Turtle

Turtles are reptiles of the order Chelonia /kɪˈloʊniə/ or Testudines /tɛˈstjuːdɪniːz/. They are characterized by a special bony or cartilaginous shell developed from their ribs that acts as a shield. 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. Turtles are one of the oldest reptile groups, more ancient than snakes or crocodilians. Of the 360 known extant species, some are highly endangered.

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, are common names and do not reflect precise biological or taxonomic distinctions. "Turtle" may denote the order as a whole, or a non-monophyletic form taxon within the order, or only aquatic species. "Tortoise" usually means any land-dwelling, non-swimming chelonian. "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", just as in Spanish, they are all called tortuga. "Tortoise" is used only in reference to fully terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises. Terrapin may refer to small semi-aquatic turtles that live in fresh and brackish water, in particular the diamondback terrapin (Malaclemys terrapin). Although the members of the genus Terrapene dwell mostly on land, they are referred to as box turtles rather than tortoises. 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.

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. Testudines, on the other hand, is based on the Latin word for tortoise, testudo. Terrapin comes from an Algonquian word for turtle.

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

Anatomy and morphology
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). 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).

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

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).

Neck retraction
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. 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. Since these vertebrae are not fused and are rounded, the neck is more flexible, bending in the backwards and sideways directions. The primary function and evolutionary implication of neck retraction is thought to be for feeding rather than protection. 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. The protection the shell provides the head when it is retracted is on this hypothesis not the main function of retraction, but an exaptation. 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.

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. 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
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.

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. 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. 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). 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. Conversely, the relaxing and flattening of the oblique abdominis muscle pulls the transversus back down which, once again, draws air back into the lungs. 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. 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. 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. When the liver is pulled down, inspiration begins. Supporting the lungs is the post-pulmonary septum, which is found in all Testudines, and is thought to prevent the lungs from collapsing.

Skin and molting
As mentioned above, the outer layer of the shell is part of the skin; each scute (or plate) on the shell corresponds to a single modified scale. The remainder of the skin has much smaller scales, similar to the skin of other reptiles. Turtles do not molt their skins all at once as snakes do, but continuously in small pieces. When turtles are kept in aquaria, small sheets of dead skin can be seen in the water (often appearing to be a thin piece of plastic) having been sloughed off when the animals deliberately rub themselves against a piece of wood or stone. Tortoises also shed skin, but dead skin is allowed to accumulate into thick knobs and plates that provide protection to parts of the body outside the shell.

By counting the rings formed by the stack of smaller, older scutes on top of the larger, newer ones, it is possible to estimate the age of a turtle, if one knows how many scutes are produced in a year. This method is not very accurate, partly because growth rate is not constant, but also because some of the scutes eventually fall away from the shell.

Limbs
Terrestrial tortoises have short, sturdy feet. Tortoises are famous for moving slowly, in part because of their heavy, cumbersome shells, which restrict stride length. Skeleton of snapping turtle (Chelydra serpentina)

Amphibious turtles normally have limbs similar to those of tortoises, except that the feet are webbed and often have long claws. These turtles swim using all four feet in a way similar to the dog paddle, with the feet on the left and right side of the body alternately providing thrust. Large turtles tend to swim less than smaller ones, and the very big species, such as alligator snapping turtles, hardly swim at all, preferring to walk along the bottom of the river or lake. As well as webbed feet, turtles have very long claws, used to help them clamber onto riverbanks and floating logs upon which they bask. Male turtles tend to have particularly long claws, and these appear to be used to stimulate the female while mating. While most turtles have webbed feet, some, such as the pig-nosed turtle, have true flippers, with the digits being fused into paddles and the claws being relatively small. These species swim in the same way as sea turtles do (see below).

Sea turtles are almost entirely aquatic and have flippers instead of feet. Sea turtles fly through the water, using the up-and-down motion of the front flippers to generate thrust; the back feet are not used for propulsion but may be used as rudders for steering. Compared with freshwater turtles, sea turtles have very limited mobility on land, and apart from the dash from the nest to the sea as hatchlings, male sea turtles normally never leave the sea. Females must come back onto land to lay eggs. They move very slowly and laboriously, dragging themselves forwards with their flippers.

