Turtles are the reptile order Testudines. They have a special bony or cartilaginous shell developed from their ribs that acts as a shield.
The order Testudines includes both living and extinct species. The earliest fossil turtles date from about 220 million years ago. So turtles are one of the oldest surviving reptile groups and a more ancient group than lizards, snakes and crocodiles.
Turtle have been very successful, and have an almost world-wide distribution. But, of the many species alive today, some are highly endangered.
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. How these names are used, if at all, depends on the type of English.
- British English 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.
- American English tends to use the word turtle as a general term for all species. “Tortoise” is used for most land-dwelling species, and oceanic species are usually referred to as sea turtles. The name “terrapin” is typically reserved only for the brackish water diamondback terrapin, Malaclemys terrapin.
- Australian English uses turtle for both the marine and freshwater species, but tortoise for the terrestrial species.
Ecology and life history
Although many turtles spend much 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.
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. Sea turtles lay their eggs on dry, sandy beaches. Turtles can take many years to reach breeding age, and in many cases breed every few years rather than annually.
In some species, there is temperature-dependent sex determination. 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. No turtle mother cares for its young.
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 almost identical with that of its young counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes.
Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells: the Cryptodira, which can draw their necks in while contracting it under their spine; and the Pleurodira, which contract their necks to the side. So, the important adaptation of head withdrawing evolved twice from ancestral turtles which did not have this ability.
Turtles have a hard 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.
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. It include parts 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 largest chelonian is a marine turtle, the great leatherback sea turtle, which reaches 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 generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have 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 60 kg (170 lb).
The lagest fossil turtle, Archelon, was more than twice the length of the leatherback, at up to 4.5 metres.