Turtles and tortoises

Turtle Facts

Classification:

• Kingdom: Animalia
• Phylum: Chordata
• Class: Reptilia
• Order: Testudines
• Geographical location: Worldwide
• Habitat: Terrestrial, aquatic, and marine habitats, except at high latitudes
• Gestational period: Varies according to temperature
• Life span: Turtles live longer than other vertebrates, with some small species living fifty years and larger forms up to two hundred years
• Special anatomy: One of the only living vertebrates with shells; lack teeth and instead have a horny beak; neck vertebrae are modified to allow the head to be extended or retracted from the shell; lungs are compressed and expanded by altering the position of other, attached internal organs

Turtles, tortoises, and terrapins are all turtles, a term that encompasses all species of the order Testudines. The term “tortoise” is used for terrestrial turtles with high-domed shells and elephantine hindlimbs, whereas the term “terrapin” is used properly for some highly aquatic turtles of eastern North America.

Turtles are easily recognized and distinguished from all other vertebrates by their shells. Shells are composed of a dorsal carapace and a ventral plastron. These are usually rigidly connected on the sides by bridges. Shells are composed of bony plates that form within the skin. These are fused to underlying vertebrae and ribs. Most shells have a covering of horny plates made of keratin, a protein which, in other vertebrates, forms scales, hair, nails, claws, or horns. In some turtles, the plates of bone and keratin are reduced or absent, and the shell is covered by leathery skin. Many turtles have one or more hinges in their shells, usually in the plastron. These allow the shell to completely enclose the withdrawn head, limbs, and tail. The plastron of males in many species is indented to accommodate the female’s shell during mating.

Turtle Lifestyles

Shell shape largely determines the lifestyle of its owner. Terrestrial (land-dwelling) turtles such as box turtles (Terrapene) and tortoises (Geochelone) have high-domed shells. These reduce surface area through which water is lost and also are difficult for predators to grasp and break. Most aquatic and all marine turtles have relatively flat, streamlined shells for ease in swimming. However, pancake tortoises (Malacochersus tornieri) have flat shells that allow them to hide in rocky crevices, and some bottom-dwelling aquatic turtles, such as the mud turtles (Kinosternon) of the southeastern United States, have high-domed shells. Snapping turtles in the genera Chelydra and Macrochelys have rough shells on which algae grow. This camouflages these turtles as they wait to ambush prey.

Limbs also provide clues to lifestyles. Aquatic turtles have webbed feet, and sea turtles have forelimbs modified into flippers that allow them to “fly” through water. In contrast, terrestrial turtles often have spadelike feet for digging and/or columnlike limbs to support them as they walk. Regardless of shape or function, the girdles to which the limbs attach are enclosed by the ribs and shell. Turtles are the only vertebrates with this skeletal arrangement.

Other anatomical modifications include nostrils on top of the snout or at the very tip of a long proboscis; these allow aquatic turtles to breathe at the surface with minimal exposure. Modern turtles, like modern birds, lack teeth. Instead, they have horny beaks of keratin variously shaped to cut leaves, tear flesh, or crush the shells of snails or clams. Because the shell prevents expansion and contraction of the thorax when breathing, turtles compress or expand the lungs by altering the location of other internal organs to which the lungs are attached. Shells limit mobility to a great extent; consequently, turtles have long and flexible necks. These allow them to reach up to browse or down to graze, or to quickly extend their necks in order to ambush quicker prey. In addition, neck vertebrae are modified to allow the head to be withdrawn into the shell, either by pulling it straight back while the neck assumes an S-shape (cryptodiran turtles) or by laying it to the side under the overhanging lip of the carapace (Austro-American side-necked turtles and Afro-American side-necked turtles).

Origins and Future of Turtles

Fossil turtles are known from the Jurassic. Most systematists (biologists who study evolutionary relationships) group turtles with some extinct relatives in a clade called the Parareptilia. Although turtles traditionally have been considered reptiles, many experts now place them a separate vertebrate class. Regardless, the ancestors of turtles arose from the first amniotes before the ancestors of other reptiles. This and their many unique features have been cited as reasons for placing turtles into their own class.

Unlike many reptiles, turtles are perceived positively by most people. Nevertheless, many species are threatened or endangered. Habitat destruction and alteration are responsible in most cases. Aquatic habitats are drained or polluted and nesting sites, especially beaches, are developed, rendering them unusable by turtles. Many species are exploited as food, either as eggs or adults, and others are killed for their shells or body parts, which are thought by some cultures to have medicinal or aphrodisiac qualities. Exotic predators, such as rats and dogs, dig up nests and kill adults. Hundreds of thousands of wild-caught turtles die each year in the pet trade, much of it illegal. Many species become roadkill when they migrate to new habitats or breeding sites. Only a few species are formally protected in at least some parts of their ranges, and several, including the sea turtles, may be nearing extinction in spite of efforts to conserve them.

Principal Terms

amniote: animals with eggs in which embryos develop within fluid-filled membranes (the amnion), allowing eggs to be laid on land; amniotes include reptiles, birds, and mammals

bridge: the portion of the shell that connects the carapace to the plastron

carapace: the portion of the shell that covers a turtle’s back (dorsum)

clade: a group of animals and their common ancestor

plastron: the portion of the shell that protects a turtle’s belly (venter)

Bibliography

Alderton, David. Turtles and Tortoises of the World. New York: Facts on File, 1988. Print.

Ernst, Carl H., and Roger W. Barbour. Turtles of the World. Washington, DC: Smithsonian Inst., 1989. Print.

Ernst, Carl H., Roger W. Barbour, and Jeffrey E. Lovich. Turtles of the United States and Canada. Washington, DC: Smithsonian Inst., 1994. Print.

Laurin, Michel, and Robert R. Reisz. “A Reevaluation of Early Amniote Phylogeny.” Zoological Journal of the Linnean Society 113 (1995): 105–225. Print.

Pough, F. Harvey, et al. Herpetology. 4th ed. Sunderland: Sinauer, 2015. Print.

Vitt, Laurie J., and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 4th ed. London: Academic, 2013. Print.