Turtle

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

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 species of turtles are highly endangered. Like birds, turtles are able to detect the Earth's magnetic field with magnetosensors, which allow them to migrate.

Evolution of the Turtle

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.

A new phylogenetic analysis agrees with prior analyses nesting turtles with pareiasaurs within the much larger clade, Lepidosauromorpha. The closest pareiasaur to turtles appears to be a rarely-studied form, Stephanospondylus. Indeed turtles are related to other reptiles without temporal openings. They are also closer to lizards than they are to archosaurormophs, including placodonts.

The earliest known modern 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).

Physical description of the Turtle

Turtles vary widely in size, although marine turtles tend to be relatively big animals. The largest chelonian is a marine turtle, the great leatherback sea turtle, which can reach a shell length of 200 cm (80 inches) and can reach a weight of over 900 kg (2,000 lb, or 1 short ton). Freshwater turtles are smaller, with the largest species being the Asian softshell turtle Pelochelys cantorii, which has been reported to measure up to 200 cm or 80 in (Das, 1991). This dwarfs even the better-known alligator snapping turtle, the largest chelonian in North America, which attains a shell length of up to 80 cm (31½ in) and a weight of about 76 kg (170 lb). Giant tortoises of the genera Geochelone, Meiolania, and others were relatively widely distributed around the world into prehistoric times, and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of Man, and it is assumed that humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 cm (50 in) in length, and weigh about 300 kg (670 lb) [1].

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long [2].

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.

Some sea turtles have evolved gills near the anus to assist with respiration. These are distinct from the pharyngeal gills (which fish have, and which are lost during embryonic development in turtles).

Neck folding of the Turtle

Turtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into their shell (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their neck in while folding it under their spine; and the Pleurodira, which have to fold their neck to the side.

Closeup head-on view of a common snapping turtle (Chelydra serpensis), taken near the St. Lawrence River in northern New York State

Head of the Turtle

Most turtles and tortoises have eyes placed on the upper sides of their heads. Species of 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 rids 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 amount of rod cells in their retinas. Normal daytime vision is marginal at best due to their color-blindness and poor visual acuity. In addition to daytime vision problems, turtles have very poor pursuit movement abilities, which is most likely due to the fact that pursuit movement abilities are normally reserved for predators that hunt quick moving prey.

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. Their ancient ancestors have teeth unlike modern-day turtles. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivourous turtles have serrated edged ridges that help them cut through tough plants. Turtles use their tongues to swallow food, but they can't, unlike most reptiles, stick out their tongues to catch food.

Shell of the Turtle

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 includes 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 sea turtle and the soft-shelled turtles have shells covered with leathery skin instead.

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.

Tortoises have rather heavy shells in contrast to aquatic and soft-shelled turtles which have lighter shells that help them avoid sinking in water and swim faster and more agilely. These lighter shells have large spaces called fontanelles between the shell bones. The shell of a leatherback turtle is extremely light because they lack scutes and contain many fontanelles.

The color of a turtle's shell may vary. Shells are commonly coloured 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.

Skin and Moulting of the Turtle

As mentioned above, the outer layer of the shell is part of the skin, each scute (or plate) on the shell corresponding 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 and terrapins do not moult their skins all in one go, 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) when it has been sloughed off, often 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.

The scutes on the shell are never moulted, and, as they accumulate over time, the shell becomes thicker. 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 you know how many scutes are produced in a year [3]. 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 of the Turtle

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

The amphibious turtles normally have limbs similar to those of tortoises except that 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 aligator 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 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, a few turtles, such as the pig-nose turtles, 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 an 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. The back flippers are used to dig the burrow and then fill it back with sand once the eggs have been deposited.

Order Testudines - turtles, tortoises, and terrapins

Gulf Coast Box Turtle, Terrapene carolina major

Leatherback Sea Turtle, Dermochelys coriacea

Green Sea Turtle, Chelonia mydas

Hawksbill turtle, Eretmochelys imbricata

Suborder Paracryptodira (extinct)
 

Suborder Cryptodira

• Familia Chelydridae (Snapping Turtles)
  Family Meiolaniidae (Horned turtle, extinct)

• Superfamily Testudinoidae

• Family Haichemydidae (extinct)
  Family Sinochelyidae (extinct)
  Family Lindholmemydidae (extinct)
  Family Testudinidae (Tortoises)
  Family Geoemydidae (Asian River Turtles, Leaf and Roofed Turtles, Asian Box Turtles)
  Family Emydidae (Pond Turtles/Box and Water Turtles)

• Superfamily Trionychoidae

• Family Adocidae (extinct)
  Family Carettochelyidae (Pignose Turtles)
  Family Trionychidae (SoftshellTurtles)

• Superfamily Kinosternoidae

• Family Dermatemydidae (River Turtles)
  Family Kinosternidae (Mud and Musk Turtles)
  Family Platysternidae (Big-headed Turtles)

• Superfamily Chelonioidae (Sea Turtles)

• Family Toxochelyidae (extinct)
  Family Cheloniidae (Green Sea Turtles and relatives)
  Family Thalassemyidae (extinct)
  Family Dermochelyidae (Leatherback Turtles)
  Family Protostegidae (extinct)

Suborder Pleurodira

• Family Proterochersidae (extinct)
  Family Chelidae (Austro-American Sideneck Turtles)
  Family Araripemydidae (extinct)

• Superfamily Pelomedusoidae

• Family Pelomedusidae (Afro-American Sideneck Turtles)
  Family Bothremydidae (extinct)
  Family Podocnemididae (Madagascan Big-headed and American Sideneck River Turtles)

See also

• Turtles and tortoises in popular culture

Further reading

• Iskandar, DT (2000). Turtles and Crocodiles of Insular Southeast Asia and New Guinea. ITB, Bandung.

External links

• Florida Sea Turtle information Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute
• UC Berkeley Museum of Paleontology
• Turtle Trax: Excellent marine turtle site
• California Turtle and Tortoise Club: Informative and entertaining in equal measure
• Gulf Coast Turtle and Tortoise Society: Care Information, Species Descriptions, and More.
• Turtles of the World: Extensive information on all known turtles, tortoises and terrapins, including key and quiz.
• Chelonia: Conservation and Care of Turtles.
• Chelidae.com All of South American Side-necked Turtles
• Chelonian studbook Collection and display of the weights/sizes of captive turtles