Turtle is an order of reptiles known as Testudines, characterized by a shell developed mainly from their ribs. Modern turtle are divided into two major groups, the side-necked turtles and hidden neck turtles which differ in the way the head retracts. There are 360 living and recently extinct species of turtles, including tortoises and terrapins.
They are found on most continents, some islands and much of the ocean. Like other reptiles, birds, and mammals, they breathe air and do not lay eggs underwater, although many species live in or around water. Genetic evidence typically places them in close relation to crocodilians and birds.
Turtle shells are made mostly of bone; the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Its outer surface is covered in scales made of keratin, the material of hair and fingernails. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body.
Turtles are ectotherms or “cold-blooded”, meaning that their internal temperature varies with their direct environment. They are generally opportunistic omnivores and mainly feed on plants and animals that do not move. Many turtles migrate short distances seasonally.
Sea turtles are the only reptiles that migrate long distances to lay their eggs on a favored beach.
Turtles have appeared in myths and folktales around the world. Some terrestrial and freshwater species are widely kept as pets. Turtles have been hunted for their meat, for use in traditional medicine, and for their shells.
Sea turtles are often killed accidentally as bycatch in fishing nets. Turtle habitats around the world are being destroyed. As a result of these pressures, many species are threatened with extinction.
Naming and etymology of a Turtle
The word turtle is derived from the French tortue or tortre (‘turtle, tortoise‘). It is a common name and may be used without knowledge of taxonomic distinctions. In North America, it may denote the order as a whole.
In Britain, the name is used for sea turtles as opposed to freshwater terrapins and heavy-footed, land-dwelling tortoises. In Australia, which lacks true tortoises (family Testudinidae), non-marine turtles were traditionally called tortoises, but more recently turtle has been used for the entire group.
Anatomy and physiology of a Turtle
The largest living species of turtle (and fourth-largest reptile) is the leatherback turtle which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb). The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg (4,900 lb).
the carapace which usually contains 50–60 bones and covers the back of the animal while the plastron has only 7–11 bones and covers the belly. They are connected by lateral extensions of the plastron.
The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs). During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace.
Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.Development of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.
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The outer surface of the shell is covered in epidermal (outer skin) scales known as scutes which are made of keratin, the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total.
Carapace scutes are divided into “marginals” around the margin, “vertebrals” over the vertebral column, though the scute that overlays the neck is called the “cervical”. “Pleurals” are present between the marginals and vertebrals. Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals.
Side-necked turtles additionally have “intergular” scutes between the gulars. Turtle scutes usually interlock like mosaic tiles, but in some species, like the hawksbill sea turtle, the scutes on the carapace can overlap.
The shapes of turtle shells vary with the adaptations of the individual species, and sometimes with sex. Land-dwelling turtles tend to have more domed shells, which appear to make them more resistant to being crushed by large animals.
Aquatic turtles have flatter, smoother shells which allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce drag and increase stability in the open ocean. Some turtle species have ridged, lumped, or spiked shells which provide extra protection from predators and camouflage against the leafy floor.
The humps of a tortoise shell may tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the lead edge of the plastron is thickened and used for butting and ramming during combat.
Shells vary in flexibility. In tortoises, the plastron and its extensions lock the sides of the carapace together, giving it even greater crushing resistance. Some species, such as box turtles, lack the extensions and instead have the carapace bones fully fused or ankylosed together, creating a single unit.
Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle hardly has any bones in its shell, instead it consists of thick connective tissue covered in leathery skin.