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 (plant-eaters) such as Apatosaurus.
The first reptiles known in the fossil record lived about 340 million years ago. The last representatives of the dinosaurs became extinct about 65 million years ago, after being the dominant large animals of Earth for more than 250 million years.
Reptiles are extremely diverse in their form and function. They characteristically have four legs, although some groups such as the snakes have become legless. They usually have a tail and a body covered with protective scales or plates. These scales are dry, not slimy as some people believe, and have developed from the animal's epidermis (skin).
Reptiles are ectotherms (cold-blooded). This means they warm their bodies by absorbing heat from their environment. Thus, a reptile's body temperature fluctuates with changes in the surrounding temperature. The body temperature of snakes, for example, cools in cold weather and warms up in hot weather. Not surprisingly, external temperature plays a major role in determining the activities of reptiles: they are active when it's warm outside and slow down when its cold.
Reptiles reproduce by internal fertilization. Their eggs (sex cells) have a series of membranes (layers) around the embryo (earliest life-form) that allow the exchange of gases and waste. These eggs, known as amniotic eggs, were an important evolutionary adaptation for conserving moisture and allowed the reptiles to adapt to living on land. Most reptiles are oviparous, meaning they lay eggs in a warm place and the eggs are kept warm until they hatch. Some species are ovoviviparous, meaning the eggs are retained within the female throughout their development so that live young reptiles are born.
Some species of reptiles are dangerous to humans and to animals, including the predatory crocodiles and alligators. Some species of snakes are venomous (poisonous) and may bite people or livestock when threatened. Many species of reptiles are economically important and are hunted as food, for their eggs, or for their skin, which can be manufactured into an attractive leather. Many species are kept as interesting pets or in zoos.
Unfortunately, some people have an unreasonable fear of reptiles that has led to many of them being killed. Additionally many species are endangered because their natural habitats have been taken over for agriculture, forestry, or residential development.