Turtle

Turtles are reptiles of the order Chelonia or Testudines . They are characterized by a bony or cartilaginous shell or carapace, developed from their ribs, that acts as a shield. Testudines include both extant (living) and extinct species. Its earliest known members date from the Middle Jurassic. They are one of the oldest reptile groups, more ancient than snakes or crocodilians. 
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.
Turtles have appeared in myths and folktales around the world. Some terrestrial and freshwater species are widely kept as pets. Turtles have been hunted for their meat, for use in traditional medicine, and for their carapaces. Marine turtles are often killed accidentally as bycatch in fishing nets. Turtle habitats around the world are being destroyed. As a result of these pressures, many species are threatened with extinction by 2100.
Naming and etymology.
"Turtle" is a common name and may be used without knowledge of taxonomic distinctions. In particular, "turtle" may denote the order as a whole, as in North American usage, or a non-monophyletic form taxon within the order, or only aquatic species, as in British usage. This article uses "turtle" for the entire order. 
Animals in the order are often called "chelonians" by veterinarians, scientists, and conservationists. The name "Chelonia", now a synonym for the order, is based on the Greek word for "turtle", "chelone"; Greek "chelys" "tortoise" is used in the formation of names of many turtle taxa. The name of the order, "Testudines", is based on the Latin word for tortoise, "testudo".
Anatomy and physiology.
Size.
The largest living species of turtle, and fourth largest reptile, is the leatherback turtle which can grow up to and weigh over . On land, the Galápagos tortoises have reached lengths of , and weights of over . The largest known turtle was "Archelon ischyros", a Late Cretaceous sea turtle up to long and estimated to have weighed around . The smallest living turtle is the speckled padloper tortoise of South Africa. It measures no more than in length, and weighs about .
Shell.
The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements. It is primarily made of bone, and consists of two parts, the carapace which usually contains 50–60 bones and covers the back of the animal while the plastron has 7–11 bones and covers the belly. They are connected by lateral extensions of the plastron. The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralium. The shoulder girdle in turtles is made up of two bones, the scapula and the procoracoid. Both the anterior and posterior pelvis of turtles are located within the shell and hence are effectively within the rib-cage.
The outer surface of the shell is covered in epidermal scales known as scutes which are made of keratin, the same substance that makes up human hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron; 54 in total. Carapace scutes are divided into "marginals" around the margin, "vertebrals" over the vertebral column, in many species an extra singular scute between the first marginals called the "cervical" is present, and "costals" between the marginals and vertebrals. Plastron scutes include gulars (throat), humerals, abdominals, femorals and anals. The side necked turtles of the Pleurodira have an extra plastral scute called the "intergular." Turtle scutes usually interlock like mosaic tiles, though in some species, like the hawksbill sea turtle, the scutes on the carapace can overlap.
Turtle shells come in different shapes and sizes based on the adaptations of the individual species. Land-dwelling turtles tend to have more domed shells, which appear to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells which allow them to cut though the water. Sea turtles in particular have streamlined shells which reduce drag and increase stability in the open ocean. Some turtle species have ridged, lumped, or spiked shells which provide extra protection from predators and camouflage against patterned backgrounds. The humps of a tortoise shell may tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the lead edge of the plastron is thickened; it is used for butting and ramming during combat.
Shells can also vary in flexibility. In tortoises, the plastron and its extensions lock the sides of the carapace together, giving it even greater crushing resistance. Some species, such as box turtles, lack the extensions and instead have the carapace bones fully fused or ankylosed together, creating a single unit. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, which instead consists of thick connective tissue covered in leathery skin.
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.
Head and neck.
The turtle's skull is unique among amniotes; it is solid and rigid with no openings for muscle attachment (temporal fenestra). Muscles instead attach to recesses in the back of the skull. Living turtles also lack teeth but have a bony cusp that may be beak-like or have serrations. Cusps are covered in keratin which usually have a sharp edge for cutting and slicing. Turtle skulls vary in shape; from the elongated skulls of softshells to the broad and flattened skull of the mata mata. Some turtle species have developed proportionally large and thick heads, allowing for greater muscle mass and stronger bites. 
