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Reptiles
Temporal range: PennsylvanianPresent, 312–0 Ma
Extant reptilia.jpg
Clockwise from above left: Green sea turtle (Chelonia mydas), Tuatara (Sphenodon punctatus), Nile crocodile (Crocodylus niloticus), and Sinai agama (Pseudotrapelus sinaitus)
Scientific classification e
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Sauropsida
Class: Reptilia
Laurenti, 1768
Extant groups

See text for extinct groups.

Global Reptile Biodiversity.png
Global reptile species distribution (excluding birds)

Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of these traditional reptile orders, historically combined with that of mod

See text for extinct groups.

Global Reptile Biodiversity.png Global reptile species distribution (excluding birds)

Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.

Because some reptiles are more closely related to birds than they are to other reptiles (e.g., crocodiles are more closely related to birds than they are to lizards), the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade (consisting of all descendants of a common ancestor). For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living diapsids.[1][2][3][4] The term reptiles is sometimes used as shorthand for 'non-avian Reptilia'.[5][6]

The earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became increasingly adapted to life on dry land. Some early examples include the lizard-like Hylonomus and Casineria. 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, ornithischians, and sauropods, alongside many species of theropods, crocodyliforms, and squamates (e.g., mosasaurs).

Modern non-avian reptiles inhabit all the continents except Antarctica, although some birds are found on the periphery of Antarctica. Several living subgroups are recognized: Testudines (turtles and tortoises), 360 species;[7][8] Rhynchocephalia (tuatara from New Zealand), 1 species;[7][9] Squamata (lizards, snakes, and worm lizards), about 10,954 species;[7][10] and Crocodilia (crocodiles, gharials, caimans, and alligators), 27 species.[7][11]

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[12] – 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 tetrapod animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.

Because some reptiles are more closely related to birds than they are to other reptiles (e.g., crocodiles are more closely related to birds than they are to lizards), the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade (consisting of all descendants of a common ancestor). For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living diapsids.[1][2]Because some reptiles are more closely related to birds than they are to other reptiles (e.g., crocodiles are more closely related to birds than they are to lizards), the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade (consisting of all descendants of a common ancestor). For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living diapsids.[1][2][3][4] The term reptiles is sometimes used as shorthand for 'non-avian Reptilia'.[5][6]

The earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became increasingly adapted to life on dry land. Some early examples include the lizard-like Hylonomus and Casineria. 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, ornithischians, and sauropods, alongside many species of theropods, crocodyliforms, and squamates (e.g., mosasaurs).

Modern non-avian reptiles inhabit all the continents except Antarctica, although some birds are found on the periphery of Antarctica. Several living subgroups are recognized: Testudines (turtles and tortoises), 360 species;[7][8] Rhynchocephalia (tuatara from New Zealand), 1 species;[7][9] Squamata (lizards, snakes, and worm lizards), about 10,954 species;[7][10] and Crocodilia (crocodiles, gharials, caimans, and alligators), 27 species.[7][11]

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[12] – 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 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

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 Vincent of Beauvais in his Mirror of Nature.[13] 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.[14] The terms reptile and amphibian were largely interchangeable, reptile (from Latin repere, 'to creep') being preferred by the French.[15] Josephus Nicolaus Laurenti was the first to formally use the term Reptilia for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.[16] Today, the two groups are still commonly treated under the single heading herpetology.

"Antediluvian monster", a Mosasaurus discovered in a Maastricht limestone quarry, 1770 (contemporary engraving)

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 Pierre André Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds, and mammals.[17] The British anatomist Thomas Henry 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.[18] 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.[19]

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.[20]

In the late 19th century, a number of definitions of Reptilia were offered. The 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.[21] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.[22]

The first reptiles had an anapsid type of skull roof, as seen in the Permian genus Captorhinus

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.[23][2

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 Pierre André Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds, and mammals.[17] The British anatomist Thomas Henry 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.[18] 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.&#

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.[19]

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.[20]

In the late 19th century, a number of definitions of Reptilia were offered. The 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.[21] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.[22]

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.[23][24] Those four subclasses were:

  • Anapsida – no fenestrae – cotylosaurs and Chelonia (turtles and relatives)[note 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 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.[25]

    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, which were both traditionally called reptiles.[26] 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'.[22]

    Despi

    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, which were both traditionally called reptiles.[26] 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 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'.[22]

    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.[1]

    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.[1] Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.[1]

    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. 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

    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. 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,[1] as they are within the clade that includes both lizards and crocodiles.[1]

    Classification to order level of the reptiles, after Benton, 2014.[27][28]

