Reptilia Classification Essay

For other uses, see Reptile (disambiguation).

Reptiles are tetrapod animals in the classReptilia, 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 (i.e., 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 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, as well as many species of theropods, including troodontids, dromaeosaurids, tyrannosaurids, and abelisaurids, along with many Crocodyliformes, and squamates (e.g. mosasaurids).

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), 350 species;[5]Rhynchocephalia (tuatara from New Zealand), 1 species;[5][6]Squamata (lizards, snakes, and worm lizards), over 10,200 species;[5]Crocodilia (crocodiles, gavials, caimans, and alligators), 24 species;[5] and Aves (birds), approximately 10,000 species.[7]

Reptiles are tetrapodvertebrates, 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[8] — the fetus develops within the mother, contained in a placenta rather than 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 may reach 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).


See also: List of reptiles

Research history[edit]

See also: Skull roof

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.[9] 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.[10] The terms "reptile" and "amphibian" were largely interchangeable, "reptile" (from Latin repere, "to creep") being preferred by the French.[11]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.[12] Today, the two groups are still commonly treated under the same heading as herptiles.

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.[13] 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.[14] 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.[15]

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" (paraphyleticbasalArchosauria), non-aviandinosaurs, pterosaurs, ichthyosaurs, and sauropterygians.[16]

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.[17] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.[18]

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.[19][20] Those four subclasses were:

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.[21]

Phylogenetics and modern definition[edit]

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 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.[22] 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'.[18]

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] 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. Because the actual relationship of turtles to other reptiles was not yet well understood at this time, Gauthier's definition came to be considered inadequate.[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 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.


See also: List of reptiles and List of snakes

Classification to order level of the reptiles, after Benton, 2014.[23][24]


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.[25] All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within archosauriformes,[25][26][27][28][29][30] though a few have recovered turtles as lepidosauriformes instead.[31] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[25]

The position of turtles[edit]

The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.[32] Molecular work has usually placed turtles within the diapsids. So far three turtle genomes have been sequenced.[33] The results place turtles as a sister clade to the archosaurs, the group that includes crocodiles, dinosaurs, and birds.[34]

Evolutionary history[edit]

Main article: Evolution of reptiles

Origin of the reptiles[edit]

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.[3]

The oldest known animal that may have been an amniote is Casineria (though it may have been a temnospondyl).[35][36][37] A series of footprints from the fossil strata of Nova Scotia dated to 7015994064400000000♠315 Ma show typical reptilian toes and imprints of scales.[38] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.[39] 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.[40] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)[41] and Paleothyris, both of similar build and presumably similar habit.

Rise of the reptiles[edit]

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.

Anapsids, synapsids, diapsids, and sauropsids[edit]

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.[32] 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.[1] 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 to give 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

Reptiles, from Nouveau Larousse Illustré, 1897-1904: Notice the inclusion of amphibians (below the crocodiles).
An "antediluvian monster", a Mosasaurus discovered in a Maastricht limestone quarry, 1770 (contemporary engraving)
Phylogenetic classifications group the traditional "mammal-like reptiles", like this Varanodon, with other synapsids, not with extant reptiles.
A = Anapsid,
B = Synapsid,
C = Diapsid

Classification of Reptilia

Based upon Zoological RecordVolume 138 (with modifications after Benton, J., Vertebrate Paleontology, 2015, pp. 438-441.)

Dinosaurs are classified as reptiles. A reptile is a cold-blooded, scaly, four-legged animal that lays eggs on land. Two formal orders: Saurischia (or lizard-hip) and Ornithischia (or bird-hip) encompass the creatures that have traditionally been called dinosaurs. Typically discussed alongside them as part of the great Age of Reptiles are the order Pterosauria (flying reptiles) and the orders Plesiosauria (great swimming reptiles). All of the animals that are highlighted in Genesis Park fit into the reptilian classification shown below.

