REPTILE

( classReptilia ) any member of a group of air-breathing vertebrates that have internal fertilization and scaly bodies. These animals occupy an intermediate position in evolutionary development between the amphibians and the birds and mammals. Living reptiles include the snakes, crocodiles, lizards, turtles, and tuatara. Extinct reptiles include the dinosaurs, pterosaurs, and ichthyosaurs. There are about 6,000 species of living reptiles widely distributed throughout the warm and temperate regions, although they are most abundant and diversified in the tropics. They range in size from a species of gecko that measures about 34 millimetres (1.3 inches) in length to the anaconda snake, which can exceed 9 metres (30 feet). The largest turtle, the marine leatherback, weighs about 680 kilograms (1,500 pounds). The skin of reptiles is characteristically dry and has few or no glands. Except for snakes, reptiles have four limbs, which project to the side. This causes both lateral and forward thrusts when the animal walks, but crocodiles and lizards can lift their bodies and run. Most snakes move by pushing backward against a solid object. Climbing mechanisms include claws or scales that contain tiny hooks, as in geckos, and tails that can cling to branches. Marine reptiles rely mostly on their lashing tails while swimming. Reptiles are also cold-blooded; i.e., their body temperatures vary with external temperature, making them dependent on the heat of the surrounding air. Except for the tuatara, males of all species have copulatory organs. Female reptiles generally lay eggs, but some species bear live young. In a few species, the young inside the female are nourished by an organ similar to the mammalian placenta. Eggs may be laid in a nest or beneath cover; the amnion, a thin sac holding a watery fluid, protects the embryo from drying out or being damaged. The allantois, another sac, covered by a shell, functions as a respiratory organ. Some turtles can lay up to 200 eggs at a time; the gecko usually lays two, and the crocodile bears from 20 to 70 eggs. In certain parts of the world, reptiles are hunted for food, and their skins are valued for leather products. As a result, many species have become extinct. Venomous species cause little harm to humans except in a few rural areas. any member of the class Reptilia, a group of air-breathing vertebrates that have internal fertilization and scaly bodies rather than hair or feathers. They occupy an intermediate position in evolutionary development between amphibians and warm-blooded vertebrates, the birds and mammals. Reptiles include the snakes, lizards, crocodiles and alligators, turtles, and tuatara, among the living forms, and a great many extinct types such as dinosaurs, pterosaurs, and ichthyosaurs. Additional reading General works Angus d'A. Bellairs, The Life of Reptiles (1970); Roger Conant and Joseph T. Collins, A Field Guide to Reptiles and Amphibians: Eastern and Central North America, 3rd ed. (1991); P.J. Darlington, Zoogeography (1957, reprinted 1982); Carl Gans et al. (eds.), Biology of the Reptilia, 3 vol. (196870); Coleman J. Goin, Olive B. Goin, and George R. Zug, Introduction to Herpetology, 3rd ed. (1978); Arthur Loveridge, Reptiles of the Pacific World (1945); Robert Mertens, The World of Amphibians and Reptiles (1960; originally published in French, 1959); J.A. Oliver, The Natural History of North American Amphibians and Reptiles (1955); J.A. Peters, Dictionary of Herpetology (1964); C.H. Pope, The Reptile World (1955, reissued 1974); C.L. Prosser (ed.), Comparative Animal Physiology, 4th ed., 2 vol. (1991); K.P. Schmidt and R.F. Inger, Living Reptiles of the World (1957, reissued 1967); M.A. Smith, The British Amphibians and Reptiles, 4th ed. (1973); R.C. Stebbins, A Field Guide to Western Reptiles and Amphibians, 2nd rev. ed. (1985); Bernard S. Martof et al., Amphibians and Reptiles of the Carolinas and Virginia (1980), a guide to 159 herpetozoan species. Paleontology E.H. Colbert, The Dinosaur Book, 2nd ed. (1951), and The Age of Reptiles (1965); A.S. Romer, Vertebrate Paleontology, 3rd ed. (1966). Classification Distinguishing taxonomic features The major reptile groups are distinguished on the basis of vertebral and skull features, particularly the number and positions of the temporal fenestrae (i.e., large openings in the temporal bone). Beyond these, the pelvic structure and that of the teeth and limbs are important. Any extraordinary structural development (e.g., the wings of pterosaurs, the shell of turtles) is also a major factor in the system of classification. In recent reptiles (four out of some 17 orders), the structure of the heart, the male secondary sex organs, the extent and kind of dermal armour, and the structure of other soft organs are used. Discoveries in serology (the study of blood serum) and karyology (the study of chromosomes) have little effect, as yet, on the systems of classification. Annotated classification The following classification of the reptiles is based on that of A.S. Romer (1956, 1966), the American vertebrate paleontologist, as later modified by himself (1968) and in minor respects by E.H. Colbert (1965), B.W. Halstead (1969), and H.G. Dowling (1971). No single system of reptile classification is acceptable to all herpetologists, and widely differing views are held by various authorities. Considerable change in the recognition and content of both major and minor categories is to be expected. Groups marked with a dagger () are extinct and known only from fossils.