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Although red Cellulose Acetate Butyrate (CAB) (generally known trade names are Cellidor, Tenite and Tenex) scaled Swiss Army knives are most common, there are many colors and alternative materials like more resilient nylon and aluminum for the scales available. Many textures, colors and shapes now appear in the Swiss A... | Swiss Army knife | Wikipedia | 496 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null | vital_articles |
Finally, scales are applied. Slightly undersized holes incorporated into the inner surface enclose the bushings, which have truncated cone cross-section and are slightly undercut, forming a one-way interference fit when pressed into the generally softer and more elastic scale material. The result is a tight adhesive-fr... | Swiss Army knife | Wikipedia | 439 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null | vital_articles |
The Soldier Knife model 1890 had a spear point blade, reamer, can-opener, screwdriver and grips made out of oak wood scales (handles) that were treated with rapeseed oil for greater toughness and water-repellency, which made them black in color. The wooden grips of the Modell 1890 tended to crack and chip so in 1901 th... | Swiss Army knife | Wikipedia | 407 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null | vital_articles |
In 2007 the Swiss Government made a request for new updated soldier knives for the Swiss military for distribution in late 2008. The evaluation phase of the new soldier knife began in February 2008, when Armasuisse issued an invitation to tender. A total of seven suppliers from Switzerland and other countries were invi... | Swiss Army knife | Wikipedia | 496 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null | vital_articles |
Space program
The Swiss Army knife has been present in space missions carried out by NASA since the late 1970s. In 1978, NASA sent a letter of confirmation to Victorinox regarding a purchase of 50 knives known as the Master Craftsman model. In 1985, Edward M. Payton, brother of astronaut Gary E. Payton, sent a letter ... | Swiss Army knife | Wikipedia | 397 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null | vital_articles |
Ichthyostega (from , 'fish' and , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion. Ichthyostega possess... | Ichthyostega | Wikipedia | 499 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals | vital_articles |
Postcranial skeleton
The legs were large compared to contemporary relatives. It had seven digits on each hind leg, along with a peculiar, poorly ossified mass which lies anteriorly adjacent to the digits. The exact number of digits on the forelimb is not yet known, since fossils of the hand have not been found. While ... | Ichthyostega | Wikipedia | 469 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals | vital_articles |
Traditionally, Ichthyostega was considered part of an order named for it, the "Ichthyostegalia". However, this group represents a paraphyletic grade of primitive stem-tetrapods and is not used by many modern researchers. Phylogenetic analysis has shown Ichthyostega is intermediate between other primitive stegocephalian... | Ichthyostega | Wikipedia | 407 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals | vital_articles |
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at... | Ichthyosauria | Wikipedia | 376 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Ichthyosaurian species varied from in length. Ichthyosaurians resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often ... | Ichthyosauria | Wikipedia | 453 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The demand by collectors led to more intense commercial digging activities. In the early nineteenth century, this resulted in the discovery of more complete skeletons. In 1804, Edward Donovan at St Donats uncovered a ichthyosaur specimen containing a jaw, vertebrae, ribs, and a shoulder girdle. It was considered to be... | Ichthyosauria | Wikipedia | 511 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Home was very uncertain how the animal should be classified. Though most individual skeletal elements looked very reptilian, the anatomy as a whole resembled that of a fish, so he initially assigned the creature to the fishes, as seemed to be confirmed by the flat shape of the vertebrae. At the same time, he considered... | Ichthyosauria | Wikipedia | 464 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The discovery of a hitherto unsuspected extinct group of large marine reptiles generated much publicity, capturing the imagination of both scientists and the public at large. People were fascinated by the strange build of the animals, especially the large scleral rings in the eye sockets, of which it was sometimes erro... | Ichthyosauria | Wikipedia | 420 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Ichthyosaurs became even more popular in 1854 by the rebuilding at Sydenham Hill of the Crystal Palace, originally erected at the world exhibition of 1851. In the surrounding park, life-sized, painted, concrete statues of extinct animals were placed, which were designed by Benjamin Waterhouse Hawkins under the directio... | Ichthyosauria | Wikipedia | 450 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
