There are about 260 species of turtles, tortoises, and terrapins. They range in size from the leatherback, a marine species reaching an upper shell length of about 190 centimeters (6.2 feet) and a weight of over 900 kilograms (1,984 pounds), to small freshwater species that average around 10 centimeters (3.9 inches) in length and weigh less than 100 grams (a few ounces). Turtles are no longer classified as reptiles but are considered a distinct and unique evolutionary lineage of terrestrial vertebrates, the class Chelonia. Possession of an upper (carapace) and lower (plastron) shell in combination with a skull that lacks temporal ("the temple") openings behind the eye socket sets turtles distinctly apart from amphibians, reptiles, tuataras, crocodilians, and birds. Over their 210-million-year history since the late Triassic, turtles have remained conservative in retaining a shell, their distinctive skeletal feature, but at the same time demonstrating an amazing diversity during their evolution, from sleek, flexible water-loving softshell turtles to high-domed, land-dwelling galapagos tortoises.
All turtles are egg-layers; females dig nests in which to lay their eggs but like amphibians provide no maternal care after hatching (as in crocodilians and birds). Some turtles, such as softshells, snapping turtles, and diamondback terrapins, have commercial value and have been regularly consumed as food by people. Turtles are popular in the pet trade, and many species have been adversely impacted by overcollecting. In addition, the natural habitats of turtles are disappearing at an alarming rate, due to human overpopulation worldwide.
Joseph T. Collins
Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. New York: Facts on File, 1986.
Pough, F. Harvey, et al. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.