Animal Diversity Web

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).