Turtles, tortoises, and terrapins are all turtles. The term "tortoise" is used for terrestrial turtles with high-domed shells and elephantine hindlimbs, whereas the term "terrapin" is used properly for some highly aquatic turtles (genus Malaclemys) of eastern North America, although it frequently is used in error for American box turtles in the genus Terrapene. Turtles are easily recognized and distinguished from all other vertebrates by their shells. Shells are composed of a dorsal carapace and a ventral plastron. These are usually rigidly connected on the sides by bridges. Shells are composed of bony plates that form within the skin. These are fused to underlying vertebrae and ribs. Most shells have a covering of horny plates made of keratin, a protein which, in other vertebrates, forms scales, hair, nails, claws, or horns. In some turtles, the plates of bone and keratin are reduced or absent, and the shell is covered by leathery skin. Many turtles have one or more hinges in their shells, usually in the plastron. These allow the shell to completely enclose the withdrawn head, limbs, and tail. The plastron of males in many species is indented to accommodate the female's shell during mating.
Shell shape largely determines the lifestyle of its owner. Terrestrial (land-dwelling) turtles such as box turtles (Testudo) and tortoises (Geochelone) have high-domed shells. These reduce surface area through which water is lost and also are difficult for predators to grasp and break. Most aquatic and all marine turtles have relatively flat, streamlined shells for ease in swimming. However, African pancake tortoises (Malacochersus) have flat shells that allow them to hide in rocky crevices, and some bottom-dwelling aquatic turtles, such as the mud turtles (Kinosternon) of the southeastern United States, have high-domed shells. Snapping turtles in the genera Chelydra and Macrochelys have rough shells on which algae grow. This camouflages these turtles as they wait to ambush prey. Limbs also provide clues to lifestyles. Aquatic turtles have webbed feet, and sea turtles have forelimbs modified into flippers that allow them to "fly" through water. In contrast, terrestrial turtles often have spadelike feet for digging and/or columnlike limbs to support them as they walk. Regardless of shape or function, the girdles to which the limbs attach are enclosed by the ribs and shell. Turtles are the only vertebrates with this skeletal arrangement. Other anatomical modifications include nostrils on top of the snout or at the very tip of a long proboscis; these allow aquatic turtles to breathe at the surface with minimal exposure. Modern turtles, like modern birds, lack teeth. Instead, they have horny beaks of keratin variously shaped to cut leaves, tear flesh, or crush the shells of snails or clams. Because the shell prevents expansion and contraction of the thorax when breathing, turtles compress or expand the lungs by altering the location of other internal organs to which the lungs are attached. Shells limit mobility to a great extent; consequently, turtles have long and flexible necks. These allow them to reach up to browse or down to graze, or to quickly extend their necks in order to ambush quicker prey. In addition, neck vertebrae are modified to allow the head to be withdrawn into the shell, either by pulling it straight back while the neck assumes an S-shape (cryptodiran turtles) or by laying it to the side under the overhanging lip of the carapace (pleurodiran or sideneck turtles).
Origins and Future of Turtles
Fossil turtles are known from the Jurassic. Most systematists (biologists who study evolutionary relationships) group turtles with some extinct relatives in a clade called the Parareptilia. Although turtles traditionally have been considered reptiles, many expertsnowplace them a separate vertebrate class. Regardless, the ancestors of turtles arose from the first amniotes before the ancestors of other reptiles. This and their many unique features justify placing turtles into their own class. Unlike many reptiles, turtles are perceived positively by most people. Nevertheless, many species are threatened or endangered. Habitat destruction and alteration are responsible in most cases. Aquatic habitats are drained or polluted and nesting sites, especially beaches, are developed, rendering them unusable by turtles. Many species are exploited as food, either as eggs or adults, and others are killed for their shells or body parts, which are thought by some cultures to have medicinal or aphrodisiac qualities. Exotic predators, such as rats and dogs, dig up nests and kill adults. Hundreds of thousands of wild-caught turtles die each year in the pet trade, much of it illegal. Many species become roadkills when they migrate to new habitats or breeding sites. Only a few species are formally protected in at least some parts of their ranges, and several, including the sea turtles,maybe nearing extinction in spite of efforts to conserve them.
Subphylum: Vertebrata (vertebral column and braincase)
Order: Chelonia (turtles), consisting of twelve families, with fifty-two genera and about 260 species
Geographical location: Worldwide
Habitat: Terrestrial, aquatic, and marine habitats, except at high latitudes
Gestational period: Varies according to temperature
Life span: Turtles live longer than other vertebrates, with some small species living fifty years and larger formsupto two hundred years
Special anatomy: The only vertebrates with shells; lack teeth and instead have a horny beak; neck vertebrae are modified to allow the head to be extended or retracted from the shell; lungs are compressed and expanded by altering the position of other, attached internal organs
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