Fossil Soft-Shelled Turtles (Family Trionychidae) of the Lake Turkana Basin, Africa

Fossil soft-shelled turtles (family Trionychidae) are described from three localities in the Lake Turkana Basin, Kenya. Data on trionychid specimens from these three localities, Pliocene of Kanapoi and Lothagam, and Pleistocene of Koobi Fora, are supplemented with data from Karungu and Chemeron, Kenya, Omo, Ethiopia, and the Chiwondo Beds of Malawi. With the exception of three specimens from Koobi Fora, which probably represent Trionyx triunguis, all of these East African fossil trionychids can be referred to the subfamily Cyclanorbinae. Three of four living cyclanorbine species in Africa, Cyclodermafrenatum, Cyclanorbis elegans and C. senegalensis, are preserved as fossils in the Lake Turkana basin. Additionally, three extinct species are now known from the Neogene of East Africa. Cycloderma victoriae Andrews was described from the Miocene Karungu beds of Kenya in 1914. Two additional cyclanorbines, C. debroinae and Cyclanorbis turkanensis, both from Pliocene localities in the Lake Turkana basin, are described in this report. Identification of these two new fossil species brings to seven the total number of cyclanorbines known from Africa. Five of these occur in the Lake Turkana Basin. This high diversity may reflect the historical association of this basin with both other Rift Valley lakes and with the greater Nile drainage system, and periodic transgressions and regressions within the basin itself. The distribution of cyclanorbine trionychids in Africa is congruent with those of freshwater fishes and is completely explained using a vicariant hypothesis.