Turtles are reptiles of the order Chelonia

Turtles are reptiles of the order CheloniaThey are characterized by a special bony or cartilaginous shell developed from their ribs that acts as a shield.[3] Colloquially, the word "turtle" is generally restricted to fresh-water and sea-dwelling Testudines. Testudines includes both extant (living) and extinct species. Its earliest known members date from the Middle Jurassic.[1] Turtles are one of the oldest reptile groups, more ancient than snakes or crocodilians. Of the 360 known extant species,[2][4] some are highly endangered.[2]

Turtles are ectotherms—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.
Naming and etymology
The common terms "turtle", "tortoise" and "terrapin", depending on the English dialect used,[5] are common names and do not reflect precise biological or taxonomic distinctions.[6] "Turtle" may name either the order as a whole, or to particular turtles that make up a form taxon that is not monophyletic, or may apply only to aquatic species. "Tortoise" usually means any land-dwelling, non-swimming chelonian.[7] "Terrapin" is used for several species of small, edible, hard-shell turtles, typically those found in brackish waters.

In North America, all chelonians are commonly called "turtles",[8][7] just as in Spanish, they are all called tortuga.[9] "Tortoise" is used only in reference to fully terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises.[8][7] 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.[6] 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.[5]

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" 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.[8][16]

This article uses "turtle" for the entire order, which is a single clade.

Anatomy and morphology

Cryptodire cervical vertebrae seen ventrally. The vertebrae have an S-shaped curve to allow for neck retraction into the shell.
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. The largest terrestrial species, the Asian softshell turtle (Pelochelys cantorii), produced a few individuals up to 200 cm (6.6 ft). This dwarfs 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).[17] 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 as humans appeared, and it is assumed humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands. They can grow to over 130 cm (51 in) in length, and weigh about 300 kg (660 lb).[18]

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long.[19]

The smallest extant (living) 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).[20]

Neck retraction
Neck retraction

Pleurodires retract their neck sideways

Cryptodires retract their neck backwards
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. In recent turtles, the cervical column consists of nine joints and eight independent vertebrae.[22] Since these vertebrae are not fused and are rounded, the neck is more flexible, bending 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 allow 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. 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 on this hypothesis not the main function of retraction, but an exaptation.[24] 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]

Head
Most turtles that spend most of their lives on land have eyes positioned to look down at objects in front of them. Some aquatic turtles, such as snapping and soft-shelled turtles, have eyes closer to the top of the head. These species can hide from predators in shallow water, where they lie entirely submerged except for eyes and nostrils. Near their eyes, sea turtles possess glands that produce salty tears that rid their body of excess salt absorbed from the water they drink.

Turtles have rigid beaks and use their jaws to cut and chew food. Turtles appear to have lost their teeth about 150–200 million years ago.[26] Their upper and lower jaws are instead 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 their tongues out to catch food.

Shell
Main article: Turtle shell
Its upper shell is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined on the turtle's sides by bony structures called bridges. The inner shell layer is made up of about 60 bones that include portions of the backbone and the ribs, meaning the turtle cannot crawl out of its shell. In most turtles, the shell's outer layer is covered by horny scales called scutes that are part of its outer skin, or epidermis. Scutes are made up of the fibrous protein keratin. Keratin also forms the scales of other reptiles. These scutes overlap the seams between the shell bones and add strength. 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 shell's shape gives clues about how a 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, and the Belawan turtle (Cirebon, West Java) has a sunken-back soft shell.

The shell's color may vary. Shells are commonly colored brown, black, or olive green. In some species, shells may have red, orange, yellow, or grey markings, often spots, lines, or irregular blotches. One of the most colorful turtles is the eastern painted turtle, with a yellow plastron and a black or olive shell with red markings around the rim.

Tortoises have rather heavy shells. In contrast, aquatic and soft-shelled turtles have lighter shells that help them avoid sinking in water and swim faster and with more agility. These lighter shells have large spaces called fontanelles between the shell bones. The shells of leatherback sea turtles are extremely light because they lack scutes and contain many fontanelles.

Jackson (2002) suggested that the turtle shell can function as a pH buffer. To endure through anoxic conditions, such as winter periods beneath ice or within anoxic mud at the bottom of ponds, turtles utilize two general physiological mechanisms: their shell releases carbonate buffers and uptakes lactic acid.[27]

Respiration
Respiration, for many amniotes, is achieved by the contraction and relaxation of specific muscle groups (i.e. intercostals, abdominal muscles, and/or a diaphragm) attached to an internal rib-cage that can expand or contract the body wall thus assisting airflow in and out of the lungs.[28] The ribs of Testudines, however, are fused with their carapace and external to their pelvic and pectoral girdles, a feature unique among turtles. This rigid shell is not capable of expansion. With their immobile rib-cage, Testudines have had to evolve special adaptations for respiration.[29][30] Turtle pulmonary ventilation occurs by using specific groups of abdominal muscles attached to their viscera and shell that pull the lungs ventrally during inspiration, where air is drawn in via a negative pressure gradient (Boyle's Law).[28] In expiration, the contraction of the transversus abdominis is the driving force by propelling the viscera into the lungs and expelling air under positive pressure.[29] Conversely, the relaxing and flattening of the oblique abdominis muscle pulls the transversus back down which, once again, draws air back into the lungs.[29] Important auxiliary muscles used for ventilatory processes are the pectoralis, which is used in conjunction with the transverse abdominis during inspiration, and the serratus, which moves with the abdominal oblique accompanying expiration.

The lungs of Testudines are multi-chambered and attached their entire length down the carapace. The number of chambers can vary between taxa, though most commonly they have three lateral chambers, three medial chambers, and one terminal chamber.[31] As previously mentioned, the act of specific abdominal muscles pulling down the viscera (or pushing back up) is what allows for respiration in turtles. Specifically, it is the turtles large liver that pulls or pushes on the lungs.[29] Ventral to the lungs, in the coelomic cavity, the liver of turtles is attached directly to the right lung, and their stomach is directly attached to the left lung by the ventral mesopneumonium, which is attached to their liver by the ventral mesentery.[29] When t