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Guided Multiple Launch Rocket System (GMLRS) 227mm rockets have an extended range and add GPS-aided guidance to their Inertial Navigation System. GMLRS rockets were introduced in 2005 and the M30 and M31 rockets are, except for their warheads, identical. 50,000 GMLRS rockets have been produced, with yearly production n... | https://en.wikipedia.org/wiki?curid=147011 |
The Army Tactical Missile System (ATACMS) is a series of 610 mm surface-to-surface missile (SSM) with a range of up to . Each rocket pod contains one ATACMS missile. , only the M48, M57, and M57E1 remain in the US military's arsenal. | https://en.wikipedia.org/wiki?curid=147011 |
The Precision Strike Missile (PrSM) is a new series of GPS-guided missiles, which will begin to replace ATACMS missiles in 2024. PrSM carries a newly designed area-effects warhead and has a range of . PrSM missiles can be launched from the M270A2 and the HIMARS, with rockets pods containing 2 missiles. the PrSM is in l... | https://en.wikipedia.org/wiki?curid=147011 |
Lockheed Martin UK and INSYS had jointly developed a demonstrator rocket artillery system similar to HIMARS for the British Army's "Lightweight Mobile Artillery Weapon System/Rocket" (LIMAWS(R)) program. The system consisted of a single MLRS pod, mounted on a Supacat SPV600 chassis. The LIMAWS(R) program was canceled i... | https://en.wikipedia.org/wiki?curid=147011 |
Lockheed Martin and Thales Australia are discussing with the Australian government, manufacturing HIMARS rockets in Australia, due in part to concerns of resupply during conflict. Australia has the ability to manufacture the rockets but it depends on the technology (specifically the guidance components) being authorize... | https://en.wikipedia.org/wiki?curid=147011 |
Occam's razor, Ockham's razor, or Ocham's razor (), also known as the principle of parsimony or the law of parsimony (), is the problem-solving principle that "entities should not be multiplied beyond necessity". It is generally understood in the sense that with competing theories or explanations, the simpler one, for ... | https://en.wikipedia.org/wiki?curid=36797 |
Similarly, in science, Occam's razor is used as an abductive heuristic in the development of theoretical models rather than as a rigorous arbiter between candidate models. In the scientific method, Occam's razor is not considered an irrefutable principle of logic or a scientific result; the preference for simplicity in... | https://en.wikipedia.org/wiki?curid=36797 |
The phrase "Occam's razor" did not appear until a few centuries after William of Ockham's death in 1347. Libert Froidmont, in his "On Christian Philosophy of the Soul", takes credit for the phrase, speaking of ""novacula occami"". Ockham did not invent this principle, but the "razor"—and its association with him—may be... | https://en.wikipedia.org/wiki?curid=36797 |
The origins of what has come to be known as Occam's razor are traceable to the works of earlier philosophers such as John Duns Scotus (1265–1308), Robert Grosseteste (1175–1253), Maimonides (Moses ben-Maimon, 1138–1204), and even Aristotle (384–322 BC). Aristotle writes in his "Posterior Analytics", "We may assume the ... | https://en.wikipedia.org/wiki?curid=36797 |
Phrases such as "It is vain to do with more what can be done with fewer" and "A plurality is not to be posited without necessity" were commonplace in 13th-century scholastic writing. Robert Grosseteste, in "Commentary on" [Aristotle's] "the Posterior Analytics Books" ("Commentarius in Posteriorum Analyticorum Libros") ... | https://en.wikipedia.org/wiki?curid=36797 |
The "Summa Theologica" of Thomas Aquinas (1225–1274) states that "it is superfluous to suppose that what can be accounted for by a few principles has been produced by many." Aquinas uses this principle to construct an objection to God's existence, an objection that he in turn answers and refutes generally (cf. "quinque... | https://en.wikipedia.org/wiki?curid=36797 |
William of Ockham ("circa" 1287–1347) was an English Franciscan friar and theologian, an influential medieval philosopher and a nominalist. His popular fame as a great logician rests chiefly on the maxim attributed to him and known as Occam's razor. The term "razor" refers to distinguishing between two hypotheses eithe... | https://en.wikipedia.org/wiki?curid=36797 |
While it has been claimed that Occam's razor is not found in any of William's writings, one can cite statements such as ("Plurality must never be posited without necessity"), which occurs in his theological work on the "Sentences of Peter Lombard" ("Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombar... | https://en.wikipedia.org/wiki?curid=36797 |
Nevertheless, the precise words sometimes attributed to William of Ockham, (Entities must not be multiplied beyond necessity), are absent in his extant works; this particular phrasing comes from John Punch, who described the principle as a "common axiom" ("axioma vulgare") of the Scholastics. William of Ockham's contri... | https://en.wikipedia.org/wiki?curid=36797 |