In the early twentieth century, ichthyosaur research was dominated by the German paleontologist Friedrich von Huene, who wrote an extensive series of articles, taking advantage of an easy access to the many specimens found in his country. The amount of anatomical data was hereby vastly increased. Von Huene also travell... | Ichthyosauria | Wikipedia | 461 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The origin of the ichthyosaurs is contentious. Until recently, clear transitional forms with land-dwelling vertebrate groups had not yet been found, the earliest known species of the ichthyosaur lineage being already fully aquatic. In 2014, a small basal ichthyosauriform from the upper Lower Triassic was described that... | Ichthyosauria | Wikipedia | 353 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Since the 1980s, a close relationship was assumed between the Ichthyosauria and the Sauropterygia, another marine reptile group, within an overarching Euryapsida, with one such study in 1997 by John Merck showing them to be monophyletic archosauromorph euryapsids. This has been contested over the years, with the Euryap... | Ichthyosauria | Wikipedia | 447 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Early Ichthyopterygia
The earliest ichthyosaurs are known from the Early and Early-Middle (Olenekian and Anisian) Triassic strata of Canada, China, Japan, and Spitsbergen in Norway, being up to 246 million years old. These first forms included the genera Chaohusaurus, Grippia, and Utatsusaurus. Even older fossils show... | Ichthyosauria | Wikipedia | 299 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The basal forms quickly gave rise to ichthyosaurs in the narrow sense sometime around the boundary between the Early Triassic and Middle Triassic; the earliest Ichthyosauria in the sense Motani gave to the concept, appear about 245 million years ago. These later diversified into a variety of forms, including the still ... | Ichthyosauria | Wikipedia | 480 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
In the Late Triassic, ichthyosaurs attained the peak of their size and diversity. They occupied many ecological niches. Some were apex predators; others were hunters of small prey. Several species perhaps specialised in suction feeding or were ram feeders; also, durophagous forms are known. Towards the end of the Late ... | Ichthyosauria | Wikipedia | 506 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Traditionally, ichthyosaurs were seen as decreasing in diversity even further with the Cretaceous, though they had a worldwide distribution. All fossils from this period were referred to a single genus: Platypterygius. This last ichthyosaur genus was thought to have become extinct early in the late Cretaceous, during t... | Ichthyosauria | Wikipedia | 342 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The demise of the ichthyosaurs has been described as a two-step process. A first extinction event in the beginning of the Cenomanian eliminated two of the three ichthyosaur feeding guilds still present: the 'soft-prey specialists' and the 'generalists', leaving only an unspecialized apex predator group. The second exti... | Ichthyosauria | Wikipedia | 330 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Phylogeny
In modern phylogeny, clades are defined that contain all species forming a certain branch of the evolutionary tree. This also allows one to clearly indicate all relationships between the several subgroups in a cladogram. In 1999, a node clade Ichthyopterygia was defined by Motani as the group consisting of th... | Ichthyosauria | Wikipedia | 370 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Size
Ichthyosaurs averaged about in length. Some individual specimens were as short as ; some species were much larger: the Triassic Shonisaurus popularis was about long and in 2004 Shonisaurus sikanniensis (classified as a shastasaurus between 2011 and 2013) was estimated to have been in length. Fragmentary finds s... | Ichthyosauria | Wikipedia | 381 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Diagnostic traits
Derived ichthyosaurs in the narrow sense, as defined by Motani in 1999, differ from their closest basal ichthyopterygian relatives in certain traits. Motani listed a number of these. The external nostril is located on the side of the skull, and is hardly visible from above. The upper rim of the eye so... | Ichthyosauria | Wikipedia | 366 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The rear of the skull is dominated by a large eye socket, often covering the major part of the rear side surface. In the socket, a large scleral ring is present; this is a circular structure of small, overlapping bone segments protecting the eye against the water pressure. Both in the relative and absolute senses, icht... | Ichthyosauria | Wikipedia | 461 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Lower jaws
