The order to which the turtles belong is usually called Testudines. Sometimes Chelonia is given as a junior synonym, but this might strictly speaking be 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 but is overall of very uncertain placement due to the lack of material. If Murrhardtia staeschei (known from a rather complete shell) and Palaeochersis talampayensis are close relatives of Australochelys, the Chelonia and Testudines are indeed likely to be effectively synonymous in content – though the former would remain a valid superorder, to which any turtle ancestors to be found in the future would be assigned.
The 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]
There are about 300 living species of Testudines, grouped into two living suborders with fourteen living and many more extinct families, and one entirely prehistoric suborder which is somewhat disputed and might just be a paraphyletic assemblage of basal Testudines.[3].
Obst, Fritz Jürgen (1998): [Testudines]. In:Cogger, H.G. & Zweifel, R.G. (eds.): Encyclopedia of Reptiles and Amphibians: 108-111. Academic Press, San Diego. ISBN 0-12-178560-2
Edit