The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as snake-necked turtles, have very long ones. Despite this, all turtle species have eight neck vertebrate; a consistency not found in other reptiles but paralleled in mammals.
Limbs and locomotion.
Turtles are slow-moving on land, because of their heavy shells; a desert tortoise moves at only . By contrast, sea turtles can swim at . The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet with short toes. The gopher tortoise has flattened front-limbs for digging in the substrate. Aquatic turtles have more flexible legs and longer toes with webbing, getting them thrust in the water. Some of these species, such as snapping turtles and mud turtles, mainly walk along the water bottom, much as they would on land. Others, such as terrapins, swim by paddling with all four limbs with the simultaneous retraction of the opposing front and hind limbs, helping them maintain their direction while thrusting.
Sea turtles and the pig-nosed turtle are the most specialized for aquatic locomotion. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers. Sea turtles such as "Chelonia mydas" rotate the front limb flippers like a bird's wings so as generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as "Mauremys caspica", which use the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine turtles is similar to that of fast-swimming fish of open water, like mackerel.
Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. They are especially large in snapping turtles and the big-headed turtle, the latter of which uses its tail to balance itself while climbing. The cloaca is at the base of the tail, and the tail itself houses the reproductive organs. Hence, males have longer tails to accommodate the penis. In sea turtles, the tail is longer and also somewhat prehensile; males use it to grasp females when mating. Several turtle species have spines on their tails.
Respiration.
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.
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 turtles, however, are, uniquely, fused with their carapace and external to their pelvic and pectoral girdles. This rigid shell is not capable of expansion, so the turtles have had to evolve special adaptations for respiration. They ventilate their lungs 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. In expiration, the contraction of the transversus abdominis muscle propels the viscera into the lungs and expels 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.
Water retention and excretion.
Charles Darwin noted that the Galapagos tortoise had a bladder which could store up to 20% of its body weight. Such Xerocole adaptations are the result of environments such as remote islands and deserts where water is very scarce. The urine is normally very low in solutes, mainly potassium ions from food plants. The bladder's wall is permeable, so when the animal becomes dehydrated, water returns by osmosis from the bladder to the blood, and the urine's solute concentration rises until it approaches that of the blood plasma. In this way the bladder serves as a reserve store of water.
Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity. The bladder is usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have calculi.
Thermoregulation.
Turtles, like other reptiles, have a limited ability to regulate their body temperature; this varies between species, and with body size. Small pond turtles regulate their temperature by crawling out of the water and basking in the sun, while small terrestrial turtles move between sunny and shady places to adjust their temperature. Large species, both terrestrial and marine, have sufficient mass to give them substantial thermal inertia, meaning that they heat up or cool down over many hours. The Aldabra giant tortoise ("Aldabrachelys gigantea") weighs up to some , and is able to allow its temperature to rise to some 33 Celsius on a hot day, and to fall naturally to around 29 Celsius by night. Some giant tortoises seek out shade to avoid overheating on sunny days. On Grand Terre Island, food is scarce inland, but shade is scarce near the coast, and the tortoises compete for space under the few trees on hot days; large males may push smaller females out of the shade, and some then overheat and die. Adult sea turtles, too, have large enough bodies that they can to some extent control their temperature. The largest, the leatherback, can swim in the waters off Nova Scotia which may be as cool as ; their body temperature has been measured at up to warmer than the surrounding water. To help keep their temperature up, they have a system of countercurrent heat exchange in the blood vessels between their body core and the skin of their flippers; the vessels supplying the head are insulated by fat around the neck. 
Some species have their sex determined by temperature, implying environmental control; however, there is experimental evidence that the embryos of "Mauremys reevesii" can move around inside their eggs to select the optimal temperature for development, thus influencing their sexual destiny.
Ageing.
A turtle's organs do not senesce, unlike those of most other animals; 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.
Behavior.
Diet and feeding.