Like the snout, the lower jaws are elongated. However, in some species, such as Eurhinosaurus and Excalibosaurus, the front of the snout far protrudes beyond the lower jaws. While the front of the lower jaw is typically low, its rear depth is very variable. The greater part of the lower jaw is formed by the ... | Ichthyosauria | Wikipedia | 359 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Basal Ichthyopterygia, like their land-dwelling ancestors, still had vertebrae that possessed a full set of processes that allowed them to interlock and articulate, forming a vertebral column supporting the weight of the body. As ichthyosaurs were fully aquatic, their bodies were supported by the Archimedes force exert... | Ichthyosauria | Wikipedia | 440 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
As derived species no longer have transversal processes on their vertebrae—again a condition unique in the Amniota—the parapophyseal and diapophysael rib joints have been reduced to flat facets, at least one of which is located on the vertebral body. The number of facets can be one or two; their profile can be circular... | Ichthyosauria | Wikipedia | 394 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Basal forms have a forelimb that is still functionally differentiated, in some details resembling the arm of their land-dwelling forebears; the ulna and radius are elongated and somewhat separated; the carpals are rounded, allowing the wrist to rotate; the number of phalanges is within the range shown by land animals. ... | Ichthyosauria | Wikipedia | 244 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
A strongly derived condition show the phalanges, small, disc-shaped elements positioned in long rows. Sometimes, the number of fingers is reduced, to as low as two. This is a rather common phenomenon within the Tetrapoda. Unique, however, for derived tetrapods, is the fact that some species show nonpathological polydac... | Ichthyosauria | Wikipedia | 308 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The ichthyosaur pelvis is typically rather reduced. The three pelvic bones: the ilium, the ischium, and the pubic bone, are not fused and often do not even touch each other. Also, the left and right pelvic sides no longer touch; only basal forms still have sacral ribs connecting the ilia to the vertebral column. The hi... | Ichthyosauria | Wikipedia | 499 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The presence of dorsal fins in ichthyosaurs has been controversial. Finely preserved specimens from the Holzmaden Lagerstätten in Germany found in the late 19th century revealed additional traces, usually preserved in black, of the outline of the entire body, including the first evidence of dorsal fins in ichthyosaurs.... | Ichthyosauria | Wikipedia | 420 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
In 2017, from the German Posidonia Shale the discovery was reported of 182.7-million-year-old vertebrae of Stenopterygius in a carbonate nodule, still containing collagen fibers, cholesterol, platelets, and red and white blood cells. The structures would not have been petrified, but represent the original organic tissu... | Ichthyosauria | Wikipedia | 298 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The colouration of ichthyosaurs is difficult to determine. In 1956, Mary Whitear reported finding melanocytes, pigment cells in which reddish-brown pigment granules would still be present, in a skin specimen of a British fossil, R 509. Ichthyosaurs are traditionally assumed to have employed countershading (dark on top,... | Ichthyosauria | Wikipedia | 466 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Ecology
Apart from the obvious similarities to fish, ichthyosaurs also shared parallel developmental features with dolphins, lamnid sharks, and tuna. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecolog... | Ichthyosauria | Wikipedia | 451 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Some early ichthyosaurs were durophagous and had flat convex teeth adapted for crushing shellfish. They thus ate benthos from the floor of shallow seas. Other species were perhaps suction feeders, sucking animals into their mouths by quickly opening their relatively short jaws. This was first assumed for Shonisaurus, w... | Ichthyosauria | Wikipedia | 465 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
The tail was bi-lobed, with the lower lobe being supported by the caudal vertebral column, which was "kinked" ventrally to follow the contours of the ventral lobe. Basal species had a rather asymmetric or "heterocercal" tail fin. The asymmetry differed from that of sharks in that the lower lobe was largest, instead of ... | Ichthyosauria | Wikipedia | 440 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
