Testudines

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 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 oldest fossils of the Testudines actually predate Australochelys. They are the basal turtle genera Chinlechelys, Odontochelys and Proganochelys which lived in Late Triassic Eurasia about 220-210 million years ago. This makes turtles one of the oldest "reptile" groups, and a much more ancient group than lizards and snakes. Priscochelys, about 230 million years old, was initially believed to represent the most ancient turtle, but actually appears to be a cyamodontoid placodont.

Characteristics
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.

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.

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.

Systematics
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. .

Basal and incertae sedis chelonians

 * Genus †Australochelys (Chelonia incertae sedis)
 * Genus †Murrhardtia (Chelonia incertae sedis)
 * Genus †Palaeochersis (Chelonia incertae sedis)
 * Genus †Chinlechelys (Proganochelydia or basal Testudines)
 * Genus †Chelycarapookus (Testudines incertae sedis)
 * Genus †Chitracephalus (Testudines incertae sedis)
 * Genus †Neusticemys (Testudines incertae sedis)
 * Genus †Scutemys (Testudines incertae sedis)

Suborder †Proganochelydia

 * Genus †Odontochelys (tentatively placed here)
 * Genus †Proganochelys

Suborder Cryptodira
Basal genera Infraorder †Paracryptodira Infraorder Eucryptodira
 * Genus †Kayentachelys
 * Genus †Indochelys
 * Basal and incertae sedis
 * Family †Kallokibotiidae
 * Family †Mongolochelyidae
 * Family †Pleurosternidae
 * Family †Solemydidae
 * Superfamily †Baenoidea
 * Family †Baenidae
 * Family †Macrobaenidae
 * Family †Neurankylidae
 * Basal and incertae sedis
 * †"Sinemys" wuerhoensis
 * Genus †Chubutemys (Meiolaniidae?)
 * Genus †Hangaiemys (Macrobaenidae?)
 * Genus †Judithemys
 * Genus †Osteopygis
 * Genus †Planetochelys
 * Family Chelydridae (snapping turtles)
 * Family †Eurysternidae
 * Family †Macrobaenidae
 * Family †Meiolaniidae (horned turtles)
 * Family †Plesiochelyidae
 * Family †Sinemydidae
 * Family †Xinjiangchelyidae
 * Superfamily Chelonioidea (sea turtles)
 * Family †Protostegidae
 * Family Thalassemyidae
 * Family †Toxochelyidae[[Image:T.h. hermanni con speroni 5.JPG|thumb|right|242px|The [[Hermann's Tortoise|Western Hermann's Tortoise]] (Testudo hermanni hermanni) is a cryptodire. Cryptodires hide their head inwards.]]
 * Family Cheloniidae (green sea turtles and relatives)
 * Family Dermochelyidae (leatherback turtles)
 * Superfamily Testudinoidea
 * Family †Haichemydidae
 * Family †Lindholmemydidae
 * Family †Sinochelyidae
 * Family Platysternidae (big-headed turtle)
 * Family Emydidae (pond, box and water turtles)
 * Family Geoemydidae (Asian river turtles, Asian leaf turtles, Asian box turtles and roofed turtles)
 * Family Testudinidae (tortoises)
 * Superfamily Trionychoidea
 * Family †Adocidae
 * Family Carettochelyidae (pignose turtles)
 * Family Dermatemydidae (river turtles)
 * Family Kinosternidae (mud turtles)[[Image:Pelomedusa subrufa.JPG|thumb|right|242px|The [[African Helmeted Turtle]] (Pelomedusa subrufa) is a pleurodire. Pleurodires hide their head sideways.]]


 * Family Trionychidae (softshell turtles)

Suborder Pleurodira

 * Basal and incertae sedis
 * Family †Araripemydidae
 * Family †Proterochersidae
 * Family Chelidae (Austro-American sideneck turtles)
 * Superfamily Pelomedusoidea
 * Family †Bothremydidae
 * Family Pelomedusidae (African sideneck turtles)
 * Family Podocnemididae (Madagascan big-headed and American sideneck river turtles)