Communication
While typically thought of as mute, turtles make various sounds when communicating. Tortoises may be vocal when courting and mating. Various species of both freshwater and sea turtles emit numerous types of calls, often short and low frequency, from the time they are in the egg to when they are adults. These vocalizations may serve to create group cohesion when migrating. The big-headed turtle is said to growl when removed from the water in an effort to scare off predators. Turtles also have non-vocal ways of communicating that may be employed during courtship. For example male slider turtles use their long foreclaws to stroke the female's face while courting.

Social behavior
Most turtle species do not display social behavior outside of mating season but a few exceptions have been noted. It has been suggested that gopher tortoises, more than most other tortoise species, exhibit social behaviour. Gopher tortoises live in well-defined colonies which are similar to those of highly social animals such as the prairie dog. The distribution and proximity of burrows might be the consequence of social relationships between tortoises. Some females have been observed visiting the burrows of a particular female repeatedly, even if there are other tortoises nearer to them. Some researchers have called this "a sort of friendship".

Hatchling Arrau turtles are thought to communicate with each other and with females that wait for the hatchlings in order to migrate from the nesting site to the foraging sites.

Intelligence
It has been reported that wood turtles are better than white rats at learning to navigate mazes. Case studies exist of turtles playing. They do, however, have a very low encephalization quotient (relative brain to body mass), and their hard shells enable them to live without fast reflexes or elaborate predator avoidance strategies. In the laboratory, turtles (Pseudemys nelsoni) can learn novel operant tasks and have demonstrated a long-term memory of at least 7.5 months. Similarly, giant tortoises can learn and remember tasks, and master lessons much faster when trained in groups. Remarkably, tortoises that were tested 9 years after the initial training still retained the operant conditioning.

Reproduction
Turtles have a wide variety of mating behaviors, but do not form pair-bonds or social groups.Once the eggs have hatched, neither parent provides care for the offspring. Females generally outnumber males in various turtle species (such as green turtles), and as a result, most males will engage in multiple copulation with multiple partners throughout their lifespan.Most terrestrial species are sexually dimorphic, with males larger than females, and fighting between males often determines a hierarchical order for access to mates. For most semi-aquatic species and bottom-walking aquatic species, combat occurs less often. Males belonging to semi-aquatic and bottom-walking species instead often use their larger size advantage to forcibly mate with a female. In fully aquatic species, males are often smaller than females, and rely on courtship displays rather than strength to gain mating access to females.

Male competition
In some terrestrial species such as wood turtles, the males have a hierarchical ranking system based on dominance through fighting; the males with the highest rank and thus the most wins in fights have the most offspring.

Galapagos tortoises are another example of a species which has a hierarchical rank that is determined by dominance displays, and access to food and mates is regulated by this dominance hierarchy. Two male saddle backs most often compete for access to cactus trees, which is their source of food. The winner is the individual who stretches their neck the highest, and that individual gets access to the cactus tree, which can attract potential mates.

Force mating
The male scorpion mud turtle is an example of a bottom-walking aquatic species that relies on overpowering females with its larger size as a mating strategy. The male approaches the female from the rear, and often resorts to aggressive methods such as biting the female's tail or hind limbs, followed by a mounting behavior in which the male clasps the edges of her carapace with his forelimbs and hind limbs to hold her in position. The male follows this action by laterally waving his head and sometimes biting the female's head in an attempt to get her to withdraw her head into her shell. This exposes her cloaca, and with it exposed, the male can attempt copulation by trying to insert his grasping tail.

Male radiated tortoises are also known to use the force mating strategy wherein they use surrounding vegetation to trap or prevent females from escaping, then pin them down for copulation.

Courtship displays
Red-eared sliders are a fully aquatic species. The male courts the female by extending his forelegs with the palms facing out and fluttering his forelegs in the female's face. Female choice is important in this method, and the females of some species, such as green sea turtles, are not always receptive. As such, they have evolved certain behaviors to avoid the male's attempts at copulation, such as swimming away, confronting the male followed by biting, or a refusal position in which the female assumes a vertical position with her limbs widely outspread and her plastron facing the male. If the water is too shallow to perform the refusal position, the females will resort to beaching themselves, which is a proven deterrent method, as the males will not follow them ashore.

