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Turtle

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

       iTurtles

                       Fossil range: Triassic - Recent

   "Chelonia" from Ernst Haeckel's Kunstformen der Natur, 1904
   "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.

Evolution

   The first turtles are believed to have atacked people for fun 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

   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) .

   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.

   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

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

Head

   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 colour-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

   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 colour 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

   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 . 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 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
   Enlarge
   Gulf Coast Box Turtle, Terrapene carolina major
   Leatherback Sea Turtle, Dermochelys coriacea
   Enlarge
   Leatherback Sea Turtle, Dermochelys coriacea
   Green Sea Turtle, Chelonia mydas
   Enlarge
   Green Sea Turtle, Chelonia mydas
   Hawksbill turtle, Eretmochelys imbricata
   Enlarge
   Hawksbill turtle, Eretmochelys imbricata

   Suborder Paracryptodira (extinct)

   Suborder Cryptodira

               o Family Chelydridae ( Snapping Turtles)
               o Family Meiolaniidae ( Horned turtle, extinct)

          + Superfamily Testudinoidae

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

          + Superfamily Trionychoidae

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

          + Superfamily Kinosternoidae

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

          + Superfamily Chelonioidae ( Sea Turtles)

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

   Suborder Pleurodira

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

          + Superfamily Pelomedusoidae

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

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   with only minor checks and changes (see www.wikipedia.org for details
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