Reptiles, as most commonly defined, are the animals in the class Reptilia, a paraphyletic grouping comprising all amniote except synapsids (mammals and their extinct relatives) and Aves (birds).The class comprises turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives.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.