Ecology and life history
Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air and must surface at regular intervals to refill their lungs. Immersion periods vary between 60 seconds and 1 hour depending on the species. Some turtles spend much or all of their lives on dry land. Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities 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 dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.

Like other reptiles, turtles lay eggs that are slightly soft and leathery. The eggs of the largest species are spherical while the eggs of the rest are elongated. Their albumen is white and contains a different protein from bird eggs, such that it will not coagulate when cooked. Turtle eggs prepared to eat consist mainly of yolk. In some species, 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. Depending on the species, the eggs will typically take 70–120 days to hatch. When the turtles hatch, they squirm their way to the surface and head toward the water. There are no known species in which the mother cares for her young.

Sea turtles lay their eggs on dry, sandy beaches. Immature sea turtles are not cared for by the adults. Turtles can take many years to reach breeding age, and in many cases, breed every few years rather than annually.

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 virtually indistinguishable from those of its immature counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes.

Diet
A turtle's diet varies greatly depending on the environment in which it lives. Adult turtles typically eat aquatic plants, invertebrates such as insects, snails, and worms, and have been reported to occasionally eat dead marine animals. Several small freshwater species are carnivorous, eating small fish and a wide range of aquatic life. However, protein is essential to turtle growth and juvenile turtles are purely carnivorous.

Sea turtles typically feed on jellyfish, sponges, and other soft-bodied organisms. Some species with stronger jaws have been observed to eat shellfish, while others, such as the green sea turtle, do not eat meat at all and, instead, have a diet largely made up of algae.

Fossil history
Based on body fossils, the first proto-turtles are believed to have existed in the late Triassic Period of the Mesozoic era, about 220 million years ago, and their shell, which has remained a remarkably stable body plan, is thought to have evolved from bony extensions of their backbones and broad ribs that expanded and grew together to form a complete shell that offered protection at every stage of its evolution, even when the bony component of the shell was not complete. This is supported by fossils of the freshwater Odontochelys semitestacea or "half-shelled turtle with teeth", from the late Triassic, which have been found near Guangling in southwest China. Odontochelys displays a complete bony plastron and an incomplete carapace, similar to an early stage of turtle embryonic development.Prior to this discovery, the earliest-known fossil turtle ancestors, like Proganochelys, were terrestrial and had a complete shell, offering no clue to the evolution of this remarkable anatomical feature. By the late Jurassic, turtles had radiated widely, and their fossil history becomes easier to read.

The earliest known fully-shelled member of the turtle lineage is the late Triassic Proganochelys. This genus already possessed many advanced turtle traits, and thus probably indicates many millions of years of preceding turtle evolution; this is further supported by evidence from fossil tracks from the Early Triassic of the United States (Wyoming and Utah) and from the Middle Triassic of Germany, indicating that proto-turtles already existed as early as the Early Triassic. Proganochelys lacked the ability to pull its head into its shell, had a long neck, and had a long, spiked tail ending in a club. While this body form is similar to that of ankylosaurs, it resulted from convergent evolution.

Turtle fossils of hatchling and nestling size have been documented in the scientific literature.Paleontologists from North Carolina State University found the fossilized remains of the world's largest turtle in a coal mine in Colombia. The specimen named as Carbonemys cofrinii is around 60 million years old and nearly 2.4 m (8 ft) long.

On a few rare occasions, paleontologists have unearthed large numbers of Jurassic or Cretaceous turtle skeletons accumulated in a single area (the Nemegt Formation in Mongolia, the Turtle Graveyard in North Dakota, or the Black Mountain Turtle Layer in Wyoming). The most spectacular find of this kind to date occurred in 2009 in Shanshan County in Xinjiang, where over a thousand ancient freshwater turtles apparently died after the last water hole in an area dried out during a major drought.

Though absent from New Zealand in recent times, turtle fossils are known from the Miocene Saint Bathans Fauna, represented by a meiolaniid and pleurodires.

