Taxonomy and Ecology of the Fossil Freshwater Molluscs from Late Miocene Palaeolake Obweruka

From Late Miocene (Tortonian, 8­9 Ma) to Late Pliocene (Piacenzian, 2.5 Ma) times, a gigantic lake, Palaeolake Obweruka, occupied the Albertine Basin. This palaeolake was geomorphologically similar to Modern Lake Tanganyika and in addition it shares the occurrence of thalassoid (marine-like) molluscan faunas. Former research on this palaeolake revealed that evolution gave rise to three different thalassoid malacofaunas, each preceded by an extinction. Over the past few years fossil-bearing deposits were discovered north of Lake Albert, dating from the earliest lake stage (8­9 Ma) from which evidence had previously been lacking. In the present study, the fossil molluscan assemblages from these deposits are treated. The earlier deposits have yielded diverse molluscan community totalling 15 species, dominated by viviparids (Neothauma) and unionids (Coelatura) that lived in a vast but relatively shallow lake. Eight of the species are new to science. Assemblages from the subsequent deeper lake stage (7­4.5 Ma) contain species that were found in other parts of the basin during earlier investigations. A total of 22 species is described from the two assemblages. The major scientific importance of the earlier assemblage is that it consists for a large part of hitherto undescribed thalassoid endemics, representing evidence for a first thalassoid evolutionary escalation at the very onset of the lake. It is clear that the four thalassoid molluscan faunas of Palaeo­lake Obweruka represent rapid escalatory punctuations, each preceded by a major environmental shift and an extinction.