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Reptiles are tetrapod animals in the class or clade Reptilia /rɛpˈtɪliə/. As a class in Linnean taxonomy, Reptilia refers to a paraphyletic grouping comprising all amniotes (vertebrates which encase their embryos in a series of protective sacs) except synapsids (mammals and their extinct relatives) and Aves (birds). The class Reptilia comprises turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of the traditional reptile orders, historically combined with that of modern amphibians, is called herpetology. Operating under the modern evolutionary principle of cladistics, many paleontologists and herpetologists have redefined Reptilia as a monophyletic clade containing all modern reptiles (in the traditional sense), and any other descendant of their last common ancestor. This would include birds, which are a subgroup of archosaurs and more closely related to crocodilians than to any other modern animal. The clade Reptilia could alternatively be defined as referring to animals more closely related to modern reptiles than to mammals. This definition would make Reptilia synonymous with the clade Sauropsida. 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. 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, ornithischians, and sauropods, alongside many species of theropods, crocodyliforms, and squamates (e.g., mosasaurs). Modern non-bird reptiles inhabit all the continents except Antarctica. Several living subgroups are recognized: Testudines (turtles and tortoises), 360 species; Rhynchocephalia (the tuatara from New Zealand), 1 species; Squamata (lizards, snakes, and worm lizards), about 10,954 species; and Crocodilia (crocodiles, gharials, caimans, and alligators), 27 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, 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).