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The class Reptilia includes over 6,000 species grouped into four orders: the turtles (Chelonia), the snakes and lizards (Squamata), the crocodiles and alligators (Crocodilia), and the tuataras (Sphenodonta). Other, now extinct, reptilian orders included Earth's largest terrestrial animals, and some enormous marine creatures. The fishlike ichthyosaurs were large marine reptiles, as were the long-necked plesiosaurs. The pterosaurs were large flying or gliding reptiles. The most famous of the extinct reptilian orders were the dinosaurs, which included immense, ferocious predators such as Tyrannosaurus rex, and enormously large herbivores such as Apatosaurus.

The first reptiles known in the fossil record occurred about 340 million years ago, during the Carboniferous period. The last representatives of the dinosaurs became extinct about 65 million years ago, after being the dominant large animals of the earth for more than 250 million years. Some paleontologists believe that the dinosaurs are not actually extinct, and that they survive today as birds, with which dinosaurs are known to have shared many anatomical, physiological, and behavioral traits.

Reptiles are extremely diverse in their form and function. They characteristically have four legs (although some groups have secondarily become legless), a tail, and a body covered by protective scales or plates developed from the epidermis. Reptiles have internal fertilization, and their eggs have a series of membranes around the embryo that allow the exchange of respiratory gases and metabolic waste (known as amniotic eggs). Amniotic eggs were an important evolutionary adaptation for conserving moisture and allowed the adoption of a terrestrial way of life. Reptiles have direct development, meaning they lack a larval stage, and their eggs produce miniature replicas of adult animals. Most reptiles are oviparous, laying eggs in a warm place that incubates the eggs until they hatch. Some species are ovoviviparous, with the female retaining the eggs inside her reproductive tract throughout their development, so that live young reptiles are born.

Some species of reptiles are dangerous to humans and to agricultural and domestic animals. Crocodiles and alligators can be predators of humans and other large animals, while some species of snakes are venomous 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 of reptiles are kept as interesting pets or in zoos.

Unfortunately, some people have an inordinate fear of reptiles, and this has commonly led to the persecution of these animals. Many species of reptiles are endangered, having suffered the loss of their natural habitat, which has been used for agriculture, forestry, or residential development.

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