Reptile
Group of animals including lepidosaurs, testudines, and archosaurs / From Wikipedia, the free encyclopedia
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Reptiles, as commonly defined, are a group of tetrapods with an ectothermic ('cold-blooded') metabolism and amniotic development. Living reptiles comprise four orders: Testudines (turtles), Crocodilia (crocodilians), Squamata (lizards and snakes), and Rhynchocephalia (the tuatara). As of May 2023, about 12,000 living species of reptiles are listed in the Reptile Database.[1] The study of the traditional reptile orders, customarily in combination with the study of modern amphibians, is called herpetology.
Reptiles | |
---|---|
Reptilians by saurian clade listed in top-to-bottom order: six lepidosaurs and six archelosaurs. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Sauropsida |
Class: | Reptilia Laurenti, 1768 |
Extant groups | |
See text for extinct groups. |
Reptiles have been subject to several conflicting taxonomic definitions.[2] In Linnaean taxonomy, reptiles are gathered together under the class Reptilia (/rɛpˈtɪliə/ rep-TIL-ee-ə), which corresponds to common usage. Modern cladistic taxonomy regards that group as paraphyletic, since genetic and paleontological evidence has determined that birds (class Aves), as members of Dinosauria are more closely related to living crocodilians than to other reptiles, and are thus nested among reptiles from an evolutionary perspective. Many cladistic systems therefore redefine Reptilia as a clade (monophyletic group) including birds, though the precise definition of this clade varies between authors.[3][2] Others prioritize the clade Sauropsida, which typically refers to all amniotes more closely related to modern reptiles than to mammals.[3]
The earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods which became increasingly adapted to life on dry land. The earliest known eureptile ("true reptile") was Hylonomus, a small and superficially lizard-like animal. Genetic and fossil data argues that the two largest lineages of reptiles, Archosauromorpha (crocodilians, birds, and kin) and Lepidosauromorpha (lizards, and kin), diverged near the end of the Permian period.[4] In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs, and all non-avian dinosaurs alongside many species of crocodyliforms, and squamates (e.g., mosasaurs). Modern non-bird reptiles inhabit all the continents except Antarctica.
Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades[5] – the fetus develops within the mother, using a (non-mammalian) placenta rather than contained in an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to the saltwater crocodile, Crocodylus porosus, which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).
Research history
In the 13th century, the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded by Beauvais in his Mirror of Nature.[6] In the 18th century, the reptiles were, from the outset of classification, grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are often found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturæ.[7] The terms reptile and amphibian were largely interchangeable, reptile (from Latin repere, 'to creep') being preferred by the French.[8] J.N. Laurenti was the first to formally use the term Reptilia for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.[9] Today, the two groups are still commonly treated under the single heading herpetology.
It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and P.A. Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds, and mammals.[10] The British anatomist T.H. Huxley made Latreille's definition popular and, together with Richard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", including dinosaurs and the mammal-like (synapsid) Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into mammals, sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of Sauropsida and Ichthyopsida for the latter two groups.[11] In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by the amniotic egg.
The terms Sauropsida ("lizard faces") and Theropsida ("beast faces") were used again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on the one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features, such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, Protosauria ("first lizards") in which he included some animals today considered reptile-like amphibians, as well as early reptiles.[12]
In 1956, D.M.S. Watson observed that the first two groups diverged very early in reptilian history, so he divided Goodrich's Protosauria between them. He also reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropterygians.[13]
In the late 19th century, a number of definitions of Reptilia were offered. The biological traits listed by Lydekker in 1896, for example, include a single occipital condyle, a jaw joint formed by the quadrate and articular bones, and certain characteristics of the vertebrae.[14] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.[15]
The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of temporal fenestrae, openings in the sides of the skull behind the eyes. This classification was initiated by Henry Fairfield Osborn and elaborated and made popular by Romer's classic Vertebrate Paleontology.[16][17] Those four subclasses were:
- Anapsida – no fenestrae – cotylosaurs and chelonia (turtles and relatives)[lower-alpha 1]
- Synapsida – one low fenestra – pelycosaurs and therapsids (the 'mammal-like reptiles')
- Euryapsida – one high fenestra (above the postorbital and squamosal) – protorosaurs (small, early lizard-like reptiles) and the marine sauropterygians and ichthyosaurs, the latter called Parapsida in Osborn's work.
