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—see also sea turtle, terrapin, tortoise, and the discussion below.

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, snakes and crocodiles. Of the many species alive today some are highly endangered.

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to long.

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than in length and weighs about . 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 in length. in the zoo of Sharm el-Sheikh.]] at a zoo in the Czech Republic.]]

Neck folding

Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their necks in while contracting it under their spine; and the Pleurodira, which contract their necks to the side.

Head

Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators in shallow water where they lie entirely submerged except for their eyes and nostrils. Sea turtles possess glands near their eyes that produce salty tears that rid their body of excess salt taken in from the water they drink.

Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near ultraviolet (UV A) to red. Some land turtles have very poor pursuit movement abilities, which are normally reserved for predators that hunt quick moving prey, but carnivorous turtles are able to move their heads quickly to snap.

Turtles have a rigid beak. Turtles use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. Turtles use their tongues to swallow food, but they cannot, unlike most reptiles, stick out their tongues to catch food.

Shell

The upper shell of the turtle is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined together on the turtle's sides by bony structures called bridges. The inner layer of a turtle's shell is made up of about 60 bones that include portions of the backbone and the ribs, meaning the turtle cannot crawl out of its shell. In most turtles, the outer layer of the shell is covered by horny scales called scutes that are part of its outer skin, or epidermis. Scutes are made up of a fibrous protein called keratin that also makes up the scales of other reptiles. These scutes overlap the seams between the shell bones and add strength to the shell. Some turtles do not have horny scutes. For example, the Leatherback turtle and the soft-shelled turtles have shells covered with leathery skin instead.

The rigid shell means turtles cannot breathe as other reptiles do, by changing the volume of their chest cavity via expansion and contraction of the ribs. Instead, turtles breathe in two ways. First, they employ buccal pumping, pulling air into their mouth then pushing it into the lungs via oscillations of the floor of the throat. Secondly, by contracting the abdominal muscles that cover the posterior opening of the shell, the internal volume of the shell increases, drawing air into the lungs, allowing these muscles to function in much the same way as the mammalian diaphragm.

The shape of the shell gives helpful clues to how the turtle lives. Most tortoises have a large dome-shaped shell that makes it difficult for predators to crush the shell between their jaws. One of the few exceptions is the African pancake tortoise, which has a flat, flexible shell that allows it to hide in rock crevices. Most aquatic turtles have flat, streamlined shells which aid in swimming and diving. American snapping turtles and musk turtles have small, cross-shaped plastrons that give them more efficient leg movement for walking along the bottom of ponds and streams.

The color of a turtle's shell may vary. Shells are commonly colored brown, black, or olive green. In some species, shells may have red, orange, yellow, or grey markings and these markings are often spots, lines, or irregular blotches. One of the most colorful turtles is the eastern painted turtle which includes a yellow plastron and a black or olive shell with red markings around the rim.

Tortoises, being land-based, have rather heavy shells. In contrast, aquatic and soft-shelled turtles have lighter shells that help them avoid sinking in water and swim faster with more agility. These lighter shells have large spaces called fontanelles between the shell bones. The shells of leatherback turtles are extremely light because they lack scutes and contain many fontanelles.

Skin and molting

As mentioned above, the outer layer of the shell is part of the skin; each scute (or plate) on the shell corresponds to a single modified scale. The remainder of the skin is composed of skin with much smaller scales, similar to the skin of other reptiles. Turtles do not molt their skins all at once, as snakes do, but continuously, in small pieces. When kept in aquaria, small sheets of dead skin can be seen in the water (often appearing to be a thin piece of plastic) having been sloughed off when the animal deliberately rubs itself against a piece of wood or stone. Tortoises also shed skin, but a lot of dead skin is allowed to accumulate into thick knobs and plates that provide protection to parts of the body outside the shell.

By counting the rings formed by the stack of smaller, older scutes on top of the larger, newer ones, it is possible to estimate the age of a turtle, if one knows how many scutes are produced in a year. This method is not very accurate, partly because growth rate is not constant, but also because some of the scutes eventually fall away from the shell.

Limbs

Terrestrial tortoises have short, sturdy feet. Tortoises are famous for moving slowly, in part because of their heavy, cumbersome shell, which restricts stride length.

The amphibious turtles normally have limbs similar to those of tortoises, except the feet are webbed and often have long claws. These turtles swim using all four feet in a way similar to the dog paddle, 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 have very long claws, used to help them clamber onto riverbanks and floating logs, upon which they like to bask. 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, some, such as the pig-nosed turtle, 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 have flippers instead of feet. 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 rudders 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.

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 , 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 from 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. 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.

Systematics and evolution

) from Ernst Haeckel's Kunstformen der Natur, 1904.]] The first proto-turtles are believed to have existed in the late 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 or "half-shelled turtle with teeth", from the late Triassic, which have been found near Guangling in south-west China. Odontochelys displays a complete bony plastron and an incomplete carapace, similar to an early stage of turtle embryonic development. Prior to this discovery, the earliest-known 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 has been disputed. It was believed that they are the only surviving branch of the ancient evolutionary grade 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 later 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; some place turtles within within Archosauria, or, more commonly, as a sister group to extant archosaurs. 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. It has been suggested that Testudines diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled.

The earliest known fully-shelled turtle is the late-Triassic Proganochelys. The genus 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, a body form similar to that of ankylosaurs resulting from convergent evolution.

Turtles are divided into two extant suborders, 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.

Turtle genera with basal or uncertain phylogenetic position

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

, one of the oldest true turtles presently known. Unlike modern Testudines, Proganochelys was not able to hide its head under the shell.]]