This principle is sometimes phrased as ("Plurality should not be posited without necessity"). In his "Summa Totius Logicae", i. 12, William of Ockham cites the principle of economy, ("It is futile to do with more things that which can be done with fewer"; Thorburn, 1918, pp. 352–53; Kneale and Kneale, 1962, p. 243.) | https://en.wikipedia.org/wiki?curid=36797 |
To quote Isaac Newton, "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, as far as possible, assign the same causes." | https://en.wikipedia.org/wiki?curid=36797 |
Bertrand Russell offers a particular version of Occam's razor: "Whenever possible, substitute constructions out of known entities for inferences to unknown entities." | https://en.wikipedia.org/wiki?curid=36797 |
Around 1960, Ray Solomonoff founded the theory of universal inductive inference, the theory of prediction based on observationsfor example, predicting the next symbol based upon a given series of symbols. The only assumption is that the environment follows some unknown but computable probability distribution. This theo... | https://en.wikipedia.org/wiki?curid=36797 |
Another technical approach to Occam's razor is ontological parsimony. Parsimony means spareness and is also referred to as the Rule of Simplicity. This is considered a strong version of Occam's razor. A variation used in medicine is called the "Zebra": a physician should reject an exotic medical diagnosis when a more c... | https://en.wikipedia.org/wiki?curid=36797 |
Ernst Mach formulated the stronger version of Occam's razor into physics, which he called the Principle of Economy stating: "Scientists must use the simplest means of arriving at their results and exclude everything not perceived by the senses." | https://en.wikipedia.org/wiki?curid=36797 |
This principle goes back at least as far as Aristotle, who wrote "Nature operates in the shortest way possible." The idea of parsimony or simplicity in deciding between theories, though not the intent of the original expression of Occam's razor, has been assimilated into common culture as the widespread layman's formul... | https://en.wikipedia.org/wiki?curid=36797 |
Prior to the 20th century, it was a commonly held belief that nature itself was simple and that simpler hypotheses about nature were thus more likely to be true. Thomas Aquinas made this argument in the 13th century, writing, "If a thing can be done adequately by means of one, it is superfluous to do it by means of sev... | https://en.wikipedia.org/wiki?curid=36797 |
Beginning in the 20th century, epistemological justifications based on induction, logic, pragmatism, and especially probability theory have become more popular among philosophers. | https://en.wikipedia.org/wiki?curid=36797 |
Occam's razor has gained strong empirical support in helping to converge on better theories (see Uses section below for some examples). | https://en.wikipedia.org/wiki?curid=36797 |
In the related concept of overfitting, excessively complex models are affected by statistical noise (a problem also known as the bias-variance trade-off), whereas simpler models may capture the underlying structure better and may thus have better predictive performance. It is, however, often difficult to deduce which p... | https://en.wikipedia.org/wiki?curid=36797 |
The razor's statement that "other things being equal, simpler explanations are generally better than more complex ones" is amenable to empirical testing. Another interpretation of the razor's statement would be that "simpler hypotheses are generally better than the complex ones". The procedure to test the former interp... | https://en.wikipedia.org/wiki?curid=36797 |
Even if some increases in complexity are sometimes necessary, there still remains a justified general bias toward the simpler of two competing explanations. To understand why, consider that for each accepted explanation of a phenomenon, there is always an infinite number of possible, more complex, and ultimately incorr... | https://en.wikipedia.org/wiki?curid=36797 |
For example, if a man, accused of breaking a vase, makes supernatural claims that leprechauns were responsible for the breakage, a simple explanation might be that the man did it, but ongoing ad hoc justifications (e.g. "... and that's not me breaking it on the film; they tampered with that, too") could successfully pr... | https://en.wikipedia.org/wiki?curid=36797 |
Any more complex theory might still possibly be true. A study of the predictive validity of Occam's razor found 32 published papers that included 97 comparisons of economic forecasts from simple and complex forecasting methods. None of the papers provided a balance of evidence that complexity of method improved forecas... | https://en.wikipedia.org/wiki?curid=36797 |
One justification of Occam's razor is a direct result of basic probability theory. By definition, all assumptions introduce possibilities for error; if an assumption does not improve the accuracy of a theory, its only effect is to increase the probability that the overall theory is wrong. | https://en.wikipedia.org/wiki?curid=36797 |