In 1994, Judy Massare concluded that ichthyosaurs had been the fastest marine reptiles. Their length/depth ratio was between three and five, the optimal number to minimise water resistance or drag. Their smooth skin and streamlined bodies prevented excessive turbulence. Their hydrodynamic efficiency, the degree to whic... | Ichthyosauria | Wikipedia | 213 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Ichthyosaurs had fin-like limbs of varying relative length. The standard interpretation is that these, together with the dorsal fin and tail fin, were used as control surfaces for directional stability, controlling yaw, and for stabilising pitch and roll, rather than propulsion. However, during the 1980s the German pal... | Ichthyosauria | Wikipedia | 490 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Additional evidence is provided by the eyes of ichthyosaurs that among vertebrates are both relatively and absolutely the largest known. Modern leopard seals can dive to up to hunting on sight. Motani suggested that ichthyosaurs, with their relatively much larger eye sockets, should have been able to reach even greate... | Ichthyosauria | Wikipedia | 426 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Indirect evidence for endothermy is provided by the body shape of derived ichthyosaurs, which with its short tail and vertical tail fin seems optimised for a high cruising speed that can only be sustained by a high metabolism: all extant animals swimming this way are either fully warm-blooded or, like sharks and tuna, ... | Ichthyosauria | Wikipedia | 307 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Ichthyosaurs were viviparous, i.e. bore live young instead of laying eggs. Although they were reptiles and descended from egg-laying, oviparous, ancestors, viviparity is not as unexpected as it first appears. Air-breathing marine creatures must either come ashore to lay eggs, like turtles and some sea snakes, or else g... | Ichthyosauria | Wikipedia | 507 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
A comprehensive multi-author study published in 2023 examined the evolution of fetal orientation of ichthyosaurs based on known specimens of gravid female ichthyosaurs. Specimens of basal ichthyosaurs, Chaohusaurus and Cymbospondylus, showed evidence of head-first birth, while Mixosaurus had evidence of both head-first... | Ichthyosauria | Wikipedia | 472 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Crocodiles, most sea turtles and some lizards determine the sex of their offspring by manipulating the temperature of the developing eggs' environment; i.e. they do not have distinct sex chromosomes. Live-bearing reptiles do not regulate sex through incubation temperature. A study in 2009, which examined 94 living spec... | Ichthyosauria | Wikipedia | 345 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
Pathologies
Though fossils revealing ichthyosaur behavior remain rare, one ichthyosaur fossil is known to have sustained bites to the snout region. Discovered in Australia, and analyzed by Benjamin Kear et alii in 2011, measurements of the wounds reveal that the bite marks were inflicted by another ichthyosaur, likely ... | Ichthyosauria | Wikipedia | 248 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals | vital_articles |
A skeleton key (also known as a passkey) is a type of master key in which the serrated edge has been removed in such a way that it can open numerous locks, most commonly the warded lock. The term derives from the fact that the key has been reduced to its essential parts.
Master keys
A skeleton key is a key that has b... | Skeleton key | Wikipedia | 434 | 4053383 | https://en.wikipedia.org/wiki/Skeleton%20key | Technology | Mechanisms | null | vital_articles |
In universal algebra and in model theory, a structure consists of a set along with a collection of finitary operations and relations that are defined on it.
Universal algebra studies structures that generalize the algebraic structures such as groups, rings, fields and vector spaces. The term universal algebra is used ... | Structure (mathematical logic) | Wikipedia | 494 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
a set of function symbols and relation symbols, along with
a function that ascribes to each symbol a natural number
The natural number of a symbol is called the arity of because it is the arity of the interpretation of
Since the signatures that arise in algebra often contain only function symbols, a signatu... | Structure (mathematical logic) | Wikipedia | 482 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
The ordinary signature for set theory includes a single binary relation A structure for this signature consists of a set of elements and an interpretation of the relation as a binary relation on these elements.
Induced substructures and closed subsets
is called an (induced) substructure of if
and have the sam... | Structure (mathematical logic) | Wikipedia | 465 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
The most obvious way to define a graph is a structure with a signature consisting of a single binary relation symbol The vertices of the graph form the domain of the structure, and for two vertices and means that and are connected by an edge. In this encoding, the notion of induced substructure is more restricti... | Structure (mathematical logic) | Wikipedia | 481 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
Induced substructures correspond to subobjects in σ-Emb. If σ has only function symbols, σ-Emb is the subcategory of monomorphisms of σ-Hom. In this case induced substructures also correspond to subobjects in σ-Hom.
