Turtles are familiar, four-legged reptiles whose body is enclosed within a bony shell. Turtles constitute the reptilian order Testudines. The approximately 290 living species inhabit all continents except Antarctica, plus many islands, and there are marine turtles in all tropical and temperate oceans.

History and fossil record

Turtles first appear in the fossil record of the Triassic period, from about 215 million years ago. This gives them an older fossil history than any other living kind of four-legged animal. Turtles were already present when the first dinosaurs appeared, and they shared the ancient seas with ichthyosaurs, watched pterosaurs soar overhead, and saw the first small, furry mammals. Undergoing relatively little change themselves, turtles witnessed the evolution of birds from feathered dinosaurs, and they were present as some of the early mammals evolved into elephants, whales, bats, and even human beings.

The earliest known fossil turtles (Proganochelys ) are from late Triassic (Norian) sedimentary deposits in Germany. These ancestral turtles had a 3 ft (1 m) shell length, and were terrestrial or marsh-dwelling animals. Like modern turtles, the shell of the fossil animals was composed of a rounded upper half (or carapace) and a flattened lower one (plastron). The carapace of the earliest fossil turtles incorporated 10 vertebrae, their associated ribs, and additional bone between the ribs. The skull was solidly constructed, without temporal openings, and the jaws were toothless and presumably beaked as in modern turtles. There were, however, small teeth in the palate, which are not found among living turtles. The eight cervical (neck) vertebrae were primitive, in that they were not modified to allow the head to withdraw into the shell.

Turtles are fairly large animals, and have a great deal of bone. In addition, they often occur in aquatic or marshy habitats where their bones are likely to be buried and preserved. Consequently, their fossils are found relatively frequently in ancient sedimentary deposits from the Jurassic and younger eras. Many fossils are found in Cretaceous deposits of North America.

Morphology

turtles have an unmistakable appearance, with a head, tail, and four legs projecting from a broad bony shell. The domed upper carapace and flattened lower plastron serve to protect the torso and its organs. In most land-dwelling and amphibious turtles the head, limbs, and tail can be withdrawn inside this shell if danger threatens. A horny beak (like that of a bird) covers the jaws. The head, legs, and tail are covered by horny scales, and the feet have horny nails. The bony shell is covered with an epidermal layer of scutes in most turtles, occurring in a regular pattern that may be diagnostic of the species. Only the leatherback sea turtle and the softshell freshwater turtles lack these horny plates.

The internal anatomy of turtles is rather typical of vertebrate animals, with two lungs, a reptilian three-chambered heart and associated circulatory system, and an unremarkable digestive system with an esophagus, stomach, small and large intestines, and an associated liver. As with other reptiles, the digestive,

urinary, and reproductive systems vent to the outside through a chamber known as the cloaca, which also encloses the penis of male animals. The nervous system comprises a well-developed brain, a spinal cord enclosed in a vertebral column, and peripheral nerves. The senses of vision (including color vision) and hearing are acute.

Species of turtles have an extreme size range. Adult American mud turtles (Kinosternon subrubum) are less than 5.0 in (12.5 cm) long, while the gigantic leatherback sea turtle (Dermochelys coriacea) can attain a length of more than 6 ft (183 cm) and weight up to 1,500 lb (680 kg).

Ecology

most modern species of turtles are semi-aquatic, living in such habitats as ponds, swamps, and marshes. Several species are marine. In fact, turtles have diversified into species that are specialized in various ways. The sea turtles, for example, are ocean-dwelling animals that fly through the water using their paddle like forelimbs, emerging on land only to lay their eggs. Others turtles, such as the soft shells, are river- and lake-dwellers, and are flattened like a pancake to hide on sandy or muddy bottom habitat. Still others, such as the tortoises, are strictly land-dwellers, with a high domed shell, elephant like feet, and ranging into grasslands and semi-desert habitats.

Many turtles are omnivores, eating both plants and animals, but others are more specialized in their food habits. The giant tortoises and some of the sea turtles are vegetarian as adults, although their young may eat invertebrates or small vertebrates. Some river turtles, such as the map turtles (Graptemys spp.), are specialists that feed only on snails and clams. Softshell turtles (Trionyx spp. ) are mainly fish-eaters, while snapping turtles (Chelydra serpentina ) will eat any animal they can subdue.

Behavior and life history

Most species of turtles mate in the spring or early summer, when the males actively search for receptive females. Courtship may include interesting behavior, such as that of the male red-eared turtle (Trachemys scripta ), which swims backward in front of the female, while stroking her head and neck with his greatly elongated front claws. The males of some tortoises make noises during courtship or mating. Smaller species, such as the Mediterranean tortoise (Testudo ), cluck like chickens, whereas the giant Galàpagos tortoise (Geochelone ) bellows.

