Reptile

Traditionally, reptiles are members of the class Reptilia comprising the amniotes that are neither birds nor mammals. (The amniotes are the vertebrates with eggs featuring an amnion, a double membrane that permits the embryo to breathe effectively on land.) Living reptiles can be distinguished from other tetrapods in that they are cold-blooded and bear scutes or scales.

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. There are many extinct groups, including dinosaurs, pterosaurs, and aquatic groups such as the ichthyosaurs. Modern reptiles inhabit every continent with the exception of Antarctica. Several living sub-groups are recognized:

• Testudines (turtles, terrapins and tortoises): approximately 320 species
• Sphenodontia (tuataras from New Zealand): 2 species
• Squamata (lizards, snakes, and worm lizards): approximately 8,920 species
• Crocodilia (crocodiles, gavials, caimans, and alligators): 25 species

Although they have scutes on their feet and lay eggs, birds have historically been excluded from the reptiles, in part because they are warm-blooded. They therefore do not appear on the list above. However, as some reptiles are more closely related to birds than they are to other reptiles — crocodiles are more closely related to birds than they are to lizards — cladistic writers who prefer a more unified (monophyletic) grouping sometimes also include the birds with over 10,000 species, see Sauropsida.

Reptiles are tetrapod vertebrates, either having four limbs or, like snakes, being descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous (egg-laying), although certain species of squamates retain the eggs until hatching and a few are viviparous — 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 that 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).

Traditionally, the reptiles are one of the three classes of amniotes, the others being the mammals and the birds. By the middle of the 20th century, it had become common to split the reptiles into four groups based on their skull openings. However, as reptiles in the traditional sense are the ancestors of both birds and mammals, many taxonomists have more recently adopted a cladistic approach, preferring to group animals based exclusively on shared ancestry rather than on shared features; this orientation has led some to abandon reptiles as a systematic unit or redefine the reptiles in a manner that includes birds.

The historically combined study of reptiles and amphibians is called herpetology.