Example
As seen above, in the standard encoding of graphs as structures the induced substructures are ... | Structure (mathematical logic) | Wikipedia | 509 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
A structure is said to be a model of a theory if the language of is the same as the language of and every sentence in is satisfied by Thus, for example, a "ring" is a structure for the language of rings that satisfies each of the ring axioms, and a model of ZFC set theory is a structure in the language of set the... | Structure (mathematical logic) | Wikipedia | 434 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
Structures as defined above are sometimes called s to distinguish them from the more general s. A many-sorted structure can have an arbitrary number of domains. The sorts are part of the signature, and they play the role of names for the different domains. Many-sorted signatures also prescribe which sorts the functions... | Structure (mathematical logic) | Wikipedia | 354 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
Both universal algebra and model theory study classes of (structures or) algebras that are defined by a signature and a set of axioms. In the case of model theory these axioms have the form of first-order sentences. The formalism of universal algebra is much more restrictive; essentially it only allows first-order sent... | Structure (mathematical logic) | Wikipedia | 509 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
Structures that are proper classes
In the study of set theory and category theory, it is sometimes useful to consider structures in which the domain of discourse is a proper class instead of a set. These structures are sometimes called class models to distinguish them from the "set models" discussed above. When the do... | Structure (mathematical logic) | Wikipedia | 104 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null | vital_articles |
The banded bullfrog (Kaloula pulchra) is a species of frog in the narrow-mouthed frog family Microhylidae. Native to Southeast Asia, it is also known as the Asian painted frog, digging frog, Malaysian bullfrog, common Asian frog, and painted balloon frog. In the pet trade, it is sometimes called the chubby frog. Adults... | Banded bullfrog | Wikipedia | 493 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals | vital_articles |
Description
The banded bullfrog is medium-sized with a stocky, triangular body and a short snout. Males grow to a snout–vent length (SVL) of and females are slightly larger, reaching an SVL of . Other than the slight difference in length, there is very limited sexual dimorphism. They have a body weight of . The back ... | Banded bullfrog | Wikipedia | 500 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals | vital_articles |
Breeding is stimulated by heavy monsoon rains, after which the frogs relocate from underground to rain pools or ponds. They are more commonly found on wetter nights, and while they are not reproductively active during dry periods, their gonads remain ripe so that they can mate soon after rainfall. In India, the male fr... | Banded bullfrog | Wikipedia | 373 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals | vital_articles |
Snakes such as the kukri snake are predators of adult banded bullfrogs. For eggs and tadpoles, predators include dragonfly larvae and snails such as the golden apple snail. Banded bullfrogs display deimatic behaviour when threatened, greatly inflating their bodies in an attempt to distract or startle predators. By infl... | Banded bullfrog | Wikipedia | 419 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals | vital_articles |
Human sexual reproduction, to produce offspring, begins with fertilization. Successful reproduction typically involves sexual intercourse between a healthy, sexually mature and fertile male and female. During sexual intercourse, sperm cells are ejaculated into the vagina through the penis, resulting in fertilization of... | Human reproduction | Wikipedia | 409 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology | vital_articles |
The male reproductive system contains two main divisions: the testicles where sperm are produced, and the penis where semen is ejaculated through the urethra. In humans, both of these organs are outside the abdominal cavity. Having the testicles outside the abdomen facilitates temperature regulation of the sperm, which... | Human reproduction | Wikipedia | 450 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology | vital_articles |
The ova, which are the female sex cells, are much larger than the spermatozoon and are normally formed within the ovaries of the female fetus before birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval... | Human reproduction | Wikipedia | 482 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology | vital_articles |
Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some 5 days before until 1 to 2 days after ovulation. For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days, or every 2 or 3 days. Studies have shown no significant difference between differ... | Human reproduction | Wikipedia | 405 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology | vital_articles |
Labor is separated into 4 stages. The first stage involves latent phase and active phase separated by the dilation of the cervix for 6 to 10 cm. The second stage is the pushing stage. The third stage involves the delivery of the placenta. And the last stage is the contraction of the uterus. Once the fetus is sufficient... | Human reproduction | Wikipedia | 258 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology | vital_articles |
A search engine is a software system that provides hyperlinks to web pages and other relevant information on the Web in response to a user's query. The user inputs a query within a web browser or a mobile app, and the search results are often a list of hyperlinks, accompanied by textual summaries and images. Users also... | Search engine | Wikipedia | 492 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Prior to September 1993, the World Wide Web was entirely indexed by hand. There was a list of webservers edited by Tim Berners-Lee and hosted on the CERN webserver. One snapshot of the list in 1992 remains, but as more and more web servers went online the central list could no longer keep up. On the NCSA site, new serv... | Search engine | Wikipedia | 439 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
