu or Qing dynasty, the number of chi was increased from 1,500 per li to 1,800. This had a value of 2115 feet or 644.6 meters. In addition, the Qing added a longer unit called the tu, which was equal to 150 li (96.7 km).
These changes were undone by the Republic of China of Chiang Kai-shek, who adopted the metric system in 1928. The Republic of China (now also known as Taiwan) continues not to use the li at all but only the kilometer (Mandarin: , gōnglǐ, lit. "common li").
Under Mao Zedong, the People's Republic of China reinstituted the traditional units as a measure of anti-imperialism and cultural pride before officially adopting the metric system in 1984. A place was made within this for the traditional units, which were restandardized to metric values. A modern li is thus set at exactly half a kilometer (500 meters). However, unlike the jin which is still frequently preferred in daily use over the kilogram, the li is almost never used. Nonetheless, its appearance in many phrases and sayings means that "kilometer" must always be specified by saying gōnglǐ in full.
Cultural use
As one might expect for the equivalent of "mile", li appears in many Chinese sayings, locations, and proverbs as an indicator of great distances or the exotic: | Li (unit) | Wikipedia | 436 | 1495421 | https://en.wikipedia.org/wiki/Li%20%28unit%29 | Physical sciences | East Asian | Basics and measurement |
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