Reptiles, as most commonly defined, are the animals in the class Reptilia, a paraphyletic grouping comprising all amniotes except synapsids and Aves. The class comprises turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. In the traditional Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians are more closely related to birds than they are to other living reptiles, and so modern cladistic classification systems include birds within Reptilia, redefining the term as a clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade Sauropsida, which refers to all animals more closely related to modern reptiles than to mammals. The study of the traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.
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 was Hylonomus, a small and superficially lizard-like animal.