Turtle

Turtles are one of the many reptiles on earth. They have a hard shell, which is useful for camouflage. For example, they can be used so predators think it's a type of big rock.
There are two types of turtles: side-necked turtles and hidden neck turtles. Side-necked turtles have very easy to see necks, but hidden neck turtles have very small necks.
There are about 356 species of turtles living on land in all continents except Antarctica and in both salt water and fresh water.
The order Testudines (which includes turtles) includes both living and extinct species. The earliest fossil of a turtle was found in China from the early Upper Triassic age. The fossil was from about 220 million years ago. Turtles are one of the oldest reptile groups that are not extinct. Turtles have been on earth for longer than lizards, snakes and crocodiles. Turtles come from ancient turtles, which had spiked shells.
Turtles are almost everywhere in the world. A lot of the turtle species alive today are endangered.
Name differentials
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. How these names are used 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 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 not as commonly heard.
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 some who work with these animals, as a catch-all name. Unfortunately, Chelonia is also the name of a particular genus of turtles, so this conflicts with its use for the entire order Testudines.
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. Some spend their whole lives on dry land.
Aquatic respiration in Australian freshwater turtles is being studied. Some species have large cloacal (the hole used for mating and excretion) holes that are lined with many finger-like projections. These projections, called papillae, have a rich blood supply, and increase the surface area of the cloaca. The turtles can take up oxygen from the water using these papillae, in much the same way that fish use gills to take in oxygen.
Like other reptiles, turtles lay eggs which are slightly soft and leathery. The eggs of the largest species are shaped like spheres (evenly round like a ball). Other turtle species have eggs that are shaped more like chicken eggs. 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.
Long-lived
Researchers discovered that a turtle's organs do not break down or become less efficient over time, unlike most other animals. They found that the liver, lungs, and kidneys of a century-old turtle are almost the same as those of a younger turtle. This has inspired genetic researchers to begin studying turtle genetics for longevity (living a long time) genes.
Anatomy
Neck folding
Turtles are parted into two groups from the way they hide their heads into their shells. The Cryptodira (hidden neck turtles) pull their heads straight into their shells by folding their necks back under their spines. They have very small necks. The Pleurodira (side-necked turtles), now found only in fresh water environments in the Southern hemisphere, fold their necks to the side, as its name states. The important adaptation of hiding their heads evolved twice from turtles in the past which were not able to do this. Obviously, this adds to their defence against predators.
Feeding
Turtles have a hard beak. They use their jaws to cut and chew food. Turtles have hard edges called horny ridges that cover their upper and lower jaws. These ridges are different from teeth, which turtles do not have. Carnivorous (meat-eating) turtles tend to have knife-sharp ridges that help them cut through meat. Herbivorous (plant-eating) turtles have serrated-edged ridges that help them cut through tough plants. Turtles use their tongues to swallow food, but unlike most reptiles, they cannot stick out their tongues to catch food.
Shell
The upper shell of the turtle is called the carapace. The lower shell that covers the belly is called the plastron. The carapace and plastron are joined 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 includes parts of the spine and ribs, meaning the turtle cannot crawl out of its shell.
Largest living
The largest chelonian is a marine turtle, the great leatherback sea turtle, which reaches a shell length of 200 cm (80 inches) and can weigh over 900 kg (2,000 lb, or 1 ton). Freshwater turtles tend to be smaller, with a few exceptions. The largest species, the Asian giant softshell turtle Pelochelys cantorii, have turtles that measured up to 200 cm or 80 in (Das, 1991). This is bigger than even the better-known alligator snapping turtle, the largest chelonian in North America, which reaches a shell length of up to 80 cm (31 in) and a weight of about 60 kg (170 lb).
The largest fossil turtle, Archelon, was more than twice as long as the leatherback, at up to 4.5 metres.
Evolution
The earliest turtle fossils come from Upper Triassic of the Mesozoic era, about 220 million years ago. Their shell evolved from bony extensions of their backbones and broad ribs that became larger and grew together to make a complete shell. This adaptation was very useful and has been around for a long time. It helped protect turtles as they evolved over many years, even when the bony part of the shell was not complete. The group as a whole has survived many changes in the seas and several extinction events.
Fossils of the freshwater Odontochelys semitestacea or "half-shelled turtle with teeth", from the later Triassic, were found in southwest China. Odontochelys shows a complete bony plastron and an incomplete carapace. This is similar to turtles in an early stage of embryonic development. By the Upper Jurassic, turtles had radiated widely, and their fossil history becomes easier to read.
Their exact ancestry is not known. Early amniotes had no openings in the skull behind the eyes. Openings started to appear in both Sauropsid and Synapsid skulls. They made the skull lighter, gave attachment points for muscles, and gave room for muscle bulges. But turtles do not have these skull openings. They were called 'anapsids', meaning 'no openings'.
Eventually it was suggested that turtles evolved from sauropsids which had skull openings, but turtles lost these openings as part of their evolution towards heavy defensive armour. All molecular studies strongly support that turtles are diapsids; some support that turtles are either archosaurs, or are a sister group to living archosaurs.
The earliest known fully shelled turtles are the late Triassic Odontochelys, Chinlechelys and Proganochelys. The first-named genus was aquatic (lived in water), but the second was probably terrestrial (lived on land). They already had many advanced turtle traits, and probably had millions of years of turtle evolution and species in their ancestry. They had a long neck and could not pull their heads into their shell. Odontochelys had a long, spiked tail ending in a club. Ankylosaurs also have these traits because of convergent evolution.
Turtles are divided into two living suborders (groups): the Cryptodira and the Pleurodira. The Cryptodira is the larger of the two groups. All marine (sea) turtles, terrestrial (land) tortoises, and many of the freshwater turtles belong to this group. The Pleurodira are sometimes known as the side-necked turtles because of the way they hide their heads into their shells. This smaller group is mostly made up of freshwater turtles.