There have also been other attempts to derive Occam's razor from probability theory, including notable attempts made by Harold Jeffreys and E. T. Jaynes. The probabilistic (Bayesian) basis for Occam's razor is elaborated by David J. C. MacKay in chapter 28 of his book "Information Theory, Inference, and Learning Algori... | https://en.wikipedia.org/wiki?curid=36797 |
William H. Jefferys and James O. Berger (1991) generalize and quantify the original formulation's "assumptions" concept as the degree to which a proposition is unnecessarily accommodating to possible observable data. They state, "A hypothesis with fewer adjustable parameters will automatically have an enhanced posterio... | https://en.wikipedia.org/wiki?curid=36797 |
The bias–variance tradeoff is a framework that incorporates the Occam's razor principle in its balance between overfitting (associated with lower bias but higher variance) and underfitting (associated with lower variance but higher bias). | https://en.wikipedia.org/wiki?curid=36797 |
Karl Popper argues that a preference for simple theories need not appeal to practical or aesthetic considerations. Our preference for simplicity may be justified by its falsifiability criterion: we prefer simpler theories to more complex ones "because their empirical content is greater; and because they are better test... | https://en.wikipedia.org/wiki?curid=36797 |
The philosopher of science Elliott Sober once argued along the same lines as Popper, tying simplicity with "informativeness": The simplest theory is the more informative, in the sense that it requires less information to a question. He has since rejected this account of simplicity, purportedly because it fails to provi... | https://en.wikipedia.org/wiki?curid=36797 |
According to Swinburne, since our choice of theory cannot be determined by data (see Underdetermination and Duhem–Quine thesis), we must rely on some criterion to determine which theory to use. Since it is absurd to have no logical method for settling on one hypothesis amongst an infinite number of equally data-complia... | https://en.wikipedia.org/wiki?curid=36797 |
In science, Occam's razor is used as a heuristic to guide scientists in developing theoretical models rather than as an arbiter between published models. In physics, parsimony was an important heuristic in Albert Einstein's formulation of special relativity, in the development and application of the principle of least ... | https://en.wikipedia.org/wiki?curid=36797 |
In chemistry, Occam's razor is often an important heuristic when developing a model of a reaction mechanism. Although it is useful as a heuristic in developing models of reaction mechanisms, it has been shown to fail as a criterion for selecting among some selected published models. In this context, Einstein himself ex... | https://en.wikipedia.org/wiki?curid=36797 |
In the scientific method, parsimony is an epistemological, metaphysical or heuristic preference, not an irrefutable principle of logic or a scientific result. As a logical principle, Occam's razor would demand that scientists accept the simplest possible theoretical explanation for existing data. However, science has s... | https://en.wikipedia.org/wiki?curid=36797 |
When scientists use the idea of parsimony, it has meaning only in a very specific context of inquiry. Several background assumptions are required for parsimony to connect with plausibility in a particular research problem. The reasonableness of parsimony in one research context may have nothing to do with its reasonabl... | https://en.wikipedia.org/wiki?curid=36797 |
It has been suggested that Occam's razor is a widely accepted example of extraevidential consideration, even though it is entirely a metaphysical assumption. Most of the time, however, Occam's razor is a conservative tool, cutting out "crazy, complicated constructions" and assuring "that hypotheses are grounded in the ... | https://en.wikipedia.org/wiki?curid=36797 |
Appeals to simplicity were used to argue against the phenomena of meteorites, ball lightning, continental drift, and reverse transcriptase. One can argue for atomic building blocks for matter, because it provides a simpler explanation for the observed reversibility of both and chemical reactions as simple separation an... | https://en.wikipedia.org/wiki?curid=36797 |
In the same way, postulating the aether is more complex than transmission of light through a vacuum. At the time, however, all known waves propagated through a physical medium, and it seemed simpler to postulate the existence of a medium than to theorize about wave propagation without a medium. Likewise, Isaac Newton's... | https://en.wikipedia.org/wiki?curid=36797 |
Three axioms presupposed by the scientific method are realism (the existence of objective reality), the existence of natural laws, and the constancy of natural law. Rather than depend on provability of these axioms, science depends on the fact that they have not been objectively falsified. Occam's razor and parsimony s... | https://en.wikipedia.org/wiki?curid=36797 |
