International Wildlife Encyclopedia - Vol. 20

International Wildlife Encyclopedia - Vol. 20

International Wildlife Encyclopedia - Vol. 20

International Wildlife Encyclopedia - Vol. 20

Excerpt

TURTLES ARE REPTILES THAT have a shell protecting most of their body. The shell is made up of two parts: the upper part is called the carapace and the lower part is called the plastron. The carapace and the plastron are joined along the sides of the body, but at the front end there is an opening for the head and legs to emerge, and at the back end an opening for the back legs and tail. Most turtles are able to withdraw their head, legs and tail into the shell so that it is almost impossible for a predator to kill and eat them unless it is large enough to swallow them whole. Mud turtles, genus Kinosternon, and musk turtles, genus Sternotherus, are notable in that their shells have hinged lobes at the front and back. When a mud or musk turtle withdraws into its shell, the lobes can be pulled shut, enabling the turtle to conceal itself completely.

The smallest turtle species is the speckled tortoise, Homopus signatus, of South Africa and Namibia, the largest specimens of which are about 3¾ inches (9.5 cm) in length and weigh just 5 ounces (140 g). At the other extreme, leatherback or leathery turtles, Dermochelys coriacea, can grow to 9 feet (2.7 m) in length and weigh more than % ton (860 kg).

Classification

Their protective shells are one of the factors that have made turtles such successful animals in evolutionary terms. They are the oldest form of reptile alive today and have survived with very little change for more than 200 million years. Scientists have dated the oldest turtle fossils back to the Triassic period, about 230 million years ago.

There are two major groups of turtles, classified according to the way in which the neck is bent when the head is withdrawn into the shell. In the main group (the hiddennecked turtles, suborder Cryptodira), the neck is bent on a vertical plane, in a similar manner to the neck of a cobra. In the side-necked turtles (suborder Pleurodira) it is folded sideways beneath the front edge of the upper shell, that is, on a horizontal plane.

Side-necked turtles usually have longer necks than hidden-necked turtles; in some species, the neck is so long that they are called snake-necked turtles (family Chelidae). The two suborders of turtles also differ from one another in various key aspects of their skull and skeleton structure.

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