Turtles are reptiles of the order Testudines characterized by a special bony or cartilaginous shell developed from their ribs and acting as a shield.[3] "Turtle" may refer to the order as a whole (American English) or to fresh-water and sea-dwelling testudines (British English).[4] The order Testudines includes both extant (living) and extinct species. The earliest known members of this group date from the Middle Jurassic,[1] making turtles one of the oldest reptile groups and a more ancient group than snakes or crocodilians. Of the 356 known species[2] alive today, some are highly endangered.[2]

Turtles are ectotherms—animals commonly called cold-blooded—meaning that their internal temperature varies according to the ambient environment. However, because of their high metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the surrounding water. Turtles are classified as amniotes, along with other reptiles, birds, and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although many species live in or around water.

A group of turtles is known as a bale.

Differences exist in usage of the common terms turtle, tortoise, and terrapin, depending on the variety of English being used.[6] These terms are common names and do not reflect precise biological or taxonomic distinctions.[7]

Turtle may either refer to the order as a whole, or to particular turtles that make up a form taxon that is not monophyletic, or may be limited to only aquatic species. Tortoise usually refers to any land-dwelling, non-swimming chelonian.[8] Terrapin is used to describe several species of small, edible, hard-shell turtles, typically those found in brackish waters.

In North America, all chelonians are commonly called turtles. Tortoise is used only in reference to fully terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises.[9][8] Terrapin may refer to small semi-aquatic turtles that live in fresh and brackish water, in particular the diamondback terrapin (Malaclemys terrapin).[10][11][12][13] Although the members of the genus Terrapene dwell mostly on land, they are referred to as box turtles rather than tortoises.[7] The American Society of Ichthyologists and Herpetologists uses "turtle" to describe all species of the order Testudines, regardless of whether they are land-dwelling or sea-dwelling, and uses "tortoise" as a more specific term for slow-moving terrestrial species.[6]

In the United Kingdom, the word turtle is used for water-dwelling species, including ones known in the US as terrapins, but not for terrestrial species, which are known only as tortoises.

The word chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia, which includes all turtles living and extinct, as well as their immediate ancestors. Chelonia is based on the Greek word for turtles, χελώνη chelone; Greek χέλυς chelys "tortoise" ( Ayaya <3) is also used in the formation of scientific names of chelonians.[14] Testudines, on the other hand, is based on the Latin word for tortoise, testudo.[15] Terrapin comes from an Algonquian word for turtle.[9][16]

Some languages do not have this distinction, as all of these are referred to by the same name. For example, in Spanish, the word tortuga is used for turtles, tortoises, and terrapins. A sea-dwelling turtle is tortuga marina, a freshwater species tortuga de río, and a tortoise tortuga terrestre.[17]

The largest living chelonian is the leatherback sea turtle (Dermochelys coriacea), which reaches a shell length of 200 cm (6.6 ft) and can reach a weight of over 900 kg (2,000 lb). Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to 200 cm (6.6 ft). 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 (2.6 ft) and weighs as much as 113.4 kg (250 lb).[18]

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 humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 cm (51 in) in length, and weigh about 300 kg (660 lb).[19]

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long.[20]

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3.1 in) in length and weighs about 140 g (4.9 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 (5.1 in) in length.

Turtles are divided into two groups, according to how they retract their necks into their shells (something the ancestral Proganochelys could not do). The mechanism of neck retraction differs phylogenetically: the suborder Pleurodira retracts laterally to the side, anterior to shoulder girdles, while the suborder Cryptodira retracts straight back, between shoulder girdles.[21] These motions are largely due to the morphology and arrangement of cervical vertebrae. Of all recent turtles, the cervical column consists of nine joints and eight vertebrae, which are individually independent.[22] Since these vertebrae are not fused and are rounded, the neck is more flexible, being able to bend in the backwards and sideways directions.[21] The primary function and evolutionary implication of neck retraction is thought to be for feeding rather than protection.[23] Neck retraction and reciprocal extension allows the turtle to reach out further to capture prey while swimming. Neck expansion creates suction when the head is thrust forward and the oropharynx is expanded, and this morphology suggests the retraction function is for feeding purposes as the suction helps catch prey.[23] The protection the shell provides the head when it is retracted is therefore not the main function of retraction, thus is an exaptation.[24] As for the difference between the two methods of retraction, both Pleurodirans and Cryptodirans use the quick extension of the neck as a method of predation, so the difference in retraction mechanism is not due to a difference in ecological niche.[25]

Most turtles that spend most of their lives 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 turtle can hide from predators in shallow water, where they lie entirely submerged except for their eyes and nostrils. Near their eyes, sea turtles possess glands that produce salty tears that rid their body of excess salt taken in from the water they drink.

Turtles have rigid beaks and use their jaws to cut and chew food. Instead of having teeth, which they appear to have lost about 150–200 million years ago,[26] 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. They use their tongues to swallow food, but unlike most reptiles, they cannot stick out their tongues to catch food.