Turtle Wallpapers

Turtle Animals - photo wallpapers; Megawallpapers.org: 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. The Order 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 are highly endangered. The first turtles are believed to have existed in the Mesozoic, [br]about 200 million years ago. Their exact ancestry is disputed. It was [br]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. [br]All molecular studies have strongly upheld this new phylogeny, though [br]some place turtles closer to Archosauria. Re-analysis of prior [br]phylogenies suggests that they classified turtles as anapsids both [br]because they assumed this classification (most of them studying what [br]sort of anapsid turtles are) and because they did not sample fossil and [br]extant taxa broadly enough for constructing the cladogram. Future [br]analyses may show the turtles to be relatives of the placodonts. A new phylogenetic analysis agrees with prior analyses nesting [br]turtles with pareiasaurs within the much larger clade, [br]Lepidosauromorpha. The closest pareiasaur to turtles appears to be a [br]rarely-studied form, Stephanospondylus. Indeed turtles are related to [br]other reptiles without temporal openings. They are also closer to [br]lizards than they are to archosaurormophs, including placodonts. The earliest known modern turtle is proganochelys, [br]though this species already had many advanced turtle traits, and thus [br]probably had many millions of years of preceding "turtle" evolution and [br]species in its ancestry. It did lack the ability to pull its head into [br]its shell (and it had a long neck), and had a long, spiked tail ending [br]in a club, implying an ancestry occupying a similar niche to the ankylosaurs (though, presumably, only parallel evolution).

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Turtle Wallpapers @ Mega Wallpapers (08 Edition)
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