Turtles are reptiles of the order Chelonia, also called the Testudines or Testudinata, characterized by a body-encasing shell of usually fused, bony plates covered by large, horny scales. Turtles occur worldwide from the tropics to temperate regions in terrestrial, freshwater, and marine habitats. In contrast to other reptiles, whose greater diversity and abundance center in tropical regions, turtle species reach their highest numbers in southeastern North America and southeastern Asia. Turtle size is conventionally measured by the length of the upper shell (carapace) or the lower shell (plastron) in a straight line from front to rear. Turtles range in size from about 7.5 cm (3 in) in some specimens of the striped mud turtle, Kinosternon baurii, to 2.4 m (8 ft) and 725 kg (1,600 lb) in the leatherback, Dermochelys coriacea. The extinct sea turtle Archelon ischyros reached 6 m (19.5 ft) in length, as did the fossil tortoise Colossochelys atlas. The earliest fossil remains of shelled vertebrates recognized as turtles are from the Triassic Period, approximately 200 million years ago. Turtles have gone essentially unchanged in anatomical appearance since that time. The pre-Triassic ancestors of turtles are unknown, but a traceable ancestry as far back as the Triassic is impressive: the direct ancestors of three entire classes of vertebrates--modern mammals, birds, and amphibians--can be reliably traced back to the Jurassic Period, or about 160 million years. CHARACTERISTICS Although the earliest-known extinct turtle had teeth on the roof of its mouth, all living turtles are toothless. The edges of the jaws are instead covered with a hard, sharp-edged, horny sheath, which may be smooth or may have toothlike serrations. The turtle's skull is distinctive in being solid, with no side openings to the rear of the eye sockets. Turtles have no external ear openings, but an ear covering (tympanum) is sometimes readily visible. Although their internal ear structures are well developed and able to detect low-frequency vibrations, turtles are generally regarded as being unresponsive to airborne sounds. Their sense of smell, however, is considered good. The Shell The turtle's upper shell, or carapace, is commonly constructed of about 50 bones. These bones are believed to have evolved as new structures (dermal plates formed in embryonic connective tissue) rather than as modifications of previously existing parts of the skeleton. Unique among backboned animals, the turtle's skeletal limb-support systems--the shoulder and hip girdles--are inside the rib cage, the shoulder girdle partly adding to the formation and strength of the shell. The turtle's backbone is also incorporated into the shell for increased solidity; shortened by the loss of vertebral bones, it contains only two sacral (pelvic) and ten trunk vertebrae. The three lengthwise rows of bony plates in the upper shell are fused to the backbone and ribs beneath. The plates of the middle row are fused to the neural arches of the trunk vertebrae and are known as neural plates. On each side of this middle row is a row of costal plates, fused to the ribs (costae) of these vertebrae. The three longitudinal rows of bony plates are surrounded by a border of smaller bony plates called peripherals. The lower shell, or plastron, typically contains nine bones: a small, single plate in front and four pairs of bony plates behind. The single plate and the first pair of plates were derived from bones of the shoulder girdle; the remaining three pairs of plates may have arisen as new structures or may have been derived, at least in part, from skeletal elements called abdominal ribs. The bridge, formed by the upward-curving outer edges of the plastron, is usually present, connecting the plastron to the carapace. The outer surface of both the upper and lower bony shell is covered by an extremely thin layer of skin, the epidermis, which contains both nerves and blood vessels. Covering this skin is a thick layer composed of large, horny scales called scutes, shields, or laminae. These scutes form the exterior shell seen on living turtles. Pressure on these scutes can be detected by nerves in the skin below. The number, position, size, and shape of the scutes are not identical to those of the bony plates beneath; indeed, it is common for the scutes to lie across the junction of two bony plates, thus lending additional strength to the shell. Although marked variations do occur and certain scutes are given specific names, the scutes of the carapace can be divided broadly into three main longitudinal rows--the vertebral scutes in the middle and a row of pleural scutes on each side--and a surrounding border of smaller scutes called marginals. Similarly, the scutes of the plastron can be broadly regarded as consisting of six pairs arranged in a double row from front to rear, plus a small scute at each leg opening. The