Reptiles, the class Reptilia, are an evolutionary grade of animals, comprising today’s turtles, crocodilians, snakes, lizards and tuatara, their extinct relatives, and some of the extinct ancestors of mammals. Due to their evolutionary history and the diversity of extinct forms, the validity of the class is not universally supported in scientific circles, though in practice, it remains in use by some biologists and more laymen, especially in mass media. The study of reptiles, historically combined with that of amphibians, is called herpetology. The earliest known reptiles originated around 315 million years ago during the Carboniferous period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. Some early examples include the lizard-like Hylonomus, Casineria and possibly Westlothiana, although the latter may be an advanced land-dwelling amphibian. 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 the pterosaurs, plesiosaurs, ornithischians, and sauropods, as well as many species of theropods (e.g. tyrannosaurs and dromaeosaurids), crocodyliforms, and squamates (e.g. mosasaurids). Modern reptiles inhabit every continent with the exception of Antarctica. Several living subgroups are recognized: Testudines (turtles, terrapins and tortoises): approximately 400 species Sphenodontia (tuatara from New Zealand): 1 species Squamata (lizards, snakes, and worm lizards): over 9,600 species Crocodilia (crocodiles, gavials, caimans, and alligators): 25 species 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, as were some extinct aquatic clades — 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 to the saltwater crocodile, Crocodylus porosus, which may reach in length and weigh over .
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