Geologic range.—Lower Cretaceous (possibly Upper Jurassic) to Recent of Asia; Upper Cretaceous to Recent of North America; Paleocene (Upper Jurassic, assuming Trionyx primoevus is a trionychid) to Pleistocene of Europe; Lower Miocene to Recent of Africa; Pleistocene to Recent in East Indies (Loveridge and Williams, op. cit.:412; Romer, 1945:594); questionable trionychid fragments from Pleistocene of Australia (Darlington, loc. cit.).
Remarks.—The genera Lissemys, Cyclanorbis and Cycloderma are distinguished from Pelochelys, Chitra and Trionyx by several characters (Loveridge and Williams, op. cit.:414). The recognition of two groups of genera caused Deraniyagala (1939:290) to erect two families, Cyclanorbidae and Trionychidae. An appraisal of fossils prompted Hummel (1929:768) to propose two corresponding subfamilies, Cyclanorbinae and Trionychinae. Williams (1950:554) considered the two groups as subfamilies (Lissemydinae and Trionychinae).
Baur (1887:97) regarded the Trionychidae as constituting a separate suborder distinct from the rest of the living turtles. Later (1891), however, he pointed out the resemblances of the Trionychidae and Carettochelyidae (having one living genus in New Guinea), and the cryptodiran affinities of Carettochelys. Bergounioux (1932:1408) mentioned the close resemblance of the Carettochelyidae to Trionyx but considered the former as having pleurodiran affinities, a view adopted by Deraniyagala (loc. cit.). Most students now consider the two families to be closely related, and conceive of both as members of the suborder Cryptodira (Hummel, 1929:768; Williams, loc. cit.; Mertens and Wermuth, 1955).
The oldest trionychid fossil, Trionyx primoevus, is from marine deposits of the Upper Jurassic (Kiméridgien) from "Cap de la Hève," and its characters do not indicate the kind of cryptodiran ancestor from which the family arose (Bergounioux, op. cit.:1409; 1937:188). Lane (1910:350) found that the entoplastron (= epiplastron) was paired in embryos of Trionyx and regarded that genus as the most primitive of the order; he also mentioned Wiedersheim's report of rudiments of teeth in embryos of Trionyx. Baur (1891:637-38) thought that the family arose directly from the Amphichelydia, that the ancestors of the Trionychidae closely resembled Carettochelys in the structure of the carapace and plastron, and that a progressive reduction in ossification of those structures occurred. Nopcsa (1926:654) also wrote that the family originated from ancestors having a well-developed plastron; he maintained that the progressive reduction in ossification of the plastron was a specialization for aquatic life, and that the more primitive trionychids had the best developed bones and callosities. Hummel (1929:772) also thought that there had been a progressive reduction in ossification. Bergounioux (1932:1408; 1936:1088, 1952:2304), on the contrary, thought that there had been a progressive increase in ossification of the marginal bones in both families as well as of the plastron (1936:1088; 1937:190). Zangerl's study of the shell elements of turtles (1939:393) indicated that Trionyx was highly specialized in having a well-developed epithecal armor (sculptured callosities, neurals and costals), and that it occurred in most aquatic turtles; the development in soft-shells suggested that members of the family had maintained an aquatic mode of life over a long period of geologic time, a view supported by Deraniyagala (1930:1066). Of interest are Stunkard's remarks (1930:214-18) concerning several Trionyx spinifer that were obtained from a commercial supply house and found to be infested with pronocephalid trematodes (Opisthoporus [= Teloporia] aspidonectes). The closest relatives of that trematode (also recorded from T. ferox) live in marine turtles. Possibly, a Mesozoic ancestor of marine and essentially fresh-water soft-shelled turtles harboured ancestors of these trematodes, but possibly the parasites may have transferred relatively recently to their present hosts. Bergounioux (1937:190) judged the Trionychidae to be an ancient group of marine origin. Hummel (1929:770) wrote that the Trionychidae originated in east Asia (the region of most differentiation) in humid climates.
Baur (1891:634, 637) pointed out that the dorsal aspect of the skull of the closely related Carettochelys resembles the skull of the Dermatemydidae, Staurotypidae and Kinosternidae; the close relationship of Carettochelys and the Dermatemydidae is also mentioned by Bergounioux (1952:2304) and Hummel (1929:769). Hummel (op. cit.:771) thought that the Carettochelyidae and "die Chelydroiden" had a common ancestor, and that (op. cit.:772) the origin of the Trionychidae was older than those two groups. Dunn (1931:109) wrote that the Kinosternidae, Carettochelyidae and Dermatemydidae represented the same general ancestry. Williams (1950:552) has shown the resemblance of the cervical articulations in members of the Chelydridae (including Staurotypinae and Kinosterninae) and the Central American family Dermatemydidae. The consensus of opinion, then, is that the families Trionychidae, Carettochelyidae, Chelydridae and Dermatemydidae are relatively closely related.
Genus Trionyx Geoffroy, 1809
Testudo Linnaeus (in part), Syst. Nat., Ed. 10, 1:197, 1758; type, Testudo graeca Linnaeus by subsequent designation (Fitzinger, 1843:29).
Trionyx Geoffroy, Ann. Mus. Hist. Nat. Paris, 14:1, August, 1809; type, Trionyx aegyptiacus (= Testudo triunguis Forskål) by original designation.
Apalone Rafinesque, Atlan. Jour., Friend of Knowledge, Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type, Apalone hudsonica (= Trionyx spiniferus Lesueur) by monotypy.