soft-shelled turtles, family Trionychidae, however, lack the horny scutes and instead have their somewhat reduced bony shells covered by a thick, fleshy layer of skin. The leatherback turtle also has a thick, leathery skin, but it is ridged and embedded with small bony plates. Respiration Because of the rigid shell, turtles are unable to expand their "chests" to breathe. Consequently, inflation of the lungs, located just beneath the upper shell, is made possible only by altering the volume of space within the shell. Two sets of muscles at the rear of the body form the primary breathing mechanism. Broadly speaking, when these muscles are effectively contracted, they pull the internal organs up against the lungs, forcing out the air; the process is reversed in inhaling. Muscles associated with the front limbs also contribute to the breathing action: when contracted, they rotate the shoulder girdle outward, adding to the expanding space; when relaxed, they allow the shoulder girdle to return inward, reducing the space. Many turtles can also obtain oxygen by other means. All aquatic turtles are believed able to use the tissues lining the insides of their mouths to absorb oxygen from the water. Soft-shelled turtles are able to obtain oxygen from water through the skin overlaying their shells, and marine turtles have sacs in the cloaca (rectum) that are also thought capable of extracting oxygen from water. Dormancy Turtles in temperate regions hibernate during colder periods of the year. The ability to respire through tissues of the cloaca, throat, and skin while underwater permits some species to go for long, cold periods (weeks or months) without breathing air directly. Turtles in tropical regions and some temperate areas may aestivate during hot, dry periods. An ability to shut down metabolism during unfavorable periods appears to be an adaptive trait among turtles in both aquatic and terrestrial forms throughout the world. Feeding Turtles as a group are omnivorous, eating both plant and animal matter, living or dead. A few species, however, are specialized feeders. Food is not chewed. The sharp cutting beak and strong throat muscles are used to force bite-sized pieces down the esophagus to the stomach. Reproduction Turtles mate after a period of courtship which may include the male chasing the female and vibrating its long claws in front of her. The females of some species, such as the Eastern box turtle, Terrapene carolina, and the diamondback terrapin, Malaclemys terrapin, are known to be able to store sperm for up to 4 years, producing fertile eggs for several seasons without additional matings. All turtles are egg layers and all deposit their white eggs on land. The small mud and musk turtles, family Kinosternidae, lay tiny, oblong eggs about 1 to 2 cm (0.5 to 0.75 in) long, with a strong calcareous shell. The largest eggs, laid by the sea turtles, are round with a tough, leathery shell and can be up to 6 cm (2.5 in) in diameter. The time between ovulation and egg deposition is usually only a few days. Egg incubation time may be as short as 45 days for some species or more than 100 days for others. Females of some species characteristically lay only one or two eggs, whereas the sea turtles average more than 100 eggs per clutch. Longevity Growth rates of juvenile turtles are relatively rapid compared to those of adults. Nonetheless, individuals of some species take as long as 10 years to reach maturity. Many species display sexual dimorphism as adults, with the female sometimes being more than twice as large as the male. A unique feature of some species is the development of annual growth rings on each shell scute. Thus, the ages of young individuals can be determined before the rings begin wearing off. Turtles live much longer than most other animals. Evidence exists for captive specimens of several species that have lived for 50 years in captivity. Even in natural populations, small forms such as box turtles, Terrapene, and painted turtles, Chrysemys, of the United States have average adult ages that have been reliably estimated at 30 to 50 years. IMPORTANCE OF TURTLES The eggs and meat of turtles are of economic importance as a food source. Turtle meat and soups range from staples to delicacies, depending on the time, the place, and the culture. Millions of eggs of marine turtles are collected each year as food. The eggs of some freshwater species are also intensively harvested. The most dramatic example is perhaps the South American river turtle, Podocnemis, from which more than 40 million eggs are taken annually to be used as food or as a source of oil. The once highly fashionable "tortoiseshell" of the hawksbill turtle, Eretmochelys imbricata, is still in demand, even with the expanding use of plastics. Aquatic turtles are often significant scavengers in natural or artificial lakes. In addition, a major pet trade has thrived