Traditionally, reptiles are members of the class Reptilia comprising the amniotes that are neither birds nor mammals. (The amniotes are the vertebrates with eggs featuring an amnion, a double membrane that permits the embryo to breathe effectively on land.) Living reptiles, in that sense, can be distinguished from other tetrapods in that they are cold-blooded and bear scutes or scales.
Reptiles originated around 320-310 million years ago during the Carboniferous period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. There are many extinct groups, including dinosaurs, pterosaurs, and aquatic groups such as the ichthyosaurs. Modern reptiles inhabit every continent with the exception of Antarctica. Several living sub-groups are recognized: