Turtles

Turtles are reptiles of the order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. more...

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The order of Testudines includes both extant (living) and extinct species, the earliest turtles being known from the early Triassic Period, making turtles one of the oldest reptile groups, and a much more ancient group than the lizards and snakes. About 300 species are alive today. Some species of turtles are highly endangered.

Turtle, tortoise, or terrapin?

In British English it is normal to describe these reptiles as turtles, terrapins, or tortoises depending on whether they live in the sea, in fresh water, or on land, respectively. Thus the green sea turtle, Chelonia mydas, is considered a turtle ; the red-eared slider, Trachemys scripta elegans, a terrapin ; and the eastern box turtle, Terrapene carolina carolina, a tortoise .

In American English it is common to refer to all freshwater chelonians as turtles. Ocean-going species are sea turtles, and members of the family Testudinidae, such as the gopher tortoise (Gopherus polyphemus), are considered tortoises. \"Terrapin\" is reserved for the diamondback terrapin (Malaclemys terrapin), a North American species whose name is derived from the Algonquian word for this animal, though the average speaker would simply refer to it as a turtle.

There are no fully-terrestrial forms in Australia; therefore, speakers of Australian English tend to use turtle for marine species and tortoise for freshwater species. This confusing nomenclature is changing as recent publications have used turtle for both marine and freshwater species .

The word chelonian, an umbrella term covering turtles, tortoises, and terrapins, is increasingly popular among scientists, conservationists, hobbyists and veterinarians working with these animals. It is based on the Greek word χελώνα ( /çeˈlona/, chelone ), meaning tortoise, and is used, for example, by the Chelonian Research Foundation.

Evolution

The first turtles are believed to have existed in the Mesozoic, around 200 million years ago. 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 procolophonoids, 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). Most anapsids became extinct in the late Permian period, except procolophonoids and possibly the precursors of the testudines (turtles).

However, it was recently suggested that the anapsid-like turtle skull may be due to reversion rather than to anapsid descent. More recent phylogenetic studies with this in mind placed turtles firmly within diapsids, slightly closer to Squamata than to Archosauria. All molecular studies have strongly upheld this new phylogeny, though some place turtles closer to archosauria. Re-analysis 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. Future analyses may show the turtles to be relatives of the placodonts.

Read more at Wikipedia.org