Trionychidae

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The carapace of these turtles lacks scutes and is covered instead with a leathery skin.

The family Trionychidae contains approximately 25 species with 12 genera. Members of the family are distributed in eastern North America, Africa, Asia, and the Indo-Australian archipelago. Habitats include slow moving streams, swift rivers, lakes, ponds, and even brackish waters, but a soft bottom is requisite. These turtles spend much time buried in the mud, and basking is not common.

Superficially, softshells can be distinguished by their dorsoventral compression, leathery shells, and elongate snout. Much of the carapacial skeleton is lost, resulting in ribs with free ends. Numerous characters diagnose the Trionychidae, including lack of articulation between the centra of the last cervical and first thoracic vertebrae, claws present on only the medial three digits, and fleshy lips covering the usual keratin beak.

Carnivory is the rule for softshells, but some species are omnivorous. Crustaceans, insects, mollusks, fish, and amphibians are common prey. As in other turtles, eggs and hatchlings are much more susceptible to predation than adults. Vertebrates of all classes and many invertebrates are known predators on eggs and hatchlings, while only alligators and humans pose a threat to mature animals.

Courtship has been observed in a few species and involves acts such as head bobbing between a pair in some and the male rubbing the carapace of the female with his head in others. Overall, however, knowledge of reproductive behavior is poor. Females reproduce annually, and nests contain around 20 eggs. More than one clutch per season is often produced.

The Trionychidae are most closely related to the Carettochelyidae (pig-nosed turtles). These two families are together recognized as the Trionychoidae on the basis of several osteological characters, including opisthocoely in all but the first and last cervical vertebrae. Within the Trionychidae, two subfamilies are recognized. The Cyclanorbinae includes the genera Lissemys, Cycloderma, and Cyclanorbis and is distinguished by the presence of plastral skin flaps that cover the rear limbs when they are retracted. The Trionychinae contains the remaining genera, which lack plastral flaps.

Many fossil species of Trionychids are recognized. The fossils suggest a much broader distribution than what is currently known and extend the range to include Europe and South America. Dating from the Late Jurassic, Sinaspideretes wimani is considered the oldest member of the Trionychidae.

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

Ernst, C.H., Lovich, J.E., and Barbour, R.W. 1994. Turtles of the United States and Canada. Smithsonian Inst. Press, Washington, D.C.

Pough, F.H., Andrews, R.M., Cadle, J.E., Crump, M.L., Savitzky, A.H., and Wells, K.D. 2000. Herpetology, 2nd ed. Prentice Hall, Upper Saddle River, NJ.

Contributors

Keith Pecor (author).

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

motile

having the capacity to move from one place to another.