In June 1993, Matthew Gray, then at MIT, produced what was probably the first web robot, the Perl-based World Wide Web Wanderer, and used it to generate an index called "Wandex". The purpose of the Wanderer was to measure the size of the World Wide Web, which it did until late 1995. The web's second search engine Aliwe... | Search engine | Wikipedia | 475 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
In 1996, Robin Li developed the RankDex site-scoring algorithm for search engines results page ranking and received a US patent for the technology. It was the first search engine that used hyperlinks to measure the quality of websites it was indexing, predating the very similar algorithm patent filed by Google two year... | Search engine | Wikipedia | 343 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
2000s–present: Post dot-com bubble
Around 2000, Google's search engine rose to prominence. The company achieved better results for many searches with an algorithm called PageRank, as was explained in the paper Anatomy of a Search Engine written by Sergey Brin and Larry Page, the later founders of Google. This iterative... | Search engine | Wikipedia | 428 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Web search engines get their information by web crawling from site to site. The "spider" checks for the standard filename robots.txt, addressed to it. The robots.txt file contains directives for search spiders, telling it which pages to crawl and which pages not to crawl. After checking for robots.txt and either findin... | Search engine | Wikipedia | 451 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Typically when a user enters a query into a search engine it is a few keywords. The index already has the names of the sites containing the keywords, and these are instantly obtained from the index. The real processing load is in generating the web pages that are the search results list: Every page in the entire list m... | Search engine | Wikipedia | 370 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
The usefulness of a search engine depends on the relevance of the result set it gives back. While there may be millions of web pages that include a particular word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines employ methods to rank the results to provide the "be... | Search engine | Wikipedia | 401 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
In Russia, Yandex has a market share of 62.6%, compared to Google's 28.3%. Yandex is the second most used search engine on smartphones in Asia and Europe. In China, Baidu is the most popular search engine. South Korea-based search portal Naver is used for 62.8% of online searches in the country. Yahoo! Japan and Yahoo!... | Search engine | Wikipedia | 498 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
There has been concern raised that search engines such as Google and Bing provide customized results based on the user's activity history, leading to what has been termed echo chambers or filter bubbles by Eli Pariser in 2011. The argument is that search engines and social media platforms use algorithms to selectively ... | Search engine | Wikipedia | 472 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Search engine submission
Web search engine submission is a process in which a webmaster submits a website directly to a search engine. While search engine submission is sometimes presented as a way to promote a website, it generally is not necessary because the major search engines use web crawlers that will eventuall... | Search engine | Wikipedia | 486 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Initially, anyone who wanted to share a file had to set up an FTP server in order to make the file available to others. Later, "anonymous" FTP sites became repositories for files, allowing all users to post and retrieve them.
Even with archive sites, many important files were still scattered on small FTP servers. Thes... | Search engine | Wikipedia | 474 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
In response to the Wanderer, Martijn Koster created Archie-Like Indexing of the Web, or ALIWEB, in October 1993. As the name implies, ALIWEB was the HTTP equivalent of Archie, and because of this, it is still unique in many ways.
ALIWEB does not have a web-searching robot. Instead, webmasters of participating sites po... | Search engine | Wikipedia | 471 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
As the number of links grew and their pages began to receive thousands of hits a day, the team created ways to better organize the data. In order to aid in data retrieval, Yahoo! (www.yahoo.com) became a searchable directory. The search feature was a simple database search engine. Because Yahoo! entries were entered an... | Search engine | Wikipedia | 501 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
In both cases, when you query a search engine to locate information, you're actually searching through the index that the search engine has created —you are not actually searching the Web. These indices are giant databases of information that is collected and stored and subsequently searched. This explains why sometime... | Search engine | Wikipedia | 403 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Modern web search engines are highly intricate software systems that employ technology that has evolved over the years. There are a number of sub-categories of search engine software that are separately applicable to specific 'browsing' needs. These include web search engines (e.g. Google), database or structured data ... | Search engine | Wikipedia | 204 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null | vital_articles |
Jaekelopterus is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Jaekelopterus have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: the type species J. rhenaniae from brackish to fresh water strata in the Rhineland, a... | Jaekelopterus | Wikipedia | 482 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
