Reptiles (Reptilia) are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago. There are two characteristics that distinguished early reptiles from amphibians and enabled them to colonize terrestrial habitats more extensively than their ancestors, scales and the ability to lay hard-shelled amniotic eggs. Scales protect reptiles from abrasion and loss of body moisture. Hard-shelled eggs provide a protective environment in which the embryo can develop.
The Reptilia includes snakes, amphisbaenians, lizards, alligators, caimans, crocodiles, tortoises, turtles, and tuataras. There are over 7900 species of reptiles. Since reptiles do not produce their own body heat (they are cold-blooded, also known as ectothermic), they modify their activity and behavior to best adapt to changing environmental temperatures.