Most turtle species are opportunistic omnivores; landing-dwelling species being more herbivorous and aquatic ones being more carnivorous. Generally lacking speed and agility, most turtles most feed on either plant material or sedentary animals like mollusks, worms and insect larvae. Some species, such as the African helmeted turtle and snapping turtles, will eat fish, amphibians, reptiles (including other turtles) birds and mammals; they make take them by ambush but will also scavenge. The alligator snapping turtle has a worm-like appendage on this tongue which it used to lure fish into its mouth.Tortoises are the most herbivorous turtle species and consume grasses, leaves, and fruits. Many turtle species, including tortoises, supplement their diet with leftover eggshells, animal bones, hair and droppings for extra nutrients.
Turtle generally eat their food in a straightforward way though some species have special feeding techniques. The yellow-spotted river turtle and the painted turtle filter feed by skimming the water surface with their mouth and throat open to collect particles of food. When their mouth closes, the throat constricts; excess water is pushed out through the nostrils and the gap in between the almost closed jaws. Some species, like the mata mata, employ a "gap-and-suck method" where the turtle opens its jaws and expands its throat widely, sucking the prey in.
Individuals within a species, may differ in diet depending on age, sex, population and between seasons. In many species, juveniles are generally carnivorous but become more herbivorous as adults. With Barbour's map turtle, the larger female mainly eats mollusks while the male eats mostly arthropods. Blanding's turtle may feed on mostly on snails or crayfish depending on the population. The European pond turtle has been recorded being mostly carnivorous much of the year but switching to water lilies during the summer. Some species have developed specialized diets such as the Mekong snail-eating turtle, the hawksbill, which specializes on sponges, and the leatherback, which feeds on jellyfish.
Migration.
Sea turtles nest in a specific area, such as a beach, leaving the eggs to hatch unattended. The young turtles leave that area, migrating long distances in the years or decades in which they grow to maturity, and then return seemingly to the same area, though the precision varies between species and populations. This "natal homing" appeared remarkable to biologists, though there is now plentiful evidence for it, including from genetics. The mechanism used in turtles remains unknown. One possibility is imprinting as in salmon, where the young learn the chemical signature, effectively the scent, of their home waters before leaving, and remember that when the time comes for them to return as adults. Another possible cue is the orientation of the earth's magnetic field at the natal beach; there is experimental evidence that turtles have an effective magnetic sense, and that they use this in navigation. Proof that homing occurs is derived from genetic analysis of populations of loggerheads, hawksbills, leatherbacks, and olive ridleys by nesting place; for each of these species, the populations in different places have their own mitochondrial DNA genetic signatures which persist over the years, showing that the populations are distinct, so that homing must be occurring reliably.
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. 
Intelligence.
Case studies exist of play behaviour in some turtle species. 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 nine 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. Galápagos tortoises have a hierarchical rank determined by dominance displays, and access to food and mates is regulated by this dominance hierarchy.
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 attemptd copulation by trying to insert his grasping tail. Male radiated tortoises 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.
Egg-laying and development.
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.
Systematics and evolution.
Fossil history.
The first proto-turtle body fossils are recorded from the late Triassic, about 220 million years ago, and their shell, which has remained a remarkably stable body plan, evolved from bony extensions of their backbones and broad ribs that expanded and grew together to offer increasingly complete protection. This is supported by fossils of the freshwater "Odontochelys semitestacea", from the late Triassic of Guangling in southwest China. It has a complete bony plastron and an incomplete carapace. 
The earliest known fully-shelled member of the turtle lineage is the late Triassic "Proganochelys". It lacked the ability to pull its head into its shell, had a long neck, and had a long, spiked tail ending in a club, somewhat like an ankylosaur. This genus already possessed many advanced turtle traits, suggesting 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 in the Early Triassic. 
Turtles underwent an adaptive radiation in the Jurassic, greatly increasing the number and diversity of fossil species. In a few places, paleontologists have unearthed large numbers of Jurassic or Cretaceous turtle skeletons accumulated in a single area, such as the Nemegt Formation in Mongolia, the Turtle Graveyard in North Dakota, the Black Mountain Turtle Layer in Wyoming), and in Shanshan County, Xinjiang, where over a thousand ancient freshwater turtles died after the last water hole in an area dried out during a major drought. Hatchling and nestling size fossils have been documented. The world's largest turtle, "Carbonemys cofrinii", from a coal mine in Colombia, lived around 60 million years ago and was nearly long. 
