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Temporal range: Late Jurassic–Present
Turtle diversity.jpg
Turtles from different families; clockwise from top-left: Red-bellied short-necked turtle, Indian flapshell turtle, Hawksbill sea turtle, and Galápagos tortoise
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Pantestudines
Clade: Testudinata
Clade: Perichelydia
Order: Testudines
Batsch, 1788[1]
14 living families
Blue: sea turtles, black: land turtles
  • Chelonii Latreille 1800
  • Chelonia Ross and Macartney 1802

Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtles), which differ in the way the head retracts. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins. They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. Like other amniotes (reptiles, birds, and mammals) they breathe air and do not lay eggs underwater, although many species live in or around water.

Turtle shells are made mostly of

plastron or belly-plate. Its outer surface is covered in scales made of keratin, the material of hair, horns, and claws. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body. Turtles are ectotherms or "cold-blooded", meaning that their internal temperature varies with their direct environment. They are generally opportunistic omnivores and mainly feed on plants and animals with limited movements. Many turtles migrate short distances seasonally. Sea turtles are the only reptiles that migrate long distances to lay their eggs
on a favored beach.

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 shells. Sea 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 extinct or threatened with extinction.

Naming and etymology

The word turtle is borrowed from the French word tortue or tortre 'turtle, tortoise'.[3] It is a common name and may be used without knowledge of taxonomic distinctions. In North America, it may denote the order as a whole. In Britain, the name is used for sea turtles as opposed to freshwater terrapins and land-dwelling tortoises. In Australia, which lacks true tortoises (family Testudinidae), non-marine turtles were traditionally called tortoises, but more recently turtle has been used for the entire group.[4]

The name of the order, Testudines (/tɛˈstjdɪnz/ teh-STEW-din-eez), is based on the Latin word testudo 'tortoise';[5] and was coined by German naturalist August Batsch in 1788.[1] The order has also been historically known as Chelonii (Latreille 1800) and Chelonia (Ross and Macartney 1802),[2] which are based on the Ancient Greek word χελώνη (chelone) 'tortoise'.[6][7] Testudines is the official order name due to the principle of priority.[2] The term chelonian is used as a formal name for members of the group.[1][8]

Anatomy and physiology


The largest

leatherback turtle, which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb).[9] The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg (4,900 lb).[10] The smallest living turtle is Chersobius signatus of South Africa, measuring no more than 10 cm (3.9 in) in length[11] and weighing 172 g (6.1 oz).[12]


Sagittal section
of a tortoise skeleton

The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements.[13][14][15] It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) carapace and the flatter, ventral (belly) plastron. They are connected by lateral (side) extensions of the plastron.[13][16]

The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs).[13] During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as fibroblast growth factors that include FGF10.[17] The shoulder girdle in turtles is made up of two bones, the scapula and the coracoid.[18] Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.[19]

Development of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.[17]

The shell is covered in

epidermal (outer skin) scales known as scutes that are made of keratin, the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals.[20] Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals. Side-necked turtles additionally have "intergular" scutes between the gulars.[16][21] Turtle scutes are usually structured like mosaic tiles, but some species, like the hawksbill sea turtle, have overlapping scutes on the carapace.[16]

The shapes of turtle shells vary with the adaptations of the individual species, and sometimes with sex. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce drag and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extra protection from predators and camouflage against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.[22]

Shells vary in flexibility. Some species, such as

Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thick connective tissue and an outer layer of leathery skin.[23]

Head and neck

Closeup of the head and neck of turtle
Head and neck of a European pond turtle

The turtle's skull is unique among living

temporal fenestrae).[24][25] Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshells to the broad and flattened skull of the mata mata.[25] Some turtle species have developed large and thick heads, allowing for greater muscle mass and stronger bites.[26]

Turtles that are carnivorous or

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

Limbs and locomotion

Due to their heavy shells, turtles are slow-moving on land. A

mud turtles, mainly walk along the water bottom, as they would on land. Others, such as terrapins, swim by paddling with all four limbs, switching between the opposing front and hind limbs, which keeps their direction stable.[13][33]

streamlined shells and limbs adapted for fast and efficient swimming.[34]

Sea turtles and the pig-nosed turtle are the most specialized for swimming. 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.[13][35] Sea turtles such as the green sea turtle rotate the front limb flippers like a bird's wings to 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 the Caspian turtle, which uses 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.[34]

Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. Snapping turtles and the

prehensile in males, who use it to grasp mates. Several turtle species have spines on their tails.[36][24]


head of a red-eared slider turtle
The red-eared slider has an exceptional seven types of color-detecting cells in its eyes.[37]

Turtles make use of vision to find food and mates, avoid predators, and orient themselves. The retina's light-sensitive cells include both rods for vision in low light, and cones with three different photopigments for bright light, where they have full-color vision. There is possibly a fourth type of cone that detects ultraviolet, as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, the red-eared slider, has an exceptional seven types of cone cell.[37][38][39]

Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, the

polarized light for orientation as many other animals do. The deep-diving leatherback turtle lacks specific adaptations to low light, such as large eyes, large lenses, or a reflective tapetum. It may rely on seeing the bioluminescence of prey when hunting in deep water.[37]

Turtles have no ear openings; the

Hz in air, but underwater they are more attuned to lower frequencies.[40] The loggerhead sea turtle has been shown experimentally to respond to low sounds, with maximal sensitivity between 100 and 400 Hz.[41]

Turtles have olfactory (smell) and vomeronasal receptors along the nasal cavity, the latter of which are used to detect chemical signals.[42] Experiments on green sea turtles showed they could learn to respond to a selection of different odorant chemicals such as triethylamine and cinnamaldehyde, which were detected by olfaction in the nose. Such signals could be used in navigation.[43]


The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration.[44][45][46] The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs.[47] They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.[48]

The lungs are ventilated using specific groups of abdominal muscles attached to the organs that pull and push on them.

transversus abdominis muscle propels the organs into the lungs and expels air. Conversely, during inhalation, the relaxing and flattening of the oblique abdominis muscle pulls the transversus back down, allowing air back into the lungs.[45]

Although many turtles spend large amounts of their lives underwater, all turtles breathe air and must surface at regular intervals to refill their lungs. Depending on the species, immersion periods vary between a minute and an hour.[50] Some species can respire through the cloaca, which contains large sacs that are lined with many finger-like projections that take up dissolved oxygen from the water.[51]


brumation, in which it buried itself in mud. Turtles have multiple circulatory and physiological adaptations to enable them to go long periods without breathing.[52]