Suborder Cryptodira

(Pelomedusa subrufa) is a pleurodire.
Pleurodires hide their head sideways.]] Basal genera

Genus †Kayentachelys

Genus †Indochelys

Infraorder †Paracryptodira

Basal and incertae sedis

* Family †Kallokibotiidae

* Family †Mongolochelyidae

* Family †Pleurosternidae

* Family †Solemydidae

Superfamily †Baenoidea

* Family †Baenidae

* Family †Macrobaenidae

* Family †Neurankylidae

Infraorder Eucryptodira

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 (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 (true tortoises)

Superfamily Trionychoidea

* Family †Adocidae

* Family Carettochelyidae (pignose turtles)

* Family Dermatemydidae (river turtles)

* Family Kinosternidae (mud turtles)

* 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)

Turtle, tortoise, or terrapin

Although the word turtle is widely used to describe all members of the order Testudines, it is also common to see certain members described as terrapins, tortoises or sea turtles as well. Precisely how these alternative names are used, if at all, depends on the type of English being used.

British English normally describes these reptiles as turtles if they live in the sea; terrapins if they live in fresh or brackish water; or tortoises if they live on land. However, there are exceptions to this where American or Australian common names are in wide use, as with the Fly River turtle.

American English tends to use the word turtle as a general term for all species. "Tortoise" is used for most land-dwelling species, including the family Testudinidae and box tortoises. Oceanic species are usually referred to as sea turtles. The name "terrapin" is typically reserved only for the brackish water diamondback terrapin, Malaclemys terrapin; the word terrapin being derived from the Algonquian word for this animal.

Australian English uses turtle for both the marine and freshwater species, but tortoise for the terrestrial species.

To avoid confusion, the word chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia, which includes all turtles, tortoises and terrapins living and extinct, as well as their immediate ancestors. It is based on the Ancient Greek word χελώνη, chelōnē; Modern Greek χελώνα, chelōna; meaning turtle/tortoise.

Distribution

Seven species of marine turtles are found worldwide. Of these, five have been recorded in Europe.

As pets

Turtles, particularly small terrestrial and freshwater turtles, are commonly kept as pets. Among the most popular are Russian tortoises, spur-thighed tortoises, and red-eared sliders.

In the United States, due to the ease of contracting salmonellosis through casual contact with turtles, the U.S. Food and Drug Administration (FDA) established a regulation in 1975 to discontinue the sale of turtles under 4 inches. It is illegal in every state in the U.S. for anyone to sell any turtles under 4 inches long. Many stores and flea markets still sell small turtles due to a loophole in the FDA regulation which allows turtles under 4 inches to be sold for educational purposes.

Some states have other laws and regulations regarding possession of red-eared sliders as pets because they are looked upon as invasive species or pests where they are not native, but have been introduced through the pet trade. As of July 1, 2007, it is illegal in Florida to sell any wild type red-eared slider. Unusual color varieties such as albino and pastel red-eared sliders, which are derived from captive breeding, are still allowed for sale.

As food, traditional medicine, and cosmetics

, decorated with a 龜 ("turtle") character.]] The flesh of turtles was, and still is, considered a delicacy in a number of cultures. Turtle soup has been a prized dish in Anglo-American cuisine, and still remains so in some parts of Asia. Gopher tortoise stew was popular with some groups in Florida.

Turtles remain a part of the traditional diet on the island of Grand Cayman, so much so that when wild stocks became depleted, a turtle farm was established specifically to raise sea turtles for their meat. The farm also releases specimens to the wild as part of an effort to repopulate the Caribbean Sea.

Fat from turtles is also used in the Caribbean and in Mexico as a main ingredient in cosmetics, marketed under its Spanish name Crema de Tortuga.

s among other plants and animals parts are used in traditional Chinese medicines. (Other items in the image are dried lingzhi, snake, luo han guo, and ginseng).]] Turtle plastrons (the part of the shell that covers a tortoise from the bottom) are widely used in traditional Chinese medicine; according to statistics, Taiwan imports hundreds of tons of plastrons every year. A popular medicinal preparation based on powdered turtle plastron (and a variety of herbs) is the guilinggao jelly; these days, though, it is typically made with only herbal ingredients.

Conservation status

Efforts have been made by Chinese entrepreneurs to satisfy increasing demand for turtle meat as gourmet food and traditional medicine with farmed turtles, instead of wild-caught ones; according to a study published in 2007, over a thousand turtle farms operated in China. Turtle farms in Oklahoma and Louisiana raise turtles for export to China as well. In the words of the biologist George Amato, "the amount and the volume of captured turtles... vacuumed up entire species from areas in Southeast Asia", even as biologists still didn't know how many distinct turtle species live in the region. It has been estimated that about 75% of Asia's 90 tortoise and freshwater turtle species have become threatened.

Nonetheless, the great majority of turtles exported from the USA are farm raised. According to one estimate by the World Chelonian Trust, about 97% of 31.8 million animals harvested in the U.S. over a three-year period (November 4, 2002 - November 26, 2005) were exported. It has been estimated (presumably, over the same 2002-2005 period) that about 47% of the US turtle exports go to People's Republic of China (predominantly to Hong Kong), another 20% to Taiwan, and 11% to Mexico.

In culture

See also

Adwaita — a giant turtle that was reportedly 250 years old when it died in 2006

Araripemys arturi

Turtle racing

Further reading

References

External links

UC Berkeley Museum of Paleontology

Chelonian studbook Collection and display of the weights/sizes of captive turtles

Biogeography and Phylogeny of the Chelonia (taxonomy, maps)

The word 'turtle' in different languages

New Scientist article (including video) on how the turtle evolved its shell

Category:Testudinidae