Class Reptilia

Infraclass Diapsida Order Pareiasauromorpha Order Younginiformes Order Testudines (turtles) Family Chelydridae (Snapping Turtles) Family Emydidae (Pond Turtles/Box and Water Turtles) Family Testudinidae (Tortoises) Family Dermatemydidae (River Turtles) Family Kinosternidae (Mud and Musk Turtles) Family Carettochelyidae (Pig-nose Turtles) Family Trionychidae (Softshell Turtles) Family Cheloniidae (Sea Turtles) Family Dermochelyidae (Leatherback Turtles) Family Pelomedusidae (Afro-American Sideneck Turtles) Family Chelidae (Austro-American Sideneck Turtles) Infraclass Lepidosauromorpha Order Ichthyosauria (Dolphin-like marine reptiles) Order Thalattosauria (Large lizard-like marine reptiles) Order Placodontia (Marine reptiles-some with hard plates resembling turtles) Order Eosauropterygia Suborder Pachypleurosauria (Small, lizard-like aquatic reptiles) Suborder Nothosauria (Huge marine reptiles with a medium neck) Order Plesiosauria Suborder Plesiosauroidea (Huge marine reptiles with a long neck) Suborder Pliosauroidae (Huge marine reptiles with a short neck) Order Rhynchocephalia Family Sphenodontidae (Tuataras) Order Squamata ("scaled reptiles") Suborder Sauria (lizards) Family Agamidae (Agama Lizards) Family Chamaeleonidae (Chameleons) Family Iguanidae (Iguanas and Spinytail Iguanas) Family Corytophanidae (Casquehead Lizards) Family Crotaphytidae (Collared and Leopard Lizards) Family Hoplocercidae (Wood Lizards including the Dwarf Iguana) Family Opluridae (Madagascar Iguanids) Family Phrynosomatidae (Earless, Spiny, Tree, Side-blotched and Horned Lizards) Family Polychrotidae (Anoles) Family Tropiduridae (Neotropical Ground Lizards) Family Gekkonidae (Geckoes) Family Eublepharidae (Eyelid Geckoes) Family Pygopodidae (Legless Lizards) Family Dibamidae (Blind Lizards) Family Cordylidae (Spinytail Lizards) Family Gerrhosauridae (Plated Lizards) Family Gymnophthalmidae (Spectacled Lizards) Family Teiidae (Whiptails and Tegus) Family Lacertidae (Wall Lizards) Family Scincidae (Skinks) Family Xantusiidae (Night Lizards) Family Anguidae (Glass Lizards and Alligator Lizards; Lateral Fold Lizards) Family Anniellidae (American Legless lizards) Family Xenosauridae (Knob-scaled Lizards) Family Helodermatidae (Gila Monsters) Family Lanthanotidae (Earless Monitor lizards) Family Varanidae (Monitor Lizards) Family Mosasauridae (Huge Mosasaur marine lizard) Suborder Amphisbaennia Family Amphisbaenidae (Worm Lizards) Family Trogonophidae (Short head Worm Lizards) Family Bipedidae (Two-legged Worm Lizards) Suborder Serpentes (snakes) Family Anomalepidae (Dawn Blind Snakes) Family Typhlopidae (Blind Snakes) Family Leptotyphlopidae/Glauconiidae (Slender Blind Snakes) Family Aniliidae/Ilysiidae (Pipe Snakes) Family Anomochilidae (Dwarf Pipe Snakes) Family Boidae (Boas and Pythons) Family Bolyeridae (Round Island Boas) Family Cylindrophiidae (Asian Pipe Snakes) Family Loxocemidae (Mexican Burrowing Pythons) Family Tropidophiidae (Woodsnakes, Round Island "Boas") Family Uropeltidae (Short-tail Snakes) Family Xenopeltidae (Sunbeam Snakes) Family Acrochordidae (File Snakes) Family Atractaspididae (Mole Vipers) Family Colubridae (Colubrids) Family Elapidae (Cobras, Kraits, Coral Snakes) Family Hydrophiidae (Sea Snakes) Family Viperidae (Vipers and Pit Vipers) Infraclass Archosauromorpha Order Rhynchosauria (Stocky, herbivorous beaked reptiles) Order Protosauria (Amphibian-like reptiles) Order Phytosauria (Narrow-snouted, armored crocodilian) OrderCrocodyliaSuborderProtosuchidae (Ancient agile crocodilian) SuborderThalattosuchia (Ancient marine crocodilian) SuborderMetasuchia (Ancient huge crocodilian) SuborderEusuchiaFamilyCrocodylidae(Modern Crocodiles) FamilyGavialidae(Modern Gharials and False Gharials) FamilyAlligatoridae(Modern Alligators and Caimans) Order Pterosauria Suborder Rhamphorhynchoidea (Long-tailed pterosaurs) Suborder Pterodactyloidea (Short-tailed pterosaurs with a headcrest) SuperorderDinosauriaOrderSaurischia(“Lizard-hipped” dinosaurs) Suborder Theropoda (Bipedal, mostly carnivorous dinosaurs) Suborder Sauropodomorpha (Long-necked herbivorous dinosaurs) Order Ornithischia (“Bird-hipped” dinosaurs) Suborder Ornithopoda (Bipedal herbivorous dinosaurs like the hadrosaurs) Suborder Stegosauria (Huge, plated dinosaurs) Suborder Ankylosauria (Squat dinosaurs with club tails) Suborder Ceratopsia (Includes Triceratops)

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