External phylogeny
The turtles' exact ancestry has been disputed. It was believed they are the only surviving branch of the ancient evolutionary grade Anapsid, which includes groups such as procolophonids, millerettids, protorothyrids, and pareiasaurs. All anapsid skulls lack a temporal opening while all other extant amniotes have temporal openings (although in mammals, the hole has become the zygomatic arch). The millerettids, protorothyrids, and pareiasaurs became extinct in the late Permian period and the procolophonoids during the Triassic.

However, it was later suggested that the anapsid-like turtle skull may be due to reversion rather than to anapsid descent. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids, slightly closer to Squamata than to Archosauria. All molecular studies have strongly upheld the placement of turtles within diapsids; some place turtles within Archosauria, or, more commonly, as a sister group to extant archosaurs, though an analysis conducted by Lyson et al. (2012) recovered turtles as the sister group of lepidosaurs instead. Reanalysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa broadly enough for constructing the cladogram. Testudines were suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled. Even the traditional placement of turtles outside Diapsida cannot be ruled out at this point. A combined analysis of morphological and molecular data conducted by Lee (2001) found turtles to be anapsids (though a relationship with archosaurs could not be statistically rejected). Similarly, a morphological study conducted by Lyson et al.. (2010) recovered them as anapsids most closely related to Eunotosaurus. A molecular analysis of 248 nuclear genes from 16 vertebrate taxa suggests that turtles are a sister group to birds and crocodiles (the Archosauria). The date of separation of turtles and birds and crocodiles was estimated to be 255 million years ago. The most recent common ancestor of living turtles, corresponding to the split between Pleurodira and Cryptodira, was estimated to have occurred around 157 million years ago. The oldest definitive crown-group turtle (member of the modern clade Testudines) is Caribemys oxfordiensis from the late Jurassic period (Oxfordian stage). Through genomic-scale phylogenetic analysis of ultraconserved elements (UCEs) to investigate the placement of turtles within reptiles, Crawford et al. (2012) suggest that turtles are a sister group to birds and crocodiles (Archosauria).

The first genome-wide phylogenetic analysis was completed by Wang et al. (2013). Using the draft genomes of Chelonia mydas and Pelodiscus sinensis, the team concluded that turtles are likely a sister group of crocodilians and birds (Archosauria). This placement within the diapsids suggests that the turtle lineage once had diapsid skull characteristics but has lost them, as turtles now possess an anapsid-like skull.

Internal phylogeny
The cladogram, from Guillon et al., 2012, shows the internal phylogeny of the Testudines, not yet fully resolved. Turtles possess diverse numbers of chromosomes (2n = 28–66) and a myriad of chromosomal rearrangements have occurred during evolution.

Taxonomy
Turtles are divided into two extant suborders: Cryptodira and Pleurodira. The Cryptodira is the larger of the two groups and includes all the marine turtles, the terrestrial tortoises, and many of the freshwater turtles. The Pleurodira are sometimes known as the side-necked turtles, a reference to the way they retract their heads into their shells. This smaller group consists primarily of various freshwater turtles. Until 3,000 years ago, the family Meiolaniidae was also extant, but this family is outside the Testudines crown group, belonging to Perichelydia.

