The aquatic softshell turtles are found today in Africa, Asia, the Indo-Australian archipelago, and North America. The family Trionychidae is an ancient one with fossils dating back to a least the lower Cretaceous of Kirgiza (Nesov, in Kordikova, 1991). Although numerous fossil species are known, only 24, belonging to 14 genera, live today (following the generic designations proposed in Meylan, 1987).
Softshells are a highly derived group with rounded, flattened carapaces lacking horny scutes and covered by a leathery skin. Their neck is long and retractile, and limbs are paddlelike, with three claws on each. The snout is usually a long proboscis. Since the shell lacks scutes, much emphasis has been placed on its bones. The arrangement of the typical carapacial and plastral bones is shown in the accompanying illustrations (Carapacial bones of Apalone, Plastral bones of Apalone). As used, the epiplastral element represents the anterior extension of the entoplastron. Bony elements of the shell are secondarily reduced; pygals and suprapygals are absent, as are also peripherals, except in the genus Lissemys, and the distal ends of the ribs project freely in juveniles and adults of some species. There are normally 7 or 8 neurals and 7 to 10 pairs of costals, with the last pair often meeting at the midline. A central lacuna occurs on the plastron. Several callosities (superficial ossifications closely connected to the underlying plastral bones) occur on the adult plastron (although barely present in adult Dogania subplana). Cutaneous femoral valves (flaps), which cover the hind limbs when they are withdrawn, are present on the plastron of several Old World genera. Neck vertebrae are slender, and none is biconvex. The 4th digit has four to six phalanges; the 5th has two to four. The temporal region of the skull is widely open. The premaxillae are fused, and neither the parietal nor the postorbital touches the squamosal. Reduction of the vomer allows palatine contact. It is usually separated from the maxillae and prefrontals (in contact in Chitra indica and Cyclanorbis elegans), but touches the premaxillae and separates the internal nares. The jugal touches the parietal. A large nasopalatine foramen is present; the palatine fenestra is small. The maxilla touches the pterygoid but not the quadratojugal, and the basisphenoid is separate from the pterygoid but touches the palatines. The quadrate encloses the stapes, and an epipterygoid is present. The jaw is very deep at the level of the coronoid process, and the mandible reaches laterally nearly to the posterior end of the jaw. The family is divided into two subfamilies: Cyclanorbinae and Trionychinae.
Jump to the key of the family Page 276: Trionychidae.