The class Reptilia is composed of about 5,100 species, organized in three very closely interrelated groups: the lizards (order Lacertilia), composed of about 3,165 species; the amphisbaenians (order Amphisbaenia), which consist of about 135 species; and the snakes (order Serpentes), which contain about 1,800 species. According to most experts, lizards appeared in the fossil record in the middle Jurassic, about 165 million years ago (although some authorities place the earliest known fossil lizards in the late Permian, about 250 million years ago). Fossil amphisbaenians have been recorded as early as the late Cretaceous, over 65 million years ago. Snakes are known from the early Cretaceous, about 135 million years ago.
Research has shown that the separation of lizards and snakes into distinct orders is an unnatural artifact of outmoded scientific methodologies and does not reflect the evolutionary history of these animals; significant changes in the classification of the class Reptilia can be expected in the future. Members of the class Reptilia all share numerous characteristics (called synapomorphies) of physiology, behavior, and functional morphology that readily set them apart from amphibians, mammals, turtles, tuataras, and birds. One of the most striking of these is the presence in males of well-developed, paired copulatory organs called hemipenes (all other classes of terrestrial vertebrates have a single penis).
Two of the orders exhibit a combination of characteristics that generally permit ready identification: Lizards generally possess four limbs, ear openings, and eyelids, and snakes lack functional limbs, ear openings, and eyelids. Amphisbaenians differ substantially from lizards and snakes in many ways, most notably by their very short tails, distinctly annulated (ringed) bodies, and the reduction of the right lung (instead of the left lung, as in snakes and limbless lizards).
From small lizards such as geckos, with a snout-vent length sometimes as small as 1.5 centimeters (.59 inches), to the reticulated python of southeastern Asia that reaches 10 meters (32.8 feet) in total length, reptiles display an amazing diversity of size, shape, color, and pattern. Their beauty and comparative ease to maintain as pets has created an entirely new venture, distinct and separate from the science of herpetology, known as herpetoculture. Herpetoculturists worldwide are devoted to the husbandry of many reptiles (mostly snakes), breeding, trading, and selling captive reptiles for fun and profit.
Joseph T. Collins
Gans, Carl, ed. Biology of the Reptilia. Chicago: University of Chicago Press, 1992.
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.