Order  Testudines  Linnaeus, 1758


 * Suborder Pleurodira Cope, 1864
 * Family †Apertotemporalidae Kühne, 1937
 * Family †Platychelyidae Brän, 1965 sensu Gaffney, Tong & Buffetaut, 2006
 * Family †Dortokidae Lapparent de Broin & Murelaga, 1996
 * Family †Notoemyidae Fernandez & Fuente, 1994
 * Superfamily Cheloides Gray, 1825 sensu Gaffney, Tong & Buffetaut, 2006
 * Family Chelidae Gray, 1825
 * Superfamily Pelomedusoides Cope, 1868 sensu Broin 1988
 * Family †Araripemydidae Price, 1973
 * Family Pelomedusidae (African sideneck turtles)
 * Family †Euraxemydidae Gaffney, Tong & Buffetaut, 2006
 * Family †Bothremydidae Baur, 1891
 * Family Podocnemididae Cope, 1868 (Madagascan big-headed and American sideneck river turtles)
 * Suborder Cryptodira Duméril & Bibron, 1835
 * Infraorder Eucryptodira Gaffney, 1975a sensu Gaffney, 1984
 * Family †Macrobaenidae Sukhanov 1964
 * Family †Xinjiangchelyidae Nesov, 1990
 * Clade Centrocryptodira
 * Family †Osteopygidae Zangerl, 1953
 * Family †Sinemydidae Yeh, 1963
 * Clade Polycryptodira Gaffney, 1988
 * Clade Pantrionychia
 * Family †Adocidae
 * Superfamily Trionychoidea Gray, 1870
 * Family Carettochelyidae Boulenger, 1887 (pignose turtles)
 * Family Trionychidae Fitzinger, 1826 (softshell turtles)
 * Clade Durocryptodira
 * Superfamily Testudinoidea Baur, 1893
 * Family †Haichemydidae Sukhanov & Narmandakh, 2006
 * Family †Lindholmemydidae Chkhikvadze, 1970
 * Family †Sinochelyidae Chkhikvadze, 1970
 * Family Emydidae (Rafinesque, 1815) (pond, box, and water turtles)
 * Family Geoemydidae Theobald, 1868 (Asian river turtles, Asian leaf turtles, Asian box turtles, and roofed turtles)
 * Family Testudinidae Batsch, 1788 (true tortoises)
 * Clade Americhelydia Crawford et al., 2014
 * Family Chelydridae Gray, 1831 (snapping turtles)
 * Superfamily Kinosternoidea Joyce, Parham, and Gauthier 2004
 * Family Dermatemydidae Gray, 1870 (river turtles)
 * Family Kinosternidae Agassiz, 1857 (mud turtles)
 * Clade †Thalassochelydia?
 * Family †Eurysternidae? Dollo, 1886
 * Family †Plesiochelyidae? Baur, 1888
 * Family †Thalassemydidae?
 * Family †Protostegidae? Cope, 1872
 * Family †Sandownidae?
 * Family †Toxochelyidae Baur, 1895
 * Superfamily Chelonioidea Bauer, 1893 (sea turtles)
 * Family Cheloniidae Oppel, 1811 (green sea turtles and relatives)
 * Family Dermochelyidae Fitzinger, 1843 (leatherback sea turtles)
 * Clade †Paracryptodira
 * Family †Pleurosternidae Cope, 1868
 * Family †Compsemyidae
 * Family †Baenidae Cope, 1882

As pets
Some turtles, particularly small terrestrial and freshwater turtles, are kept as pets. Among the most popular are Russian tortoises, spur-thighed tortoises, and red-eared sliders.

In the United States, due to the ease of contracting salmonellosis through casual contact with turtles, the U.S. Food and Drug Administration (FDA) made it illegal to sell turtles under 4 inches (10 cm) long as pets, but many stores and flea markets exploit a loophole which allows turtles under 4 in (100 mm) to be sold for educational purposes.

Some states have other laws and regulations regarding possession of red-eared sliders as pets because they are looked upon as invasive species or pests where they are not native, but have been introduced through the pet trade. As of July 1, 2007, it is illegal in Florida to sell any wild type red-eared slider.

In Europe, turtle and tortoise keeping became popular in the 1960s and 1970s, when large numbers of wild-caught turtles and tortoises were imported. This was especially devastating to the Mediterranean tortoise population. In the 1980s the import of wild-caught tortoises started to be banned in various countries. Most turtles and tortoises for sale in Europe today are captive-bred.

Turtles and tortoises are seen by some people as cheap pets that need little care. The complexity and expense of proper turtle and tortoise husbandry is often underestimated. Most species of tortoise need a spacious outdoor enclosure with areas at different temperatures so they can thermoregulate. They also need opportunities to climb, dig and forage. Most species of tortoise should be fed dark, leafy greens with calcium and vitamin supplements.

Turtles require a large tub or aquarium with land areas where they can dry off completely and other areas where they can rest near the water's surface, on a piece of submerged driftwood for example. Like tortoises, turtles need access to UVB lighting and a varied diet rich in calcium.