- Diapsida – two fenestrae – most reptiles, including lizards, snakes, crocodilians, dinosaurs and pterosaurs
The composition of Euryapsida was uncertain. Ichthyosaurs were, at times, considered to have arisen independently of the other euryapsids, and given the older name Parapsida. Parapsida was later discarded as a group for the most part (ichthyosaurs being classified as incertae sedis or with Euryapsida). However, four (or three if Euryapsida is merged into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th century. It has largely been abandoned by recent researchers: In particular, the anapsid condition has been found to occur so variably among unrelated groups that it is not now considered a useful distinction.[18]
Phylogenetics and modern definition
By the early 21st century, vertebrate paleontologists were beginning to adopt phylogenetic taxonomy, in which all groups are defined in such a way as to be monophyletic; that is, groups which include all descendants of a particular ancestor. The reptiles as historically defined are paraphyletic, since they exclude both birds and mammals. These respectively evolved from dinosaurs and from early therapsids, both of which were traditionally called "reptiles".[19] Birds are more closely related to crocodilians than the latter are to the rest of extant reptiles. Colin Tudge wrote:
Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the clade Amniota: The section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synapomorphies, as is the proper way. Instead, it is defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian amniotes'.[15]
Despite the early proposals for replacing the paraphyletic Reptilia with a monophyletic Sauropsida, which includes birds, that term was never adopted widely or, when it was, was not applied consistently.[2]
When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988, Jacques Gauthier proposed a cladistic definition of Reptilia as a monophyletic node-based crown group containing turtles, lizards and snakes, crocodilians, and birds, their common ancestor and all its descendants. While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.[2] Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.[2] Gauthier 1994 and Laurin and Reisz 1995's definition of Sauropsida defined the scope of the group as distinct and broader than that of Reptilia, encompassing Mesosauridae as well as Reptilia sensu stricto.[3][20]
A variety of other definitions were proposed by other scientists in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the standards of the PhyloCode, was published by Modesto and Anderson in 2004.[2] Modesto and Anderson reviewed the many previous definitions and proposed a modified definition, which they intended to retain most traditional content of the group while keeping it stable and monophyletic. They defined Reptilia as all amniotes closer to Lacerta agilis and Crocodylus niloticus than to Homo sapiens. This stem-based definition is equivalent to the more common definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles.[2]
Taxonomy
General classification of extinct and living reptiles, focusing on major groups.[21][22]
- Reptilia/Sauropsida
- †Parareptilia
- Eureptilia
- †Captorhinidae
- Diapsida
- †Araeoscelidia
- Neodiapsida
- †Drepanosauromorpha (placement uncertain)
- †Younginiformes (paraphyletic)
- †Ichthyosauromorpha (placement uncertain)
- †Thalattosauria (placement uncertain)
- Sauria
- Lepidosauromorpha
- Lepidosauriformes
- Rhynchocephalia (tuatara)
- Squamata (lizards and snakes)
- Lepidosauriformes
- †Choristodera (placement uncertain)
- †Sauropterygia (placement uncertain)
- Pantestudines (turtles and kin, placement uncertain)
- Archosauromorpha
- †Protorosauria (paraphyletic)
- †Rhynchosauria
- †Allokotosauria
- Archosauriformes
- †Phytosauria
- Archosauria
- Pseudosuchia
- Crocodilia (crocodilians)
- Avemetatarsalia/Ornithodira
- †Pterosauria
- Dinosauria
- †Ornithischia
- Saurischia (including birds (Aves))
- Pseudosuchia
- Lepidosauromorpha
Phylogeny
The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[23] All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,[23][24][25][26][27][28] though a few have recovered turtles as Lepidosauromorpha instead.[29] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[23]
Amniota |
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The position of turtles
The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.[30] Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced.[31] The results place turtles as a sister clade to the archosaurs, the group that includes crocodiles, non-avian dinosaurs, and birds.[32] However, in their comparative analysis of the timing of organogenesis, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within Sauropsida, outside the saurian clade altogether.[33]
Origin of the reptiles
The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs.[20][failed verification]
The oldest known animal that may have been an amniote is Casineria (though it may have been a temnospondyl).[34][35][36] A series of footprints from the fossil strata of Nova Scotia dated to 315 Ma show typical reptilian toes and imprints of scales.[37] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.[38] It was a small, lizard-like animal, about 20 to 30 centimetres (7.9 to 11.8 in) long, with numerous sharp teeth indicating an insectivorous diet.[39] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)[40] and Paleothyris, both of similar build and presumably similar habit.
However, microsaurs have been at times considered true reptiles, so an earlier origin is possible.[41]
Rise of the reptiles
The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until the Carboniferous Rainforest Collapse.[42] This sudden collapse affected several large groups. Primitive tetrapods were particularly devastated, while stem-reptiles fared better, being ecologically adapted to the drier conditions that followed. Primitive tetrapods, like modern amphibians, need to return to water to lay eggs; in contrast, amniotes, like modern reptiles – whose eggs possess a shell that allows them to be laid on land – were better adapted to the new conditions. Amniotes acquired new niches at a faster rate than before the collapse and at a much faster rate than primitive tetrapods. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.[42] From this point forward, reptiles dominated communities and had a greater diversity than primitive tetrapods, setting the stage for the Mesozoic (known as the Age of Reptiles).[43] One of the best known early stem-reptiles is Mesosaurus, a genus from the Early Permian that had returned to water, feeding on fish.
