Turtle

Turtles are reptiles of the order Testudines (the crown group of the superorder Chelonia), characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic&mdash;see also sea turtle, terrapin, tortoise.

The order Testudines includes both extant (living) and extinct species. The earliest known turtles date from 215 million years ago, making turtles one of the oldest reptile groups and a more ancient group than lizards and snakes.

Systematics and evolution
The first proto-turtles are believed to have existed in the early Triassic Period of the Mesozoic era, about 220 million years ago, and their shell, which has remained a remarkably stable body plan, is thought to have evolved from bony extensions of their backbones and broad ribs that expanded and grew together to form a complete shell that offered protection at every stage of its evolution, even when the bony component of the shell was not complete. This is supported by fossils of the freshwater Odontochelys semitestacea, the "half-shelled turtle with teeth", have been found near Guangling in south-west China, which displays a complete bony plastron and an incomplete carapace, similar to an early stage of turtle embryonic development. Prior to this discovery, the earliest fossil turtles were terrestrial and had a complete shell, offering no clue to the evolution of this remarkable anatomical feature. By the late Jurassic, turtles had radiated widely, and their fossil history becomes easier to read.

Their exact ancestry is disputed. It was believed that they are the only surviving branch of the ancient clade Anapsida, which includes groups such as procolophonids, millerettids, protorothyrids, and pareiasaurs. All anapsid skulls lack a temporal opening, while all other extant amniotes have temporal openings (although in mammals the hole has become the zygomatic arch). The millerettids, protorothyrids, and pareiasaurs became extinct in the late Permian period, and the procolophonoids during the Triassic.

However, it was recently suggested that the anapsid-like turtle skull may be due to reversion rather than to anapsid descent. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids, slightly closer to Squamata than to Archosauria. All molecular studies have strongly upheld the placement of turtles within diapsids, though some place turtles closer to Archosauria than Squamata. Reanalysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa broadly enough for constructing the cladogram. As of 2003, the consensus is that Testudines diverged from other diapsids between 200 and 279 million years ago.

The earliest known fully-shelled turtle is the late-Triassic Proganochelys, though this species already had many advanced turtle traits, and thus probably had many millions of years of preceding turtle evolution and species in its ancestry. It did lack the ability to pull its head into its shell (and it had a long neck), and had a long, spiked tail ending in a club, implying an ancestry occupying a similar niche to the ankylosaurs (though only through parallel evolution).

Turtles are divided into three suborders, one of which, the Paracryptodira, is extinct. The two extant suborders are the Cryptodira and the Pleurodira. The Cryptodira is the larger of the two groups and includes all the marine turtles, the terrestrial tortoises, and many of the freshwater turtles. The Pleurodira are sometimes known as the side-necked turtles, a reference to the way they withdraw their heads into their shells. This smaller group consists primarily of various freshwater turtles.

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|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|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)