Encyclopedia of Science | Reptiles

A reptile is an organism in the kingdom Animalia and the class Reptilia. (Kingdoms are the main divisions into which scientists classify all living things on Earth; kingdoms are further subdivided into phylums [or divisions], classes, and orders.) The reptiles include more than 6,000 species grouped into four orders: the turtles (Chelonia), the snakes and lizards (Squamata), the crocodiles and alligators (Crocodilia), and the tuataras (Sphenodonta), large lizardlike animals found only on islands off the coast of New Zealand.

A number of other reptilian orders are now extinct. These include some of the largest animals ever to occupy the planet. Examples include the fishlike ichthyosaurs, the long-necked plesiosaurs, and the huge flying and gliding pterosaurs. The most famous of the extinct reptilian orders were the dinosaurs, that included immense, ferocious predators such as Tyrannosaurus rex and enormously large herbivores...