Jaekelopterus is the largest known eurypterid and the largest known arthropod to have ever existed. This was determined based on a chelicera (claw) from the Emsian Klerf Formation of Willwerath, Germany, that measures long, but is missing a quarter of its length, suggesting that the full chelicera would have been lon... | Jaekelopterus | Wikipedia | 387 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
J. howelli, known from over 30 specimens, has an almost identical pattern of denticulation on the chelicerae as J. rhenaniae and also preserves a flattened posterior margin of the telson, which results in a triangular shape, as in J. rhenaniae. Its serrated telson margin and the massive elongation of the second interme... | Jaekelopterus | Wikipedia | 388 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
British palaeontologist Charles D. Waterston erected the genus Jaekelopterus in 1964 to accommodate Pterygotus rhenaniae, which he considered sufficiently distinct from other species of Pterygotus to warrant its own genus, primarily due to the abdominal appendages of Jaekelopterus being segmented as opposed to those of... | Jaekelopterus | Wikipedia | 456 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
Jaekelopterus is classified within the family Pterygotidae in the superfamily Pterygotioidea. Jaekelopterus is similar to Pterygotus, virtually only distinct in features of its genital appendage and potentially its telson. The close similarities between the two genera have prompted some researchers to question if the p... | Jaekelopterus | Wikipedia | 366 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa (Slimonia acuminata and Hughmilleria socialis). Jaekelopterus had previously been classified as a basal sister taxon to the rest of the Pterygotidae since its description as a separate genus by Waterston in 1964 due to its supp... | Jaekelopterus | Wikipedia | 511 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
Despite being the largest arthropods, the lightweight build of Jaekelopterus and other giant pterygotid eurypterids meant they likely were not the heaviest. Other giant eurypterids, particularly the deep-bodied walking forms in the Hibbertopteridae, such as the almost 2-metre-long Hibbertopterus, may have rivalled the ... | Jaekelopterus | Wikipedia | 368 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
Like all other arthropods, eurypterids matured through a sequence of stages called "instars" consisting of periods of ecdysis (moulting) followed by rapid growth. Unlike many arthropods, such as insects and crustaceans, chelicerates (the group to which eurypterids like Jaekelopterus belongs, alongside other organisms s... | Jaekelopterus | Wikipedia | 234 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
Though several fossilised instars of Jaekelopterus howelli are known, the fragmentary and incomplete status of the specimens makes it difficult to study its ontogeny in detail. Despite this, there are some noticeable changes occurring in the chelicerae, telson and metastoma. Four of the J. howelli specimens studied by ... | Jaekelopterus | Wikipedia | 451 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
The cheliceral morphology and visual acuity of the pterygotid eurypterids separates them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their compound eyes and the interommatidial angle (IOA), which is the angle between the optic... | Jaekelopterus | Wikipedia | 472 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
The unique eyes of modern horseshoe crabs are highly distinct from eyes of other modern arthropods and allow increased edge-perception and enhance contrasts, important for animals in low and scattered light conditions. As the eyes of Jaekelopterus were very similar, it too likely had the same adaptations. With its high... | Jaekelopterus | Wikipedia | 460 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
A powerful and active predator, Jaekelopterus was likely highly agile and possessed high maneuverability. The hydromechanics of the swimming paddles and telsons of Jaekelopterus and other pterygotids suggest that all members of the group were capable of hovering, forward locomotion and quick turns. Though they were not... | Jaekelopterus | Wikipedia | 95 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals | vital_articles |
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, subspecies, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a... | Hybrid (biology) | Wikipedia | 493 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women.
Significance
In evolution
Hybridization between ... | Hybrid (biology) | Wikipedia | 492 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
The term hybrid is derived from Latin , used for crosses such as of a tame sow and a wild boar. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century. Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s wit... | Hybrid (biology) | Wikipedia | 436 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis (the white admiral) and L. a. astyanax (the red-spotted purple). The white admir... | Hybrid (biology) | Wikipedia | 453 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
From the point of view of taxonomy, hybrids differ according to their parentage.
Hybrids between different subspecies (such as between the dog and Eurasian wolf) are called intra-specific hybrids. Interspecific hybrids are the offspring from interspecies mating; these sometimes result in hybrid speciation. Intergeneric... | Hybrid (biology) | Wikipedia | 511 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
Speciation
A few animal species are the result of hybridization. The Lonicera fly is a natural hybrid. The American red wolf appears to be a hybrid of the gray wolf and the coyote, although its taxonomic status has been a subject of controversy. The European edible frog is a semi-permanent hybrid between pool frogs an... | Hybrid (biology) | Wikipedia | 358 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either par... | Hybrid (biology) | Wikipedia | 469 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null | vital_articles |
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