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Turtles, Tortoises, and Terrapins are ectothermic reptiles, most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. They belong to an anapsid lineage, as can be seen from their solid skullcap. About 300 species are alive today; some are highly endangered. The order to which the turtles belong is usually called Testudines. Sometimes Chelonia is given as a junior synonym, but this is strictly speaking incorrect. For most purposes, the differences are minor however: The Chelonia are simply the Testudines plus the "proto-turtle" Australochelys africanus, a turtlelike anapsid from the Jurassic which lacked some of the Testudines' apomorphies. The oldest fossils of the Testudines actually predate Australochelys. They are the basal turtle genus Proganochelys[1] which lived in Late Triassic Eurasia about 210 million years ago, making turtles one of the oldest reptile groups, and a much more ancient group than lizards and snakes.
[edit] CharacteristicsThe defining feature of the testudines is the presence of a bony shell comprising an upper carapace and a lower plastron, both composed of solid bony plates. In most families, this bony shell is further covered by a layer of thick horny scales, which form the visible part of the shell, but some have a softer, leathery, covering.[2] Living testudines lack teeth, having a horny beak, which is sharpened or serrated to cut flesh or plant matter. Testudines also have strong limbs to lift their heavy bodies. The shape of the limb is variable, however, with land dwelling species typically fusing the toes into a solid clump, freshwater species having distinct, but webbed, toes, and marine species having powerful flippers.[2] Testudines lay eggs in a nest chamber, but no species displays parental care. Instead, the young are left to fend for themselves from the moment they hatch. Incubation time varies greatly between species, but two to three months is typical.[2] [edit] SystematicsThere are about 300 living species of Testudines, grouped into two suborders and fourteen families[3]. [edit] Basal chelonians
Unlike modern Testudines, Proganochelys was not able to hide its head under the shell. [edit] Suborder CryptodiraBasal genera
Infraorder †Paracryptodira
Infraorder Eucryptodira
[edit] Suborder Pleurodira
[edit] Footnotes[edit] References Wikimedia Commons has media related to:
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