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Reptiles, as most commonly defined, are the animals in the class Reptilia (/rɛpˈtɪliə/), a paraphyletic grouping comprising all sauropsid amniotes except Aves (birds).[1] Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians (tuatara). In the traditional Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians are more closely related to birds than they are to other living reptiles, and so modern cladistic classification systems include birds within Reptilia, redefining the term as a clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade Sauropsida, which refers to all animals more closely related to modern reptiles than to mammals. The study of the traditional reptile orders, historically combined with that of modern amphibians, is called herpetology. The study of all reptiles (including birds) combines herpetology (when excluding amphibians) and ornithology.

Reptiles
Temporal range: Pennsylvanian–Present, 312–0 Ma
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Reptiles 2021 collage.jpg
About this image
Collage of reptilians by saurian clade listed in top-to-bottom order: six lepidosaurs and six archosaurs.
Scientific classification e
Kingdom:
Animalia
Phylum:
Chordata
Clade:
Sauropsida
Class:
Reptilia
Laurenti, 1768
Extant groups
Lepidosauria (lepidosaurs)
Rhynchocephalia (tuatara and relatives)
Squamata (lizards, snakes and amphisbaenians)
Archosauria (archosaurs)
Crocodilia (crocodilians)
Testudines (turtles)
See text for extinct groups.

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.[2] 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[3] – 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). 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.[126] 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
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A camouflaged Phelsuma deubia on a palm frond
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.[127] 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.[128]

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.[129] If danger arises so suddenly that flight is useless, crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy.