turtle
turtle a reptile of the order Chelonia, with strong, beaked, toothless jaws and, usually, an armorlike shell. The shell normally consists of bony plates overlaid with horny shields. The upper portion, or carapace, covers the turtle's back and sides, and the lower portion, or plastron, covers the belly; the two parts are joined at the sides. Exceptions are the rare plateless turtles of New Guinea and the marine leatherback turtle, which is encased in a thick, ossified skin resembling a carapace. When startled, most turtles withdraw their heads straight back into their shells, the neck folding into an S-shaped curve. However, in the side-necked turtles of the Southern Hemisphere, the head moves sideways and tucks next to the shoulder.
Turtles are found throughout most of the temperate and tropical world and in the open ocean; of the 270 known species, 42% are rare or threatened with extinction. Many turtles and their eggs are valued as food. Edible species include several marine turtles, the green turtle (traditional ingredient of turtle soup), the diamondback terrapin, and the soft-shelled turtles. Catching females when they lay eggs on land has contributed to a serious decline in many species, since it can take 10 to 30 years for some turtles to reach sexual maturity.
Different types of turtle are variously adapted to living on land, in freshwater, or in the ocean, but all turtles breathe by means of lungs, and all lay eggs on land. The land-living species, especially those of the family Testudinidae, are commonly called tortoises. The name terrapin is generally applied to large freshwater or brackish water species, especially those used for food. Turtle species are either herbivorous or carnivorous but rarely both. They range in length from a few inches to over 6 ft (2 m), most being between 5 in. and 15 in. (13-38 cm) long. Many specimens have survived more than 50 years in captivity; one giant tortoise is known to have lived for 176 years, and another is believed to have lived about 250 years.
Turtles existed 200 million years ago, at the time of the earliest dinosaurs; these early land-dwelling turtles could not retract their necks. By 120 million years ago some turtles had adapted to an aquatic life. Many of the living families of turtles existed in the Cretaceous period and have undergone very little change since then. On the basis of morphological (body structure) evidence, turtles were thought to be the oldest surviving group of reptiles. However, molecular studies comparing genes in different reptile groups indicate that turtles, along with crocodiles, are the most modern of reptiles.
Types of Turtles
Turtles are classified in 12 families. The Northern Hemisphere's largest family is that of common freshwater turtles (Emydidae), which includes about a third of all turtle species and is abundant in S and E Asia, E North America, and Central America. Members of this group have webbed feet; many spend most of the time in freshwater ponds or marshes; some live in brackish estuaries. They include such well-known North American turtles as the pond turtles (including the spotted, wood, and Muhlenberg's turtles), the painted turtle, the sliders, the diamondback terrapin, and the Blanding's turtle. The box turtle , which is primarily terrestrial, belongs to this family. Land tortoises (Testudinidae) form the second largest family. Tortoises have high-domed shells, move on club-shaped feet, are vegetarian, and live in warm regions throughout the world. The musk turtles and mud turtles (family Kinosternidae) are common small turtles of the E United States, and are found only in the Americas. The soft-shelled turtles (family Trionychidae) are flat-bodied, carnivorous freshwater turtles of the Northern Hemisphere, with a leathery covering instead of horny shields on their shells. The snapping turtle family (Chelydridae) is a North American group that includes the common snapper and the alligator snapper.
Marine turtles are classified in two families. The family Chelonidae includes five sea turtle species of tropical and subtropical distribution: the green turtle, the loggerhead, the hawksbill (or tortoiseshell turtle), the Kemp's ridley, and the olive ridley. The family Dermochelidae includes only one species, the leatherback , or leatherneck, largest and heaviest of all turtles, weighing as much as 1100 lbs (500 kg). Marine turtles lack toes, and their legs are oarlike, allowing speeds of nearly 20 mph (32 kph) in the water. With the exception of the loggerhead, all are endangered, either by pollution with plastic debris, which some turtles eat by mistake, or by commercial fishing, especially shrimp trawling. Commercial trade in all endangered sea turtles is banned; however, many wild turtles are skinned for leather and tortoiseshell ornaments, or taken for food.
Classification
Turtles are classified in the phylum Chordata , subphylum Vertebrata, class Reptilia, order Chelonia.
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