Palaeobiology and evolution of the late cenozoic freshwater Molluscs of the Turkana basin: Iridinidae Swainson, 1840 and Etheriidae Deshayes, 1830 (Bivalvia: Etherioidea)

Abstract

Synopsis The palaeobiology of the Late Cenozoic freshwater molluscs that inhabited the Omo‐Turkana Basin, situated in the eastern branch of the East African Rift System (Ethiopia, Kenya), remains poorly documented. Here we revise the taxonomy and palaeobiology of the bivalve superfamily Etherioidea from this region and discuss some palaeohydrological implications. In the Iridininae (Mutela, Pleiodon), the genus Iridina Lamarck, 1819 is revived for elongated iridinids with a denticulated hinge, since all fossil Iridininae of the Omo‐Turkana Basin and most Miocene‐Early Pleistocene Iridininae elsewhere in Africa have denticulate hinges that are not comparable to those of modern Mutela. In addition to the ubiquitous Etheria elliptica (Etheriidae), 11 iridinids are described, five of which are new to science, namely Chambardia feibeli, Iridina turkanica, I. omoensis, I. browni and Pleiodon bentoni. Most species do not show lacustrine adaptations and are/were part of a widespread East African fauna. This confirms the highly unstable character of the Pliocene‐Holocene aquatic ecosystems in the Omo‐Turkana Basin. Indications for intralacustrine speciation are only observed in the Late Pliocene‐Early Pleistocene long‐lived (∼25o ka or longer) Palaeolake Lorenyang. Williamson's (1981) evolutionary model for the Omo‐Turkana Basin molluscs does not apply to the Etherioidea.