If multiple models of natural law make exactly the same testable predictions, they are equivalent and there is no need for parsimony to choose a preferred one. For example, Newtonian, Hamiltonian and Lagrangian classical mechanics are equivalent. Physicists have no interest in using Occam's razor to say the other two a... | https://en.wikipedia.org/wiki?curid=36797 |
Biologists or philosophers of biology use Occam's razor in either of two contexts both in evolutionary biology: the units of selection controversy and systematics. George C. Williams in his book "Adaptation and Natural Selection" (1966) argues that the best way to explain altruism among animals is based on low-level (i... | https://en.wikipedia.org/wiki?curid=36797 |
However, more recent biological analyses, such as Richard Dawkins' "The Selfish Gene", have contended that Morgan's Canon is not the simplest and most basic explanation. Dawkins argues the way evolution works is that the genes propagated in most copies end up determining the development of that particular species, i.e.... | https://en.wikipedia.org/wiki?curid=36797 |
Zoology provides an example. Muskoxen, when threatened by wolves, form a circle with the males on the outside and the females and young on the inside. This is an example of a behavior by the males that seems to be altruistic. The behavior is disadvantageous to them individually but beneficial to the group as a whole an... | https://en.wikipedia.org/wiki?curid=36797 |
Systematics is the branch of biology that attempts to establish patterns of relationship among biological taxa, today generally thought to reflect evolutionary history. It is also concerned with their classification. There are three primary camps in systematics: cladists, pheneticists, and evolutionary taxonomists. Cla... | https://en.wikipedia.org/wiki?curid=36797 |
It is among the cladists that Occam's razor is applied, through the method of "cladistic parsimony". Cladistic parsimony (or maximum parsimony) is a method of phylogenetic inference that yields phylogenetic trees (more specifically, cladograms). Cladograms are branching, diagrams used to represent hypotheses of relativ... | https://en.wikipedia.org/wiki?curid=36797 |
Other methods for inferring evolutionary relationships use parsimony in a more general way. Likelihood methods for phylogeny use parsimony as they do for all likelihood tests, with hypotheses requiring fewer differing parameters (i.e., numbers or different rates of character change or different frequencies of character... | https://en.wikipedia.org/wiki?curid=36797 |
Francis Crick has commented on potential limitations of Occam's razor in biology. He advances the argument that because biological systems are the products of (an ongoing) natural selection, the mechanisms are not necessarily optimal in an obvious sense. He cautions: "While Ockham's razor is a useful tool in the physic... | https://en.wikipedia.org/wiki?curid=36797 |
In biogeography, parsimony is used to infer ancient vicariant events or migrations of species or populations by observing the geographic distribution and relationships of existing organisms. Given the phylogenetic tree, ancestral population subdivisions are inferred to be those that require the minimum amount of change... | https://en.wikipedia.org/wiki?curid=36797 |
In the philosophy of religion, Occam's razor is sometimes applied to the existence of God. William of Ockham himself was a Christian. He believed in God, and in the authority of Scripture; he writes that "nothing ought to be posited without a reason given, unless it is self-evident (literally, known through itself) or ... | https://en.wikipedia.org/wiki?curid=36797 |
St. Thomas Aquinas, in the "Summa Theologica", uses a formulation of Occam's razor to construct an objection to the idea that God exists, which he refutes directly with a counterargument: | https://en.wikipedia.org/wiki?curid=36797 |
Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all volu... | https://en.wikipedia.org/wiki?curid=36797 |
In turn, Aquinas answers this with the "quinque viae", and addresses the particular objection above with the following answer: | https://en.wikipedia.org/wiki?curid=36797 |
Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all ... | https://en.wikipedia.org/wiki?curid=36797 |
Rather than argue for the necessity of a god, some theists base their belief upon grounds independent of, or prior to, reason, making Occam's razor irrelevant. This was the stance of Søren Kierkegaard, who viewed belief in God as a leap of faith that sometimes directly opposed reason. This is also the doctrine of Gordo... | https://en.wikipedia.org/wiki?curid=36797 |
Various arguments in favor of God establish God as a useful or even necessary assumption. Contrastingly some anti-theists hold firmly to the belief that assuming the existence of God introduces unnecessary complexity (Schmitt 2005, e.g., the Ultimate Boeing 747 gambit). | https://en.wikipedia.org/wiki?curid=36797 |
Another application of the principle is to be found in the work of George Berkeley (1685–1753). Berkeley was an idealist who believed that all of reality could be explained in terms of the mind alone. He invoked Occam's razor against materialism, stating that matter was not required by his metaphysics and was thus elim... | https://en.wikipedia.org/wiki?curid=36797 |