For food, traditional medicine, and cosmetics
The flesh of turtles, calipash or calipee, has long been considered a delicacy in a number of cultures. Turtle soup has been a prized dish in Anglo-American cuisine. Gopher tortoise stew has been popular with some groups in Florida.

Turtles remain a part of the traditional diet on the island of Grand Cayman, so much so that when wild stocks became depleted, a turtle farm was established specifically to raise sea turtles for their meat. The farm releases some turtles to the wild to help repopulate the Caribbean Sea.

Fat from turtles is used in the Caribbean and in Mexico as a main ingredient in cosmetics, marketed under its Spanish name crema de tortuga. The supposed aphrodisiac or medicinal properties of turtle eggs created a large trade for them in Southeast Asia.

Turtle plastrons are widely used in traditional Chinese medicine; Taiwan imports hundreds of tons of plastrons every year. A popular medicinal preparation based on herbs with or without powdered turtle plastron is guilinggao jelly. The shell of the hawksbill turtle has been used for centuries to make jewellery, tools and ornaments around the Western Pacific.

Conservation
With between 48 and 54% of all 328 species considered threatened, turtles and tortoises are at a much higher risk of extinction than many other vertebrates. Of the 263 species of freshwater and terrestrial turtles, 117 species are considered threatened, 73 are either endangered or critically endangered and 1 is extinct. Of the 58 species in the family Testudinidae, 33 are threatened, 18 are either endangered or critically endangered, 1 is extinct in the wild and 7 are extinct. 71% of all tortoise species are either gone or almost gone. Asian species are the most endangered, closely followed by the five endemic species of Madagascar. Turtles face many threats, including habitat destruction, harvesting for consumption, and the pet trade. The high extinction risk for Asian species is primarily due to the long-term unsustainable exploitation of turtles and tortoises for consumption and traditional Chinese medicine, and to a lesser extent for the international pet trade. Turtle extinction is progressing much faster than during the Cretaceous-Paleogene Extinction; at the current rate, all turtles could be extinct in less than a century.

Turtle hatcheries can be set up when protection against flooding, erosion, predation or heavy poaching is required. Efforts have been made by Chinese entrepreneurs to satisfy increasing demand for turtle meat as gourmet food and traditional medicine with farmed turtles, instead of wild-caught ones; according to a study published in 2007, over a thousand turtle farms operated in China. Turtle farms in Oklahoma and Louisiana raise turtles for export to China. All the same, wild turtles continue to be caught and sent to market in large number (as well as to turtle farms, to be used as breeding stock), resulting in a situation described by conservationists as "the Asian turtle crisis". In the words of the biologist George Amato, "the amount and the volume of captured turtles ... vacuumed up entire species from areas in Southeast Asia", even as biologists still did not know how many distinct turtle species live in the region. About 75% of Asia's 90 tortoise and freshwater turtle species are estimated to have become threatened.

Harvesting wild turtles is legal in some American states. In Florida, a single seafood company in Fort Lauderdale was reported in 2008 as buying about 5,000 pounds of softshell turtles a week. The hunters are paid about $2 a pound; some catch as many as 30–40 turtles (500 pounds) on a good day. Some is eaten in local restaurants, while most is exported to Asia. The Florida Fish and Wildlife Conservation Commission estimated in 2008 that around 3,000 pounds of softshell turtles were exported each week via Tampa International Airport. However, the great majority of turtles exported from the US are farmed. According to one estimate by the World Chelonian Trust, about 97% of 31.8 million animals harvested in the U.S. over a three-year period (November 4, 2002 – November 26, 2005) were exported. It has been estimated that about 47% of the US turtle exports go to the People's Republic of China (predominantly to Hong Kong), another 20% to Taiwan, and 11% to Mexico.

In Australia, Queensland's shark culling program, which uses shark nets and drum lines, has since 1962 killed over 5,000 turtles as bycatch. The program has killed 719 loggerhead turtles and 33 critically endangered hawksbill turtles. New South Wales's shark control program has similarly killed at least 5,000 turtles.