All turtles lay eggs, which vary in shape from cylindrical to spherical. Smaller species may lay only two or three eggs in a clutch, while sea turtles may lay three or more clutches of 100-150 eggs in a year. Most turtles dig a nesting cavity with their hind feet, lay their eggs inside, and cover the entrance, leaving the eggs to be incubated by the heat of the sun. Typically, the eggs hatch in 60-90 days.

Most turtles are long-lived. The eastern box turtle (Terrapene carolina ) may live 100 years, and giant tortoises have been reported to live for more than 150 years. Studies of the vigor of populations of turtles must take the age structure into account. For example, although populations of giant tortoises on certain Galàpagos Islands have numerous large individuals, they may nevertheless be endangered if no young are being produced because of excessive predation by introduced mammals.

Classification

the turtles are separated into two major groups (subclasses) that can be readily identified by the way they retract their head into their shell.

Sideneck turtles

The “sideneck” turtles (Pleurodira) fold their neck into a lateral S-shape, so when the head is retracted one side is tucked between the shells. This is a relatively small group of mainly pond-dwelling animals found in South America, Africa (including Madagascar), and Australia. The African sideneck turtles (Pelomedusidae) are found in Africa, Madagascar, and the Seychelles Islands in the Indian Ocean, while the Australo-American sideneck turtles (Chelidae) inhabit South America, Indonesia, Australia, and New Guinea.

One of the best-known sidenecks is the bizarre-looking matamata (Chelus fimbriatus ) of South America, which lies in wait on the bottom of ponds or rivers until a fish comes near, then suddenly opens its mouth and expands its throat to “suck in”; its prey.

Hidden-neck turtles

The “hidden-neck”; turtles (Cryptodira) retract their head with the neck in a vertical S-shape, appearing to pull the head directly into the shell with nothing showing but the snout. This is a larger and much more diversified group, and includes the pond turtles, the land-dwelling tortoises, and the large sea turtles. Members of this group are found throughout the temperate and tropical regions of the world.

Three of the more primitive families are found mainly in North America, with some ranging also into Central and South America. The snapping turtles (Chelydridae) comprise two genera, each with a single species. The common snapper (Chelydra serpentina ) is best known, and ranges from Canada to Ecuador. It is a large turtle, with a shell length up to 18 in (47 cm), and a long tail. The even larger alligator snapper (Macrochelys temminckii ) can exceed 24 in (66 cm) in shell length. It feeds mainly on fish that it attracts with a worm like “bait”; on its tongue. Both species lay 20-80 spherical eggs in a flask-shaped hole dug into a sandy bank.

The mud and musk turtles (Kinosternidae) include about 25 small- to medium-sized species found from Canada to Brazil. Most of these are carnivorous, feeding on insects, worms, and other small animals. They lay only a few (two to 10) elongate, brittle-shelled eggs.

The highly aquatic Central American river turtle (Dermatemys mawii ) is the sole member of its family, Dermatemydidae. It is a large species, with a shell length of up to 25 in (65 cm). It is seldom found more than a few feet from water, is herbivorous, and highly prized as food by people living in its range.

The pond turtles (Emydidae) and tortoises (Testudinidae) comprise the largest numbers of turtle species, and occur throughout the tropical and temperate regions of the world, other than Australasia. The American pond turtles are closely related to the pond turtles of southern Asia, as are the European turtles.

The pelagic sea turtles comprise only a few genera. They are extremely large, conspicuous animals, and once occurred in huge numbers. Sea turtles spend almost all of their lives in the open sea, but must come to land to lay their eggs. They nest on certain tropical and subtropical beaches, and return year-after-year to these same places. Once the nesting beaches were in remote locations, but no longer; Miami Beach, for example, was once an important nesting place.

The largest of the sea turtles (and the biggest living turtle) is the giant leatherback. (Some taxonomists place this species in its own family, Dermochelyidae.) It lacks the horny plates that cover the shells of most turtles, and instead has a smooth leathery covering. It largely feeds on jellyfish and is the fastest swimmer of all turtles. It has a thick fatty layer under the skin that helps to retain body heat, as well as a heat-exchanging circulatory mechanism that conserves heat generated by muscular effort. These allow this species to range into cool waters during the northern summer, when jellyfish are abundant there.