Occam's razor may also be recognized in the apocryphal story about an exchange between Pierre-Simon Laplace and Napoleon. It is said that in praising Laplace for one of his recent publications, the emperor asked how it was that the name of God, which featured so frequently in the writings of Lagrange, appeared nowhere ... | https://en.wikipedia.org/wiki?curid=36797 |
In his article "Sensations and Brain Processes" (1959), J. J. C. Smart invoked Occam's razor with the aim to justify his preference of the mind-brain identity theory over spirit-body dualism. Dualists state that there are two kinds of substances in the universe: physical (including the body) and spiritual, which is non... | https://en.wikipedia.org/wiki?curid=36797 |
In penal theory and the philosophy of punishment, parsimony refers specifically to taking care in the distribution of punishment in order to avoid excessive punishment. In the utilitarian approach to the philosophy of punishment, Jeremy Bentham's "parsimony principle" states that any punishment greater than is required... | https://en.wikipedia.org/wiki?curid=36797 |
Marcus Hutter's universal artificial intelligence builds upon Solomonoff's mathematical formalization of the razor to calculate the expected value of an action. | https://en.wikipedia.org/wiki?curid=36797 |
There are various papers in scholarly journals deriving formal versions of Occam's razor from probability theory, applying it in statistical inference, and using it to come up with criteria for penalizing complexity in statistical inference. Papers have suggested a connection between Occam's razor and Kolmogorov comple... | https://en.wikipedia.org/wiki?curid=36797 |
One of the problems with the original formulation of the razor is that it only applies to models with the same explanatory power (i.e., it only tells us to prefer the simplest of equally good models). A more general form of the razor can be derived from Bayesian model comparison, which is based on Bayes factors and can... | https://en.wikipedia.org/wiki?curid=36797 |
Statistical versions of Occam's razor have a more rigorous formulation than what philosophical discussions produce. In particular, they must have a specific definition of the term "simplicity", and that definition can vary. For example, in the Kolmogorov–Chaitin minimum description length approach, the subject must pic... | https://en.wikipedia.org/wiki?curid=36797 |
The minimum instruction set of a universal Turing machine requires approximately the same length description across different formulations, and is small compared to the Kolmogorov complexity of most practical theories. Marcus Hutter has used this consistency to define a "natural" Turing machine of small size as the pro... | https://en.wikipedia.org/wiki?curid=36797 |
One possible conclusion from mixing the concepts of Kolmogorov complexity and Occam's razor is that an ideal data compressor would also be a scientific explanation/formulation generator. Some attempts have been made to re-derive known laws from considerations of simplicity or compressibility. | https://en.wikipedia.org/wiki?curid=36797 |
According to Jürgen Schmidhuber, the appropriate mathematical theory of Occam's razor already exists, namely, Solomonoff's theory of optimal inductive inference and its extensions. See discussions in David L. Dowe's "Foreword re C. S. Wallace" for the subtle distinctions between the algorithmic probability work of Solo... | https://en.wikipedia.org/wiki?curid=36797 |
In software development, the rule of least power argues the correct programming language to use is the one that is simplest while also solving the targeted software problem. In that form the rule is often credited to Tim Berners-Lee since it appeared in his design guidelines for the original Hypertext Transfer Protocol... | https://en.wikipedia.org/wiki?curid=36797 |
Occam's razor is not an embargo against the positing of any kind of entity, or a recommendation of the simplest theory come what may. Occam's razor is used to adjudicate between theories that have already passed "theoretical scrutiny" tests and are equally well-supported by evidence. Furthermore, it may be used to prio... | https://en.wikipedia.org/wiki?curid=36797 |
Another contentious aspect of the razor is that a theory can become more complex in terms of its structure (or syntax), while its ontology (or semantics) becomes simpler, or vice versa. Quine, in a discussion on definition, referred to these two perspectives as "economy of practical expression" and "economy in grammar ... | https://en.wikipedia.org/wiki?curid=36797 |
Galileo Galilei lampooned the "misuse" of Occam's razor in his "Dialogue". The principle is represented in the dialogue by Simplicio. The telling point that Galileo presented ironically was that if one really wanted to start from a small number of entities, one could always consider the letters of the alphabet as the f... | https://en.wikipedia.org/wiki?curid=36797 |
