Reptiles Reptiles, as most commonly defined, are the animals in the class Reptilia, a paraphyletic grouping comprising all sauropsid amniotes except Aves. In addition to the living reptiles, many diverse groups are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs, and all non-avian dinosaurs alongside many species of crocodyliforms, and squamates. Modern non-bird reptiles inhabit all the continents except Antarctica. Reptiles are tetrapod vertebrates, creatures that either has 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 placenta rather than contained in an eggshell. As amniotes, reptile eggs are surrounded by...
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