Reptiles are a group (Reptilia) of tetrapod animals comprising today's turtles, crocodilians, snakes, lizards, tuatara, and their extinct relatives. The study of these traditional reptile groups, historically combined with that of modern amphibians, is called herpetology. Because crocodilians are more closely related to birds than to any other group of reptiles, birds are also often included as a sub-group of reptiles by modern scientists.The earliest known proto-reptiles originated around 315 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became increasingly adapted to life on dry land. Some early examples include the lizard-like Hylonomus and Casineria. 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): 2 species Squamata (lizards, snakes, and worm lizards): over 9,600 species Crocodilia (crocodiles, gavials, caimans, and alligators): 25 speciesBecause 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), many modern scientists prefer to make Reptilia a monophyletic grouping and so also include the birds, which today contain over 10,000 species.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 — 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 (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).