Reptiles (Reptilia) are members of a class of air-breathing, ectothermic (cold-blooded) vertebrates which are characterized by skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors. Modern reptiles inhabit every continent with the exception of Antarctica. Reptiles originated around 320-310 million years ago during the Carboniferous period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. Four living orders are typically recognized:
Aves (birds), which are not cold-blooded or scaly, are not included in this list. Crocodiles are more closely related to birds than they are to lizards, however, so some writers who prefer a more unified (monophyletic) grouping also include the birds: over 10,000 species.
Unlike amphibians, reptiles do not have an aquatic larval stage. As a rule, reptiles are oviparous (egg-laying), although certain species of squamates retain the eggs until hatching and a few are viviparous. 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).
The study of reptiles and amphibians is called herpetology.
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. The terms "reptile" and "amphibian" were largely interchangeable, "reptile" (from Latin repere, "to creep") being preferred by the French.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. Today, it is still common to treat the two groups under the same heading as herptiles.
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.
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 two.
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") which included some Paleozoic amphibians as well as early reptiles.
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 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.
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. The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.
The synapsid/sauropsid division supplemented, but was never as popular during the 20th century as a Linneaean approach splitting 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. 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, the scheme with four (or three if Euryapsida is sunk into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th century, and has only been challenged with the rising popularity of phylogenetic nomenclature.
By the 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 would be paraphyletic, since they exclude both birds and mammals, although these also evolved from animals like dinosaurs and early therapsids that were traditionally called 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. 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'.
Despite the early proposals for replacing the paraphyletic Reptilia for a monophyletic Sauropsida, that term was never adopted widely or, when it was, applied consistently. 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. 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 more well known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds.
Classification to order level, after Benton, 2005.