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An order of the class Reptilia (subclass Archosauria) which is composed of large, voracious, aquatic species which include the alligators, caimans, crocodiles, and gavials. The group has a long fossil history from Late Triassic times and its members are the closest living relatives of the extinct dinosaurs and the birds. The 21 or 22 living species are found in tropic areas of Africa, Asia, Australia, and the Americas. One form, the salt-water crocodile (Crocodylus porosus), has traversed oceanic barriers from the East Indies as far east as the Fiji Islands.
The order is distinguished from other living reptiles in that it has two temporal foramina, an immovable quadrate, a bony secondary palate, no shell, a single median penis in males, socketed teeth, a four-chambered heart, and an oblique septum that completely separates the lung cavities from the peritoneal region. Certain of these unique features and other salient characteristics of the Crocodylia are intimately associated with their aquatic life. For example, there is a special pair of fleshy flaps at the posterior end of the mouth cavity which form a valvular mechanism which separates the mouth from the region where the air passage opens into the throat. This complex arrangement allows crocodilians to breathe even though most of the head is under water, or the mouth is open holding prey or full of water.
During the breeding season male crocodylians set up territories on land which they defend against intruders of the same species. Fertilization is internal and the hard-shelled eggs are deposited in excavations in the sand or in large nests of decaying vegetation, depending upon the species.
The living species are placed in two families and eight genera. The family Crocodylidae contains two subgroups: the true crocodiles, Crocodylinae, including the genera Crocodylus found in all tropic areas, Osteolaemus in central Africa, and the false gavial (Tomistoma) in Malaya and the East Indies; the alligators and caimans, Alligatorinae, including the genera Alligator of the southeastern United States and near Shanghai, China, the Caiman from Central and South America, and Melanosuchus and Paleosuchus of South America. The gavial (Gavialis gangeticus) of India and north Burma is the only living member of the family Gavialidae. Crocodiles differ most obviously from alligators and caimans in head shape and in the position of the teeth, although other technical details also separate them.
Crocodylians first appear in deposits of Late Triassic or possibly Early Jurassic age in North America and South Africa. They formerly were much more widely distributed in temperate latitude than today; abundant paleobotanical evidence confirms a warmer climate in mid-latitudes at this time, It seems probable that the restriction of crocodilians to the tropics was a direct result of cooling climate in the late Cenozoic. See Reptilia