Reptiles are some of the oldest life forms on Earth, with fossils dating back about 320–310 million years. Reptiles are cold-blooded, four-legged vertebrates that inhabit every continent of the world, except Antarctica. A reptile’s skin is covered with a tough external plate or scute, such as the shell of a turtle, or the scales on a crocodile. Although many reptiles spend considerable time in water, all reptiles breathe using lungs. As cold-blooded animals, reptiles don’t internally regulate their body temperature. Their body temperature varies with that of the surrounding air or water. Reptiles are egg laying animals, with the embryo protected by a tough, leathery shell.
There are four classes of reptiles: crocodilia (e.g., crocodiles, caimans, and alligators), squamata (e.g., lizards and snakes), testudines (e.g, turtles and tortoises), and sphenodontia (e.g., tuataras).