    • Class Reptilia
      • Subclass Parareptilia
      • 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.[29] All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within archosauriformes,[29][30][31][32][33][34] though a few have recovered turtles as lepidosauriformes instead.[35] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[29]

        Amniota

        Synapsida (mammals and their extinct relatives) Varanops brevirostris flipped.jpg

        Reptilia

        Millerettidae Milleretta BW flipped.jpg

        unnamed

        Eunotosaurus

        Synapsida (mammals and their extinct relatives) Varanops brevirostris flipped.jpg

        Reptilia

        Millerettidae Milleretta BW flipped.jpg

        unnamed

        Millerettidae Milleretta BW flipped.jpg

        unnamed

        Eunotosaurus

        Lanthanosuchidae Lanthanosuchus NT flipped.jpg

        Procolophonoidea Sclerosaurus1DB.jpg

        Pareiasauromorpha Scutosaurus BW flipped.jpgPareiasauromorpha Scutosaurus BW flipped.jpg

        Captorhinidae Labidosaurus flipped.jpg

        Romeriida

        Paleothyris

        Paleothyris

        Diapsida

        Araeoscelidia Spinoaequalis schultzei reconstruction.jpg

        Neodiapsida
        ClaudiosaurusClaudiosaurus white background.jpg

        Younginiformes Hovasaurus BW flipped.jpg

        Reptilia

        Kuehneosauridae Icarosaurus white background.jpg

        Lepidosauria

        Rhynchocephalia (tuatara and their extinct relatives) Hatteria white background.jpg<p><a style=Rhynchocephalia (tuatara and their extinct relatives) Hatteria white background.jpg

        Squamata (lizards and snakes) Zoology<p><a style=Squamata (lizards and snakes) Zoology of Egypt (1898) (Varanus griseus).png Python natalensis Smith 1840 white background.jpg

        Choristodera Hyphalosaurus mmartyniuk wiki.png

        Archosauriformes (crocodiles, birds, dinosaurs and extinct relatives) Description des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpgMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

        Prolacertiformes Prolacerta broomi.jpgProlacertiformes Prolacerta broomi.jpg

        TrilophosaurusTrilophosaurus buettneri (flipped).jpg

        Rhynchosauria Hyperodapedon BW2 white background.jpgRhynchosauria Hyperodapedon BW2 white background.jpg

        Eosauropterygia Dolichorhynchops BW flipped.jpg

        Placodontia Psephoderma BW flipped<p><abbr title=Placodontia Psephoderma BW flipped.jpg

        Sinosaurosphargis

        Odontochelys

        Testudinata
        Proganochelys

        Testudines (turtles) Psammobates geometricus 1<p><a style=Testudines (turtles) Psammobates geometricus 1872 white background.jpg

          The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.[36] Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced.[37] The results place turtles as a sister clade to the archosaurs, the group that includes crocodiles, dinosaurs, and birds.[38] 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.[39]

        Evolutionary history

        Origin of the reptiles

        An early reptile Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs.[3]

        The oldest known animal that may have been an amniote is Casineria (though it may have been a temnospondyl).[40][41][42] A series of footprints from the fossil strata of Nova Scotia dated to 315 Ma show typical reptilian toes and imprints of scales.[43] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.[44] 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.[45] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)[46] and Paleothyris, both of similar build and presumably similar habit.

        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.[47] This sudden collapse affected several large groups. Primitive

        The oldest known animal that may have been an amniote is Casineria (though it may have been a temnospondyl).[40][41][42] A series of footprints from the fossil strata of Nova Scotia dated to 315 Ma show typical reptilian toes and imprints of scales.[43] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.[44] 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.[45] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)[46] and Paleothyris, both of similar build and presumably similar habit.

        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.[47] 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.[47] 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).[48] One of the best known early stem-reptiles is Mesosaurus, a genus from the Early Permian that had returned to water, feeding on fish.

        Anapsids, synapsids, diapsids, and sauropsids

        secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains against damage by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.[93]

        How turtles and tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how those turtles breathe. The varied results indicate that turtles and tortoises have found a variety of solutions to this problem.

        The difficulty is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles, such as the Indian flapshell (Lissemys punctata), have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction).[94]

        The difficulty is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles, such as the Indian flapshell (Lissemys punctata), have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction).[94]

        Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements.[95] This is because they use their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells.[95]

        Reptilian skin is covered in a horny epidermis, making it watertight and enabling reptiles to live on dry land, in contrast to amphibians. Compared to mammalian skin, that of reptiles is rather thin and lacks the thick dermal layer that produces leather in mammals.[96] Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base (osteoderms), forming armor. In lepidosaurians, such as lizards and snakes, the whole skin is covered in overlapping epidermal scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurians.[citation needed] The scales found in turtles and crocodiles are of dermal, rather than epidermal, origin and are properly termed scutes.[citation needed] In turtles, the body is hidden inside a hard shell composed of fused scutes.

        Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.

        Shedding

        Reptiles shed their skin through a process called ecdysis which occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every 5–6 weeks while adults shed 3–4 times a year.[97] Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one. Proteolytic enzymes and lymphatic fluid is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.

        Reptiles shed their skin through a process called ecdysis which occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every 5–6 weeks while adults shed 3–4 times a year.[97] Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one. Proteolytic enzymes and lymphatic fluid is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.[98] Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".[98] Dysecdysis, a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.[99] There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.[98] Nutritional deficiencies decrease proteolytic enzymes while dehydration reduces lymphatic fluids to separate the skin layers. Traumatic injuries on the other hand, form scars that will not allow new scales to form and disrupt the process of ecdysis.[99]

        ExcretionExcretion is performed mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, excrete mainly urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure called a loop of Henle, which is present in the nephrons of birds and mammals. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt glands in some reptiles.

        In all reptiles the urinogenital ducts and the anus both empty into an organ called a cloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles

        In all reptiles the urinogenital ducts and the anus both empty into an organ called a cloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in the monitor lizard, the legless lizards. It is absent in the snakes, alligators, and crocodiles.[100]

        Many turtles, tortoises, and lizards have proportionally very large bladders. Charles Darwin noted that the Galapagos tortoise had a bladder which could store up to 20% of its body weight.[101] Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.[102]:143 Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in osmoregulation.[103]

        Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.[104] Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have calculi.[105]

        Most reptiles are insectivorous or carnivorous and have simple and comparatively short digestive tracts due to meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting their lower resting metabolism and their inability to divide and masticate their food.[106] Their poikilotherm metabolism has very low energy requirements, allowing large reptiles like crocodiles and large constrictors to live from a single large meal for months, digesting it slowly.[83]

        While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorous megafauna: in the Paleozoic, the pareiasaurs; and in the Mesozoic several lines of dinosaurs.[48] Today, turtles are the only predominantly herbivorous reptile group, but several lines of agamas and iguanas have evolved to live wholly or partly on plants.[107]

        Herbivorous reptiles face the same problems of mastication as herbivorous mammals but, lacking the complex teeth of mammals, many species swallow rocks and pebbles (so called gastroliths) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter.[107] Fossil gastroliths have been found associated with both ornithopods and sauropods, though whether they actually functioned as a gastric mill in the latter is disputed.[108][109] Salt water crocodiles also use gastroliths as ballast, stabilizing them in the water or helping them to dive.[110] A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found in plesiosaurs.[111]

        Nerves

        Most reptiles are diurnal animals. The vision is typically adapted to daylight conditions, with color vision and more a

        Most reptiles are diurnal animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visual depth perception than in amphibians and most mammals.

        Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have only a few Rod cells and have poor vision in low-light conditions. At the same time they have cells called “double Rod cells and have poor vision in low-light conditions. At the same time they have cells called “double cones” which give them sharp color vision and enable them to see ultraviolet wavelengths.[121] In some species, such as blind snakes, vision is reduced.

        Many lepidosaurs have a photosensory organ on the top of their heads called the parietal eye, which are also called third eye, pineal eye or pineal gland. This “eye” does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is however sensitive to changes in light and dark and can detect movement.[121]

        Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of pits sensitive to infrared radiation (heat). Such heat-sensitive pits are particularly well developed in the pit vipers, but are also found in boas and pythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.[122]

        Most reptiles including birds possess a nictitating membrane, a translucent third eyelid which is drawn over the eye from the inner corner. Notably, it protects a crocodilian's eyeball surface while allowing a degree of vision underwater.[123] However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called the brille, spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.[124]

        Reptiles generally reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs through the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamates, including snakes and lizards, possess a pair of hemipenes, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm.[125]

        Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. The eggshell (1) protects the crocodile embryo (11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion (6) aids in gas exchange between the inside and outside of the egg. It allows carbon dioxide to exit the egg and oxygen gas to enter the egg. The albumin (9) further protects the embryo and serves as a reservoir for water and protein. The allantois (8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac (10) contains amniotic fluid (12) which protects and cushions the embryo. The amnion (5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac (2) surrounding the yolk (3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels (4) that allow the embryo to grow and metabolize. The air space (7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are no larval stages of development. Viviparity and ovoviviparity have evolved in many extinct clades of reptiles and in squamates. In the latter group, many species, including all boas and most vipers, utilize this mode of reproduction. The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian placenta. The earliest documented case of viviparity in reptiles is the Early Permian mesosaurs,[126] although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found. Several groups of Mesozoic marine reptiles also exhibited viviparity, such as mosasaurs, ichthyosaurs, and Sauropterygia, a group that include pachypleurosaurs and Plesiosauria.[12]

        Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of asexual reproduction, called parthenogenesis, occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo dragons (Varanidae) have reproduced by parthenogenesis.

        Parthenogenetic species are suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

        Some reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodi

        Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. The eggshell (1) protects the crocodile embryo (11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion (6) aids in gas exchange between the inside and outside of the egg. It allows carbon dioxide to exit the egg and oxygen gas to enter the egg. The albumin (9) further protects the embryo and serves as a reservoir for water and protein. The allantois (8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac (10) contains amniotic fluid (12) which protects and cushions the embryo. The amnion (5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac (2) surrounding the yolk (3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels (4) that allow the embryo to grow and metabolize. The air space (7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are no larval stages of development. Viviparity and ovoviviparity have evolved in many extinct clades of reptiles and in squamates. In the latter group, many species, including all boas and most vipers, utilize this mode of reproduction. The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian placenta. The earliest documented case of viviparity in reptiles is the Early Permian mesosaurs,[126] although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found. Several groups of Mesozoic marine reptiles also exhibited viviparity, such as mosasaurs, ichthyosaurs, and Sauropterygia, a group that include pachypleurosaurs and Plesiosauria.[12]

        Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of asexual reproduction, called parthenogenesis, occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo dragons (Varanidae) have reproduced by parthenogenesis.

        Parthenogenetic species are suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

        Some reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodiles, but also occurs in lizards and tuatara.[127] To date, there has been no confirmation of whether TDSD occurs in snakes.[128]

        Many small reptiles, such as snakes and lizards that live on the ground or in the water, are vulnerable to being preyed on by all kinds of carnivorous animals. Thus avoidance is the most common form of defense in reptiles.[129] At the first sign of danger, most snakes and lizards crawl away into the undergrowth, and turtles and crocodiles will plunge into water and sink out of sight.

        Camouflage and warning<

        Reptiles tend to avoid confrontation through camouflage. Two major groups of reptile predators are birds and other reptiles, both of which have well developed color vision. Thus the skins of many reptiles have cryptic coloration of plain or mottled gray, green, and brown to allow them to blend into the background of their natural environment.[130] Aided by the reptiles' capacity for remaining motionless for long periods, the camouflage of many snakes is so effective that people or domestic animals are most typically bitten because they accidentally step on them.[131]

        When camouflage fails to protect them, blue-tongued skinks will try to ward off attackers by displaying their blue tongues, and the frill-necked lizard will display its brightly colored frill. These same displays are used in territorial disputes and during courtship.[132] If danger arises so suddenly that flight is useless, crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy. Rattlesnakes rapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads to ward of approaching danger.

        In contrast to the normal drab coloration of most reptiles, the lizards of the genus Heloderma (the Gila monster and the beaded lizard) and many of the coral

        When camouflage fails to protect them, blue-tongued skinks will try to ward off attackers by displaying their blue tongues, and the frill-necked lizard will display its brightly colored frill. These same displays are used in territorial disputes and during courtship.[132] If danger arises so suddenly that flight is useless, crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy. Rattlesnakes rapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads to ward of approaching danger.

        In contrast to the normal drab coloration of most reptiles, the lizards of the genus Heloderma (the Gila monster and the beaded lizard) and many of the coral snakes have high-contrast warning coloration, warning potential predators they are venomous.[133] A number of non-venomous North American snake species have colorful markings similar to those of the coral snake, an oft cited example of Batesian mimicry.[134][135]

        Camouflage does not always fool a predator. When caught out, snake species adopt different defensive tactics and use a complicated set of behaviors when attacked. Some first elevate their head and spread out the skin of their neck in an effort to look large and threatening. Failure of this strategy may lead to other measures practiced particularly by cobras, vipers, and closely related species, which use venom to attack. The venom is modified saliva, delivered through fangs from a venom gland.[136][137] Some non-venomous snakes, such as American hognose snakes or European grass snake, play dead when in danger; some, including the grass snake, exude a foul-smelling liquid to deter attackers.[138][139]

        Defense in crocodilians

        When a crocodilian is concerned about its safety, it will gape to expose the teeth and yellow tongue. If this doesn't work, the