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 Anapsida, 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 could not be ruled out at that time. 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 . The most recent common ancestor of living turtles, corresponding to the split between Pleurodira and Cryptodira, was estimated to have occurred around . 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 a diapsid-like skull with temporal openings behind the eye socket, whereas turtles now possess an anapsid-like skull without such openings. The external phylogeny of the turtles is shown in the cladogram below.
Internal phylogeny.
The cladogram, from Thompson et al, 2021, shows the internal phylogeny of the Testudines down to the level of families. Thompson and colleagues comment that extant turtles have very low diversity, given the group's age. Diversity increased steadily in their analysis, speciation occurring at a greater rate than extinction, except for a single rapid increase around the Eocene-Oligocene boundary some 30 million years ago, and a major regional extinction at roughly the same time. They suggest that global climate change caused both events, as the cooling and drying caused land to become arid and turtles to become extinct there, while new continental margins exposed by the climate change provided habitats for other species to evolve.
Differences between the two suborders.
Turtles are divided into two extant suborders: Cryptodira and Pleurodira. Turtles in the two groups differ in the way the neck is retracted, and in the shape of the head. The Cryptodira is the larger group, and includes all the marine turtles, the terrestrial tortoises, and many of the freshwater turtles. The Pleurodira or side-necked turtles is a smaller group, consisting primarily of freshwater turtles. Until 3,000 years ago, there was a third group: the family Meiolaniidae was then extant, but it lay outside the Testudines crown group, belonging to Perichelydia.
Pleurodirans retract their neck laterally to the side, anterior to the shoulder girdles, whereas cryptodirans retract their neck back into their shell by bending the neck in an S-like shape. These motions are largely due to the morphology and arrangement of cervical vertebrae. The cervical column consists of nine joints and eight independent vertebrae. These vertebrae are round and not fused, making the neck more flexible and allowing it to bend backwards and sideways. The primary function and evolutionary implication of neck retraction is thought to be for feeding rather than protection. Neck retraction and extension allow the turtle to reach out further to capture prey while swimming. Neck extension 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 shape of the head differs between the two suborders, as the jaw musculature is associated with different bones in the two groups. The adductor muscles in the lower jaw create a pulley-like system in both subgroups; however the bone in which the muscles articulate with differ. In Pleurodira, the pulley is formed with the pterygoid bones, but in Cryptodires the pulley is formed with the quadrate bones. Both of these systems help to vertically redirect the adductor muscles in order to create a powerful bite.
Distribution and habitat.
Turtles are widely distributed across the world's continents and oceans, being absent mainly from the polar regions, the northern parts of North America and Eurasia, and the driest regions of the Sahara, Arabia and Australia. There are terrestrial, fully aquatic, and semi-aquatic species, and within those realms they live in a wide range of habitats from pelagic (open ocean) to rivers, ponds, rainforest and deserts. Individual species have much narrower habitat preferences, so that few ecoregions have more than 5 species living together. 
The two major groups of turtles have different distributions and habitat ranges. The Pleurodira are all semi-aquatic or fully aquatic, and are found only in the Southern Hemisphere. 
The Cryptodira include terrestrial, freshwater and marine species; these are found across the Northern Hemisphere, and in South America and Africa within the Southern Hemisphere.
Turtles that are mainly terrestrial have eyes positioned either side of the middle of the head to allow the animal to look down at what is in front of it. Aquatic turtles that raise the top of their head from the water to look out for predators, like snapping turtles, have eyes towards the top of their head.
Richest regions.
The world regions richest in turtle species are the Amazon basin, the Southeastern United States, the coastal countries of tropical West Africa, and an extended area of South and Southeast Asia from the Himalayas to Bengal, Myanmar and Thailand to the Malay peninsula, Sumatra, the island of Borneo, Cambodia, Vietnam, and the southern coastal area of China (the south of Guangxi and Guangdong provinces).