A 2021 examination of reptile diversity in the Carboniferous and Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.[41]
Anapsids, synapsids, diapsids, and sauropsids
It was traditionally assumed that the first reptiles retained an anapsid skull inherited from their ancestors.[44] This type of skull has a skull roof with only holes for the nostrils, eyes and a pineal eye.[30] The discoveries of synapsid-like openings (see below) in the skull roof of the skulls of several members of Parareptilia (the clade containing most of the amniotes traditionally referred to as "anapsids"), including lanthanosuchoids, millerettids, bolosaurids, some nycteroleterids, some procolophonoids and at least some mesosaurs[45][46][47] made it more ambiguous and it's currently uncertain whether the ancestral amniote had an anapsid-like or synapsid-like skull.[47] These animals are traditionally referred to as "anapsids", and form a paraphyletic basic stock from which other groups evolved.[2] Very shortly after the first amniotes appeared, a lineage called Synapsida split off; this group was characterized by a temporal opening in the skull behind each eye giving room for the jaw muscle to move. These are the "mammal-like amniotes", or stem-mammals, that later gave rise to the true mammals.[48] Soon after, another group evolved a similar trait, this time with a double opening behind each eye, earning them the name Diapsida ("two arches").[44] The function of the holes in these groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.[30]
Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.[49] The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls, improving their armor.[20] Later morphological phylogenetic studies with this in mind placed turtles firmly within Diapsida.[50] All molecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant archosaurs.[25][26][27][28]
Permian reptiles
With the close of the Carboniferous, the amniotes became the dominant tetrapod fauna. While primitive, terrestrial reptiliomorphs still existed, the synapsid amniotes evolved the first truly terrestrial megafauna (giant animals) in the form of pelycosaurs, such as Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian period, the climate became drier, resulting in a change of fauna: The pelycosaurs were replaced by the therapsids.[51]
The parareptiles, whose massive skull roofs had no postorbital holes, continued and flourished throughout the Permian. The pareiasaurian parareptiles reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).[51]
Early in the period, the modern reptiles, or crown-group reptiles, evolved and split into two main lineages: the Archosauromorpha (forebears of turtles, crocodiles, and dinosaurs) and the Lepidosauromorpha (predecessors of modern lizards and tuataras). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.
Mesozoic reptiles
The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event), an event prolonged by the combination of two or more distinct extinction pulses.[52] Most of the earlier parareptile and synapsid megafauna disappeared, being replaced by the true reptiles, particularly archosauromorphs. These were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles. The archosaurs became the dominant group during the Triassic period, though it took 30 million years before their diversity was as great as the animals that lived in the Permian.[52] Archosaurs developed into the well-known dinosaurs and pterosaurs, as well as the ancestors of crocodiles. Since reptiles, first rauisuchians and then dinosaurs, dominated the Mesozoic era, the interval is popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smaller theropods. In the Cretaceous period, these gave rise to the first true birds.[53]
The sister group to Archosauromorpha is Lepidosauromorpha, containing lizards and tuataras, as well as their fossil relatives. Lepidosauromorpha contained at least one major group of the Mesozoic sea reptiles: the mosasaurs, which lived during the Cretaceous period. The phylogenetic placement of other main groups of fossil sea reptiles – the ichthyopterygians (including ichthyosaurs) and the sauropterygians, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs[3] or to archosauromorphs,[54][55][56] and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.[57]
Cenozoic reptiles
The close of the Cretaceous period saw the demise of the Mesozoic era reptilian megafauna (see the Cretaceous–Paleogene extinction event, also known as K-T extinction event). Of the large marine reptiles, only sea turtles were left; and of the non-marine large reptiles, only the semi-aquatic crocodiles and broadly similar choristoderes survived the extinction, with last members of the latter, the lizard-like Lazarussuchus, becoming extinct in the Miocene.[59] Of the great host of dinosaurs dominating the Mesozoic, only the small beaked birds survived. This dramatic extinction pattern at the end of the Mesozoic led into the Cenozoic. Mammals and birds filled the empty niches left behind by the reptilian megafauna and, while reptile diversification slowed, bird and mammal diversification took an exponential turn.[43] However, reptiles were still important components of the megafauna, particularly in the form of large and giant tortoises.[60][61]
After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic. Squamates took a massive hit during the K–Pg event, only recovering ten million years after it,[62] but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).[63][64] Approximately 10,000 extant species of traditional reptiles are known, with birds adding about 10,000 more, almost twice the number of mammals, represented by about 5,700 living species (excluding domesticated species).[65]
Reptile group | Described species | Percent of reptile species |
---|---|---|
Squamates | 9193 | 96.3% |
- Lizards | 5634 | 59% |
- Snakes | 3378 | 35% |
- Amphisbaenians | 181 | 2% |
Turtles | 327 | 3.4% |
Crocodilians | 25 | 0.3% |
Rhynchocephalians | 1 | 0.01% |
Total | 9546 | 100% |