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An order of the Reptilia, subclass Anapsida, including the turtles, terrapins, and tortoises. This order is also known as the Testudines. The group first appeared in the Triassic, and its representatives are among the commonest fossils from that time on. Members of the order are most frequently found in fresh-water streams, lakes, and ponds or in marshy areas. However, a number of strictly terrestrial species are known, and several are marine. Turtles occur on all the major continents and continental islands in tropic and temperature regions. The marine forms are basically tropical in distribution, but some individuals stray into temperate waters.
The living turtles are usually divided into two major groups, the suborders Pleurodira and Cryptodira, based upon the structures of the head and neck. The pleurodires have spines on the most posterior cervicals (neck vertebrae) and the head is retractile laterally. In the cryptodires the cervical spines are uniformly reduced and the head is folded directly back to within the shell. In several cryptodires, notably in the marine turtles, the neck is secondarily nonretractible because of a reduction in the shell.
The Chelonia differ from most other vertebrates in possessing a hard bony shell which encompasses and protects the body. The shell is made up of a dorsal portion, the carapace, and a ventral segment, the plastron, connected by soft ligamentous tissue or a bony bridge. The carapace is composed of the greatly expanded ribs and dorsal vertebrae overlain by a series of enlarged dermal ossifications and an outer covering of tough skin or horny scales. The plastron is similarly arranged with remnants of the interclavicle, clavicles, and gastralia fused with dermal ossifications and covered by skin or scales. Other peculiarities associated with the shell include the fusion of the ribs to the vertebrae and the reduction of trunk muscles, the presence of the pectoral girdle completely inside the ribs (found in no other animal), the highly modified short, thick humerus and femur, and the lungs attached dorsally to the shell. In addition, living turtles differ from the tuatara, snakes, lizards, and crocodilians in having an anapsid skull, no true teeth but horny beaks on the jaws, an immovable quadrate, and a single median penis in males.
The rigid bony shell of turtles imposes a basic body plan subject to relatively little variation. The principal obvious differences between them are in the shell shape, limbs, head, and neck. The general outline of the shell is variable, but in most forms the shell is moderately high-arched and covered with epidermal scales. The majority of turtles have limbs more or less adapted to aquatic or semiaquatic life with moderate to well-developed palmate webbed feet. However, in strictly terrestrial forms such as the tortoises, the limbs are elephantine with the weight of the body being borne on the flattened sole of the feet.
Associated with the tendency of most turtles toward a life in or near water is the auditory apparatus. Even though an eardrum is present, hearing is adapted to picking up sounds transmitted through the water or substratum. However, there is evidence that airborne sound can be heard. Vision is also important, and turtles have color vision. Most of the species are gregarious and diurnal, and territoriality is unknown in the order.
The courtship patterns of various turtles are distinctive. In general, aquatic forms mate in the water, but tortoises breed on land. The male mounts the female from above, and fertilization is internal. Sperm may be stored in the cloacal region of the female for extended periods before fertilization. Because of the presence of the large median penis in males, the sexes of many species can be distinguished by the longer and broader tail of the male. All turtles lay shelled eggs which are buried in sand or soil in areas where females congregate. The shell is calcareous in most forms, but the marine turtles have leathery shells. Eggs are rather numerous, as many as 200 being laid by a single individual in some species, and are highly valued as human food. Incubation takes 60–90 days, and the little turtle cuts its way out of the shell with a small horny egg caruncle at the end of the snout.
Turtles feed on all types of organisms. Aquatic species may eat algae, higher plants, mollusks, crustaceans, insects, or fishes; terrestrial forms are similarly catholic in tastes. Most species are omnivores, but some have very specialized diets. The edges and internal surfaces of the horny beaks of these reptiles are frequently denticulate and modified to form specialized mechanisms adapted to handle particular food items.