Habitat range.
Some turtles are found at high altitude; for example, the species "Terrapene ornata" occurs up to in New Mexico. Conversely, the leatherback sea turtle, "Dermochelys coriacea", can dive to .
The desert tortoises, "Gopherus" spp. can tolerate body temperatures from below freezing to at least , though they are inactive (remaining in their burrows) at the lowest and highest temperatures.
Human uses.
In culture.
Turtles have featured in human cultures across the world since ancient times. The ancient Greeks named their lyre, a stringed musical instrument, the chelys or "tortoise"; it was turtle-shaped, and its domed back was made either of tortoiseshell or of wood in the same shape. It was said in the "Homeric Hymn to Hermes" to have been invented by the god Hermes, when he came across a tortoise near his mother's home and used it as the soundbox of a seven-stringed instrument. The army of Ancient Rome used the "testudo" ("tortoise") formation in battles, especially sieges; it consisted of a tight pack of infantry, their shields held overhead to form a shield wall like the interlocking scutes of a tortoise's shell. In Aesop's Fables, "The Tortoise and the Hare" tells how an unequal race may be won by the slower partner. In Hindu mythology, the World Turtle Kurma or Kacchapa, an avatar of Vishnu, supports the whole world on his back; the yoga pose Kurmasana is named for the avatar. Marine turtles feature significantly in Australian Aboriginal art.
Lewis Carroll's 1865 "Alice's Adventures in Wonderland" features a Mock Turtle, named for a soup meant to imitate the expensive soup made from real turtle meat. More recently, turtles have featured in comic books and animations such as of the 1984 Teenage Mutant Ninja Turtles. Since the start of the 20th century, people in North America have organized turtle races, sometimes with children riding them.
As pets.
Some turtles, particularly small terrestrial and freshwater species, are kept as pets. They are seen by some people as cheap pets that need little care, underestimating the complexity and expense of proper turtle and tortoise husbandry. In Europe, turtle 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. In the United States, because of the risk of contracting salmonellosis through casual contact with turtles, it is illegal to sell turtles under long as pets; but many stores and flea markets exploit a loophole which allows turtles under to be sold for educational purposes.
As food and other uses.
The flesh of turtles, "calipash" or "calipee", has long been considered a delicacy in Asian cultures, while turtle soup was once a prized dish in Anglo-American cuisine. Gopher tortoise stew has been popular with some groups in Florida. 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.
Tortoiseshell, usually from the hawksbill turtle, has been used for centuries to make jewellery, tools and ornaments around the Western Pacific. It was widely used around the world, including in Europe, until the trade was banned in 2014. The material was sliced thinly to manufacture practical and decorative items such as combs, spectacle frames, furniture inlays, and guitar picks.
Conservation.
With between 48 and 54% of all 328 species considered threatened, turtles 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, the pet trade, light pollution, and climate change. The high extinction risk for Asian species is primarily due to their long-term unsustainable exploitation 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-Tertiary 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. Chinese entrepreneurs have sought to satisfy increasing demand for turtle meat as gourmet food and traditional medicine with farmed turtles, instead of wild-caught ones; according to a 2007 study, 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 what conservationists have called "the Asian turtle crisis". In the words of the biologist George Amato, the hunting of turtles "vacuumed up entire species from areas in Southeast Asia", even as biologists still did not know how many species lived in the region. About 75% of Asia's 90 tortoise and freshwater turtle species are considered threatened. In 2000, all the Asian box turtles ("Cuora" spp.) were placed on the CITES list of endangered species.
Harvesting wild turtles is legal in some American states; most of the catch 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.
Large numbers of marine turtles are accidentally killed in the nets of fishing trawlers as bycatch. A 2010 study suggested that over 8 million had been killed in 20 years; the Eastern Pacific and the Mediterranean were identified as among the areas worst affected. In 1987, the United States required all shrimp trawlers to fit their nets with turtle excluder devices; these have bars preventing turtles from being swept into the back of the net and drowning.
More locally, other human activities are affecting marine turtles. 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.