Afro-American side-necked turtles
These semi-aquatic side-necked turtles are today found naturally only in South America, Africa, Madagascar, and on some of the Seychelles Islands in the Indian Ocean. Their fossil record extends back to the late Triassic (Proterochersis robustum) and Jurassic (Platychelys oberndorfi, Notoemys laticentralis) (De Broin, 1988). Fossil records from the Cretaceous show the family to have been wide spread in North America, Europe, Africa (south of the Sahara), and Asia (De Broin, 1988). The oldest fossil pelomedusids are North American species assigned to the genus Bothremys (Gaffney and Zangerl, 1968). Among the numerous species of fossil pelomedusids is included what may be the largest freshwater turtle that ever lived, Stupendemys geographicus, which attained a carapace length of 230 cm (Wood, 1976).
The skulls are only moderately posteriorly emarginated and the ventral emargination is variable. A quadratojugal is present and the bony temporal arch is complete. This bone may contribute to the temporal roof lying above and in front of the quadrate and touching the parietal. Jugal-parietal contact may occur; however, there is a loss of contact between the parietals and squamosals. The prefrontals touch dorsally, but nasal bones are absent. In the roof of the mouth, the palatines meet, but the vomer is small or absent, and the premaxillae are unfused. In the lower jaw a single fused dentary bone extends posteriorly almost to the articular region between the angular and well-developed supraangular bones. There is no splenial bone. The carapace may be arched or depressed; it contains five to eight neural bones, and one or more pairs of the posterior costal bones meet dorsally. There is no cervical scute. On the plastron, a single intergular scute lies between the paired gulars. A pair of mesoplastral bones is also present on the plastron. These are usually positioned laterally, but do meet medially in the genus Pelusios. A movable transverse hinge is present between the pectoral and abdominal scutes in Pelusios; all other genera have rigid plastra. The 2nd cervical vertebra is convex; the others are procoelous or amphicoelous. The hind toes are heavily webbed. The five genera and 25 species are placed in two subfamilies: Podocnemidinae and Pelomedusinae.
Some turtle taxonomists divide the family Pelomedusidae into two families, Pelomedusidae (Pelomedusa, Pelusios) and Podocnemidae (Podocnemis, Peltocephalus, Erymnochelys) (Meylan, 1996).