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Extant reptilia

Clockwise from above left: Green turtle, Tuatara, Nile crocodile and Sinai agama.

Terran Reptiles, class Reptilia, are an evolutionary grade of animals, comprising today's birds, turtles, crocodilians, snakes, lizards and tuatara, their extinct relatives, and some of the extinct ancestors of mammals. The  class is not universally supported, though in practice, it remains in use by some biologists and more laymen. The study of reptiles, historically combined with that of amphibians, is called herpetology. Sentient reptile-like species are called reptiloids.

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 K–Pg extinction wiped out most reptile species existant then.

Modern reptiles inhabit every Terran continent with the exception of Antarctica. Several living subgroups are recognized:

Reptiles are tetrapod vertebrates, creatures that either have 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, although several species of squamates 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, 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 to the saltwater crocodile, Crocodylus porosus, which may reach 6 m in length and weigh over 1,000 kg.

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