This is the more derived of the two pleurodiran families. These semi-aquatic side-necked turtles are now found in South America and the Australia-New Guinea region. The family dates back to unnamed fossils from the late Cretaceous of Argentina (De Broin, 1988, 1993). Other old fossils include the Paleocene Naiadochelys patagonia and an upper Eocene Hydromedusa (De Broin, 1988).
The skull is flattened and either broad or long. It is but little emarginated posteriorly, and the squamosal still touches the parietal (except in Chelodina) but is cut away ventrally with the loss of the quadratojugal. The small prefrontals are usually medially divided by the frontal bones. Nasal bones are present (except in Chelus). In the roof of the mouth, the palatines are separated by the vomer, and the premaxillae are usually not fused. In the lower jaw, the unfused dentaries are not as well-developed as in the Pelomedusidae, but a splenial bone is present. The carapace varies from arched with a medial keel (sometimes also a pair of lateral keels) to depressed with a medial groove. Neural bones are absent in several genera, but may total eight in others; where the neurals are missing the costal bones meet dorsally. A cervical scute is usually present (absent in Elseya). On the hingeless plastron there is a single intergular scute which may separate or lie behind the gulars, which may meet anterior to it. No mesoplastral bones are present. The 5th and 8th cervical vertebrae are biconvex, all others are procoelous. The hind toes are heavily webbed.
The family includes 11 genera with 43 species.
Wells and Wellington (1984, 1985) proposed mass changes to the taxonomy of Australian reptilians. In 1984, they stated that Emydura Bonaparte, 1836 was a nomen nudum, and resurrected the name Chelymys Gray, 1844 for the genus, in which they included the species krefftii, macquarrii, signata, subglobosa, and victoriae. They also suppressed the genus Rheodytes and assigned its species, leukops, to the genus Elseya without justification. In 1985, Wells and Wellington created the new genera Hesperochelonia (for Chelodina steindachneri), Macrochelodina (for Chelodina expansa, C. oblonga, C. rugosa, and C. siebenrocki), moved Chelymys victoriae to the newly created genus Tropicochelymys, and described the "new" species Chelodina rankini, Chelymys cooki, C. johncanni, C. windorah, Elseya purvisi, E. sterlingi, Macrochelodina billabong, Tropicochelymys goodei, T. leichhardti, T. insularis, and T. worrelli. An application was made to have the International Commission on Zoological Nomenclature (ICZN) suppress the 1984 and 1985 papers (and a third paper) of Wells and Wellington. Responses both for and against suppression were reviewed by the ICZN. The applicants and those supporting the application argued that the 1984 and 1985 works were so deficient in taxonomic standards that they seriously destabilized the nomenclature of a major regional group of animals, and that the works, published privately and without proper peer review, were excessively derivative and partly fictitious, and contained false claims of originality (the 1985 paper listed hundreds of unverifiable and apparently non-existent references). Those opposing the application thought that suppression for nomenclatural purposes of the two contemporary works (and the third) would constitute a serious infringement of the freedom of taxonomic thought or action, and that the issues raised were taxonomic rather than nomenclatural, and therefore outside the jurisdiction of the ICZN. The ICZN concluded (Anonymous, 1991) that, while the arguments for promoting stability and universality by suppression of the papers are both strong, the problems are mainly taxonomic rather than nomenclatural, resulting primarily from poor scientific and editorial practice, and that the goal of the application would not be effectively achieved by suppression of the three works. The ICZN further thought that the aim of the application would be best achieved by leaving the taxonomic issues to specialists to be settled through usage. The ICZN also commented that, by departing from the voluntary Code of Ethics of the International Code of Zoological Nomenclature, Wells and Wellington displayed a contempt for the Code and its arbitration provisions. The ICZN closed the case by simply not voting on it. Subsequent usage has been almost unanimously for the older taxonomy of chelid turtles, with recognition of the genera Emydura and Rheodytes (Cogger, 1992; Legler, 1993; Legler and Georges, 1993; David, 1994; and numerous subsequent papers on individual chelid species). In an effort to maintain this stability we follow the generic taxonomy of David (1994).
Several morphologically and genetically diagnosable chelids await description; see Genus Chelodina, Genus Elseya, and Genus Emydura for a short summary of these undescribed[l][m]Text Key[/m][r]Page 65: Genus Acanthochelys[/r] forms.