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Turtles are reptiles of the order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. The order of Testudines includes both extant (living) and extinct species, the earliest turtles being known from the early Triassic Period, making them one of the oldest reptile groups, and a much more ancient group than the lizards and snakes. About 300 species are alive today. Some species of turtles are highly endangered.

Turtles vary widely in size, although marine turtles tend to be relatively big animals. The largest chelonian is a marine turtle, the great leatherback sea turtle, which can reach a shell length of 200 cm (72 in) and can reach a weight of over 750 kg (2,000 lb). Freshwater turtles are smaller, with the largest species being the Asian softshell turtle Pelochelys bibroni, which has been reported to measure up to 130 cm (51 inches) and weight about 180 kg (400 lb). This dwarfs even the better-known alligator snapping turtle, the largest chelonian in North America, which attains a shell length of up to 80 cm (31.5 in) and a weight of about 76 kg (170 lb). Giant tortoises of the genera Geochelone, Meiolania, and others were relatively widely distributed around the world into prehistoric times, and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of Man, and it is assumed that humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 cm (50 in) in length, and weigh about 300 kg (670 lb) [2].

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long [3].

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3 in) in length and weighs about 140 g (5 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 cm (5 in) in length.

The upper shell of the turtle is called the carapace. The lower shell that incases the belly is called the plastron. The carapace and plastron are joined together on the turtle's sides by bony structures called bridges. The inner layer of a turtle's shell is made up of about 60 bones that includes portions of the backbone and the ribs, meaning the turtle can not crawl out of its shell. In most turtles, the outer layer of the shell is covered by horny scales called scutes that are part of its outer skin, or epidermis. Scutes are made up of a fiberous protein called keratin that also makes up the scales of other reptiles. These scutes overlap the seams between the shell bones and add strength to the shell. Some turtles do not have horny scutes. For example, the leatherback sea turtle and the soft-shelled turtles have shells covered with leathery skin instead.

The shape of the shell gives helpful clues to how the turtle lives. Most tortoises have a large domed-shaped shell that makes it difficult for predators to crush them between their jaws. One of the few exceptions is the African pancake tortoise which has a flat, flexible shell that allows it to hide in rock crevices. Most aquatic turtles have flat, streamlined shells that aid with swimming and diving. American snapping turtles and musk turtles have small, cross-shaped plastrons that give the turtle more efficient leg movement for walking along the bottom of ponds and streams.

Tortoises have rather heavy shells in constrast to aquatic and soft-shelled turtles that have lighter shells that help them avoid sinking in the water and swim faster and more agile. These light shells have large spaces called fontanelles between the shell bones. The shell of a leatherback turtle is extremely light because they lack scutes and contain many fontanelles. The color of a turtle's shell may vary. Shells are commonly coloured brown, black, or olive green. In some species, shells may have red, orange, yellow, or grey markings and these markings are often spots, lines, or irregular blotches. One of the most colorful turtles is the eastern painted turtle which includes a yellow plastron and a black or olive shell with red markings around the rim.

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