Turtles and humans
Turtles as food

Large tortoises have long been used by humans as a source of meat. Some species have become extinct because of overhunting for this purpose. The only surviving giant tortoises live on islands that were relatively recently discovered by people, such as Aldabra in the Indian Ocean, and the Galàpagos Islands in the Pacific. The giant tortoises of the Galàpagos Islands were hunted by whalers because they could be kept alive for months in the hold of a ship, providing a source of fresh meat during the long whaling season. Female tortoises were preferred for this purpose, because the mature males were too heavy to carry. As a result, some islands were left only with large male tortoises. Predation of young tortoises by goats and rats introduced from the ships contributed to the population decline, and most of the giant tortoises are now endangered.

According to accounts of sailors of the sixteenth and seventeenth centuries, sea turtles used to occur in great flotillas in regions such as the Caribbean. However, all species of sea turtles, but especially the green turtle (Chelonia mydas ), were (and are) hunted for their meat. In addition, the hawksbill (Eretmochelys imbricata ) was killed for its beautiful “tortoise shell,”; which can be made into combs and ornaments (these are now illegal in the United States). The sea turtles are most vulnerable on their nesting beaches, where people and other predators may easily take the eggs and the female turtles. In spite of conservation measures initiated by many countries, sea turtle populations are continuing to decline throughout their range.

The American saltwater terrapin (Malaclemmys terrapin ) has also been eaten in large numbers, and declined precipitously in abundance. Fortunately, the initiation of conservation measures resulted in the survivors increasing to a greater abundance today.

Even the snapping turtle, one of the most common turtles in North America, has been over-exploited as a source of food. Turtle soup from these animals has been especially popular in Philadelphia, and for several decades a major soup company used thousands of turtles per year to supply the commercial demand, and others were used by restaurant chefs.

Captive turtles

Zoological parks have helped to save some turtle species from almost certain extinction. Some of these are large or brightly colored tortoises that provide good public exhibits. There is also a huge trade in turtles as pets. Turtle farmers in Louisiana and elsewhere have provided millions of baby red-eared sliders (Trachemys scripta ) to the pet trade. However, some turtle farms became contaminated with disease microorganisms such as Salmonella, and the sale of baby turtles was made illegal in many states.

Some rare species of turtles and tortoises are being bred and reared commercially, and can bring prices of more than $2,000 each. Europeans and Americans are major purchasers of captive-bred turtles. However, wild-caught animals are also being illegally sold, and this is an extremely serious risk to the survival of rare species.

The future of turtles

In addition to the hazards mentioned above, almost all species of turtles are suffering serious losses of their habitat because of the actions of humans. For example, the sand-hill habitat of the endangered Florida gopher tortoise (Gopherus polyphemus ) is prime land for development into residential areas and shopping malls. Similarly, the semi-desert habitat of the desert tortoise (Gopherus agassizii ) in California and Arizona is being damaged by motorcycles and off-road vehicles. And all over the world, wetlands are being dredged or drained for various reasons, so that valuable habitat for turtles and other wildlife is being destroyed. The automobile is another important threat to turtles. In the United States, many thousands of turtles are run over each year while trying to cross

KEY TERMS

Carapace —The upper part of a turtle’s shell.

Cryptodires —Hidden-necked turtles, found in most temperate and tropical regions; they fold their neck vertically.

Pelagic —Occurring in the open ocean.

Plastron —The bottom part of a turtle’s shell.

Pleurodires —Side-necked turtles, found only on the southern continents; they fold their neck laterally.

highways. There is no question that most species of turtles have been severely depleted in abundance, and are continuing to decline.

Resources

BOOKS

Dodd, C.K., Jr. North American Box Turtles: A Natural
History. Norman: University of Oklahoma Press, 2001.

Ernst, C.H., R.W. Barbour, and J.E. Lovich. Turtles of the
United States. Washington, DC: Smithsonian Books, 1994.

Ernst, C.H., and R.W. Barbour. Turtles of the World.
Washington D.C.: Smithsonian Institution Press, 1989.

Minton, S.A., Jr., and M.R. Minton. Giant Reptiles. New York: Scribner’s Sons, 1973.

Obst, F.J. Turtles, Tortoises and Terrapins. 2nd ed. New York: St. Martin’s Press, 1988.

Orenstein, R. Turtles, Tortoises and Terrapins: Survivors in Armor. Richmond Hill, ON: Firefly Books, 2001.

Spotila, J.R. Sea Turtles: A Complete Guide to Their Biology, Behavior and Conservation. Baltimore: Johns Hopkins University Press, 2004.

Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 2nd ed. New York: Academic Press, 2001.

Herndon G. Dowling

The Gale Encyclopedia of Science