Occam's razor has met some opposition from people who have considered it too extreme or rash. Walter Chatton () was a contemporary of William of Ockham who took exception to Occam's razor and Ockham's use of it. In response he devised his own "anti-razor": "If three things are not enough to verify an affirmative propos... | https://en.wikipedia.org/wiki?curid=36797 |
Anti-razors have also been created by Gottfried Wilhelm Leibniz (1646–1716), Immanuel Kant (1724–1804), and Karl Menger (1902–1985). Leibniz's version took the form of a principle of plenitude, as Arthur Lovejoy has called it: the idea being that God created the most varied and populous of possible worlds. Kant felt a ... | https://en.wikipedia.org/wiki?curid=36797 |
Karl Menger found mathematicians to be too parsimonious with regard to variables, so he formulated his Law Against Miserliness, which took one of two forms: "Entities must not be reduced to the point of inadequacy" and "It is vain to do with fewer what requires more." A less serious but even more extremist anti-razor i... | https://en.wikipedia.org/wiki?curid=36797 |
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adulthood stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five synapomorphies include a notochord, dorsal hollow nerve cord, endostyle ... | https://en.wikipedia.org/wiki?curid=5131 |
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalo... | https://en.wikipedia.org/wiki?curid=5131 |
Chordates are divided into three subphyla: Craniata (fish, amphibians, reptiles, birds, and mammals); Tunicata or Urochordata (sea squirts, salps); and Cephalochordata (which includes lancelets). The Craniata and Tunicata compose the clade Olfactores, which is sister to Cephalochordata. (See diagram under Phylogeny.) E... | https://en.wikipedia.org/wiki?curid=5131 |
Chordate fossils have been found from as early as the Cambrian explosion, 539 million years ago. Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are a subgroup of the clade Craniata, which consists of chordates with a skull. Of the more th... | https://en.wikipedia.org/wiki?curid=5131 |
Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: | https://en.wikipedia.org/wiki?curid=5131 |
There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition: | https://en.wikipedia.org/wiki?curid=5131 |
The following schema is from the 2015 edition of "Vertebrate Palaeontology". The invertebrate chordate classes are from "Fishes of the World". While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy. | https://en.wikipedia.org/wiki?curid=5131 |
Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. These burrowing filter-feeders compose the earliest-branching chordate sub-phylum. | https://en.wikipedia.org/wiki?curid=5131 |
Most tunicates appear as adults in two major forms, known as "sea squirts" and salps, both of which are soft-bodied filter-feeders that lack the standard features of chordates. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus; salps float in mid-water, feeding on plankton, and have... | https://en.wikipedia.org/wiki?curid=5131 |
Craniates all have distinct skulls. They include the hagfish, which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". | https://en.wikipedia.org/wiki?curid=5131 |
Most craniates are vertebrates, in which the notochord is replaced by the vertebral column. These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no... | https://en.wikipedia.org/wiki?curid=5131 |
but as members of the craniates, the group from which vertebrates are thought to have evolved. However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns. | https://en.wikipedia.org/wiki?curid=5131 |
The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish. However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that ... | https://en.wikipedia.org/wiki?curid=5131 |
There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some ch... | https://en.wikipedia.org/wiki?curid=5131 |
Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates' nearest relatives a... | https://en.wikipedia.org/wiki?curid=5131 |
All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that they are vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergen... | https://en.wikipedia.org/wiki?curid=5131 |
It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic. | https://en.wikipedia.org/wiki?curid=5131 |
While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux. | https://en.wikipedia.org/wiki?curid=5131 |
The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates. It seems very likely the "Kimberella" was a member of the protostomes. If so, this means the protostome and deuterostome lineages must have split... | https://en.wikipedia.org/wiki?curid=5131 |
Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, . The Mid Cambrian fossil "Rhabdotubus johanssoni" has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang... | https://en.wikipedia.org/wiki?curid=5131 |
The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates,... | https://en.wikipedia.org/wiki?curid=5131 |
Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before and the earliest chordates around . However, mol... | https://en.wikipedia.org/wiki?curid=5131 |
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