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	===Limbs===
	Terrestrial tortoises have short, sturdy feet. Tortoises are famous for moving slowly, in part because of their heavy, cumbersome shell but also because of the relatively inefficient ] that they have, with the legs being bent, as with ] rather than being straight and directly under the body, as is the case with ].

	The amphibious turtles normally have limbs similar to those of tortoises except that the feet are webbed and often have long ]. These turtles swim using all four feet in a way similar to the ], with the feet on the left and right side of the body alternately providing thrust. Large turtles tend to swim less than smaller ones, and the very big species, such as alligator snapping turtles, hardly swim at all, preferring to simply walk along the bottom of the river or lake. As well as webbed feet, turtles also have very long claws, used to help them clamber onto riverbanks and floating logs, upon which they like to ]. Male turtles tend to have particularly long claws, and these appear to be used to stimulate the female while mating. While most turtles have webbed feet, a few turtles, such as the ], have true flippers, with the digits being fused into paddles and the claws being relatively small. These species swim in the same way as sea turtles (see below).

	Sea turtles are almost entirely aquatic and instead of feet they have flippers. Sea turtles "fly" through the water, using the up-and-down motion of the front flippers to generate thrust; the back feet are not used for propulsion but may be used as ]s for steering. Compared with freshwater turtles, sea turtles have very limited mobility on land, and apart from the dash from the nest to the sea as hatchlings, male sea turtles normally never leave the sea. Females must come back onto land to lay eggs. They move very slowly and laboriously, dragging themselves forwards with their flippers. The back flippers are used to dig the burrow and then fill it back with sand once the eggs have been deposited.

	==Ecology and life history==		==Ecology and life history==

Revision as of 08:59, 3 March 2008

For other uses, see Turtle (disambiguation).

Turtle Temporal range: Triassic - Recent

"Chelonia" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Order:	Testudines Linnaeus, 1758 Cryptodira Pleurodira See text for families.

Turtles are reptilians of the Order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilaginous shell developed from their ribs. 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. About 300 species are alive today, and some are highly endangered.

Turtles cannot breathe in water however the hold their breath for various periods of time.

Like other reptiles, turtles are poikilothermic (or "of varying temperature"). Like other amniotes (reptiles, dinosaurs, birds, and mammals), they breathe air and don't lay eggs underwater, although many species live in or around water. The largest turtles are aquatic.

(670 lb).

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long.

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3 in) in length and weighs about 140 g (5 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 cm (5 in) in length.

Ecology and life history

Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air, and must surface at regular intervals to refill their lungs. They can also spend much of their lives on dry land. Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called "papillae", have a rich blood supply, and increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.

Turtles lay eggs, like other reptiles, which are slightly soft and leathery. The eggs of the largest species are spherical, while the eggs of the rest are elongated. Their albumen is white and contains a different protein than bird eggs, such that it will not coagulate when cooked. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male. Large numbers of eggs are deposited in holes dug into mud or sand. They are then covered and left to incubate by themselves. When the turtles hatch, they squirm their way to the surface and head toward the water. There are no known species in which the mother cares for the young.

Sea turtles lay their eggs on dry, sandy beaches. Immature sea turtles are not cared for by the adults. Most are endangered largely as a result of beach development and over harvesting. Turtles can take many years to reach breeding age, and in many cases breed every few years rather than annually.

Researchers have recently discovered a turtle’s organs do not gradually break down or become less efficient over time, unlike most other animals. It was found that the liver, lungs and kidneys of a centenarian turtle are virtually indistinguishable from those of its immature counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes.

Taxonomy

Main article: Testudines

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.

Evolutionary history

The first turtles are believed to have existed in the early Triassic Period of the Mesozoic era, about 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). 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 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. As of 2003, the consensus is that Testudines diverged from other diapsids between 200 and 279 million years ago.

The earliest known turtle is 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, presumably, only parallel evolution).

Turtle, tortoise or terrapin?

Differences between turtles, tortoises and terrapins

Turtles as pets

Main article: Pet turtles

Turtles, particularly small terrestrial and freshwater turtles, are commonly kept as pets. Among the most popular are Russian Tortoises, Greek spur-thighed tortoises and red-ear sliders (or terrapin).

References

http://www.enchantedlearning.com/subjects/dinosaurs/dinos/Archelon.shtml
Reptile blood isn't necessarily cold, as reptiles sun themselves and take other measures to stay warm.
http://www.tortoise.org/general/wildfaqs.html#largest
http://www.oceansofkansas.com/Turtles.html
All but Ageless, Turtles Face Their Biggest Threat: Humans
http://www.ucmp.berkeley.edu/anapsids/procolophonoidea.html
Rieppel, O., and DeBraga, M. (1996). "Turtles as diapsid reptiles." Nature, 384: 453-455.
Zardoya, R., and Meyer, A. (1998). "Complete mitochondrial genome suggests diapsid affinities of turtles." Proceedings of the National Academy of Sciences of the United States of America, 95(24): 14226-14231.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=538573
David Alderton (1986). An Interpret Guide to Reptiles & Amphibians, Salamander Books Ltd., London & New York.

External links

UC Berkeley Museum of Paleontology
Turtles of the World: Extensive information on all known turtles, tortoises and terrapins, including key and quiz.
Chelonian studbook Collection and display of the weights/sizes of captive turtles
John M. Legler & Arthur Georges, Biogeography and Phylogeny of the Chelonia (taxonomy, maps)

Category:

Turtles

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