The class Crocodylia consists of twenty-two species of alligators, caimans, gharials, and crocodiles worldwide, and is most closely related to birds (class Aves). Like birds (and mammals), crocodilians have the ventricle of their heart divided into left and right compartments (unlike amphibians, turtles, and reptiles, whose ventricles have but a single, undivided compartment). In addition, like mammals and birds, crocodilians demonstrate much parental care of their young, a behavior not found in amphibians, turtles, reptiles, and tuataras.
Crocodilians are covered with scales, a trait they share with reptiles (and to some extent with turtles, but not with amphibians, whose skin is scaleless and permeable), and their cloacal opening is a longitudinal slit (not transverse as in the classes Reptilia and Rhynchocephalia). Crocodilians are no longer classified as reptiles, but are considered a distinct and unique evolutionary lineage , the class Crocodylia. Crocodilians are tropical and subtropical in distribution. Some species, such as the saltwater crocodile, can attain lengths of up to 7 meters (23 feet). Crocodilians are carnivorous in diet, and females build nests in which to lay eggs.
During their 215-million-year evolutionary history, beginning in the middle Triassic, these magnificent beasts invaded diverse habitats, from ocean to swamp, from wet tropical forest to cascading mountain rivers. Today's comparatively small remnant of this once diverse group still live in these areas, but their numbers grow smaller with poaching and the continuing, unstoppable destruction of their habitat by world overpopulation.
see also Amphibian; Circulatory Systems; Reptile; Tuatara; Turtle
Joseph T. Collins
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Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. New York: Facts on File, 1986.
Pough, F. Harvey, R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.
Ross, Charles A., ed. Crocodiles and Alligators. New York: Facts on File, 1989.