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Reptile

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"Reptilia" redirects here. For other uses, see Reptile (disambiguation).
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Reptiles
Fossil range: 320–0 Ma
Carboniferous – Recent
Clockwise from above left: Spectacled Caiman (Caiman crocodilus), Green Sea Turtle (Chelonia mydas), Tuatara (Sphenodon punctatus) and Eastern Diamondback Rattlesnake (Crotalus adamanteus).
Clockwise from above left: Spectacled Caiman (Caiman crocodilus), Green Sea Turtle (Chelonia mydas), Tuatara (Sphenodon punctatus) and Eastern Diamondback Rattlesnake (Crotalus adamanteus).

Scientific classification
Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

(unranked) Amniota

Class: Reptilia
Laurenti, 1768
Included groups

Excluded groups

Reptiles are animals in the (Linnaean) class Reptilia characterized by breathing air, a "cold-blooded" (poikilothermic) metabolism, laying tough-shelled amniotic eggs (or retaining the same membrane system in species with live birth), and skin usually covered in scales or scutes. They are tetrapods (either having four limbs or being descended from four-limbed ancestors) and lay amniotic eggs, in which the embryo is surrounded by a membrane called the amnion. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:

The majority of reptile species are oviparous (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). 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, that grows to only 1.6 cm (0.6 in) to the saltwater crocodile, Crocodylus porosus, that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called herpetology.

Classification

History of classification

Reptiles (green field) are a paraphyletic group comprising all non-avian and non-mammalian amniotes.

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 Naturae.[1] The terms "reptile" and "amphibian" were largely interchangeable, "reptile" (from Latin repere, "to creep") being preferred by the French.[2] 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.[3] Not until the beginning of the 19th century did it become 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.[4]

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 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 two.[5]

Around the end of the 19th century, the class Reptilia had come to include all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes the extant crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, as well as fossil groups like dinosaurs, synapsids and the primitive pareiasaurs. This is still the usual definition of the term. However, in recent years, many taxonomists[who?] have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also evolved from the original reptile. Colin Tudge writes:

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. It is instead 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'.[6]

The terms "Sauropsida" ("lizard faces") and "Theropsida" ("beast faces") were taken up 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, the Protosauria ("first lizards") which included some Paleozoic amphibians as well as early reptiles.[7]

In 1956 D.M.S. Watson observed that the first two groups diverged very early in reptilian history, and so he divided Goodrich's Protosauria between them. He also reinterpreted the 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.[8]

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology[9]) into four subclasses according to the number and position of temporal fenestrae, openings in the sides of the skull behind the eyes. Those subclasses were:

  • Anapsida – no fenestrae
  • Synapsida – one low fenestra (no longer considered true reptiles)
  • Euryapsida – one high fenestra (now included within Diapsida)
  • Diapsida – two fenestrae

All of the above except Synapsida and the earliest anapsid stem-amniotes are included within Sauropsida.

Taxonomy

Classification to order level, after Benton, 2004.[10]

Phylogeny

The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by Laurin and Gauthier (1996), presented as part of the Tree of Life Web Project.[11]

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