on the sale of untold thousands of baby turtles of a dozen species across the world. The sale of pet turtles has greatly diminished in recent years because of the discovery of transmissible Salmonella disease in many of the specimens. Efforts are now being made to sell "sterilized" and rather inhumanely, individually packaged baby turtles. Conservation Despite their protective shells, some turtle species are in grave danger. The sea turtles' plight has been widely publicized. Thousands of nests are destroyed by humans each year. Habitat change, however, is perhaps the major threat to most turtles. At this time more than 30 species of turtles are officially recognized as in potential danger of extinction. In addition, some species are recognized by particular countries or states as being threatened. TAXONOMY AND CLASSIFICATION Because there are not native land turtles in Britain, the British traditionally refer only to the sea turtles as turtles, and to all other members of this reptile group as tortoises, even if the latter are aquatic forms. In the United States, however, turtle refers to all members of the order Chelonia, whereas tortoise has a more restricted usage, referring only to the land-dwelling forms. Terrapin, derived from an American Indian word, commonly refers to the diamondback terrapin, Malaclemys terrapin, a brackish-water inhabitant of the U.S. Atlantic and Gulf coasts. Approximately 250 species of turtles exist in the world today. Living species are assigned to one of two suborders. Members of the suborder Pleurodira, the side-necked turtles, are characterized by the withdrawal of their heads into their shells by a sideways bending of the neck. This suborder contains two families, four genera, and about 50 species. The suborder Cryptodira, whose members withdraw their heads into their shells by a vertical bending of the neck into an S-shaped curve, consists of about 12 families, 60 genera, and 200 species. Families Pelomedusidae and Chelidae The side-necked turtles are restricted predominantly to the Southern Hemisphere. With the one rare exception of the pitted-shell turtle, Carettochelys, the sole member of its family, the only turtles occurring in Australia are species in the family Chelidae. Podocnemis, of the family Pelomedusidae, occurs in South America and Madagascar but not in Africa. Family Chelydridae The common snapper, Chelydra, and alligator snapper, Macroclemys, are the only species in this family. Common snappers are found throughout eastern North America and range to northern South America. Alligator snappers are confined to southeastern North America. Family Emydidae More than one-third of the world's turtle species are in this family. The heaviest concentrations are in North America and Asia, with few representatives in Europe, South America, or Africa and none in Australia. Most emydids are at least partially aquatic, but the North American box turtle, Terrapene, is terrestrial. Family Testudinidae The high-domed terrestrial tortoises have become almost legendary because of the famous giants of the Galapagos Islands, but desert regions are also successfully occupied by these tortoises, who can adjust to midday heat and aridity by retreating into deep burrows. Family Trionychidae The species in this family represent an extreme in shell shape and texture. The flattened, leathery carapace is adapted for fast swimming. The soft-shell turtles are almost exclusively aquatic; they go onto land only for nesting purposes. Families Cheloniidae and Dermochelyidae The \Lsea turtle\ls are adapted to their marine environment by their generally large size, expert swimming abilities, and the physiological capacity to excrete salt through the lacrimal glands around their eyes. The six species of chelonids have the typical hard shells of other turtles, but the leatherback, Dermochelys, lacks the epidermal scutes. Other Families The Platysternidae, Dermatemydidae, and Carettochelydidae are each represented by a single species occurring, respectively, in Asia, Central America, and New Guinea and Australia. The family Kinosternidae includes some of the smallest turtles in the world, the mud and musk turtles. The two dozen species are confined to the Americas. Whit Gibbons Bibliography: Alderton, David, Turtles and Tortoises of the World (1988); Carr, A. F., Handbook of Turtles (1952); Cobb, Jo., A Complete Introduction to Turtles and Terrapins (1987); Ernst, C. H., and Barbour, R. W., Turtles of the United States (1972) and Turtles of The World (1989); Harless, Marion, and Morlock, Henry, Turtles: Perspectives and Research (1979); Mehrtens, John M., Turtles (1984); Nicholls, Richard E., Book of Turtles (1977); Obst, F., Turtles, Tortoises and Terrapins (1986); Pope, Clifford H., Turtles of the United States and Canada (1939); Pritchard, Peter, Living Turtles of the World (1966) and Encyclopedia of Turtles (1979).