From Late Miocene to Early Pleistocene times, i. e. from ca. 9 Ma to ca. 2 Ma, a vast palaeolake – Lake Obweruka (and its short-lived subdivisions, Palaeolakes Kaiso and Lusso) existed in the Albertine Rift Valley. This lake was similar in size, configuration and depth to present day Lake Tanganyika. The richly fossiliferous deposits left by this lake are exposed on the rift floor on both sides of Lake Albert and Lake Edward. They have been investigated since the beginning of the 20th Century and although geological and geographic investigations indicated that this palaeolake probably extended as far north as the Uganda-Sudan border, surveys in the region north of Lake Albert started only in 2008 following the discovery of substantial oil reserves in that region (Van Damme et al. 2010; Pickford, 2011; Pickford & Senut, 2011, 2012). Preliminary studies of the abundant molluscan material suggest that these assemblages, contrary to former opinion are not of Pleistocene age but date from the earliest phase of the palaeolake’s existence (i. e. around 8-9 Ma) a period from which hitherto virtually nothing was known (Pickford et al. 1993). This study provides information about the composition and evolution of the lacustrine molluscan community of Palaeolake Obweruka soon after it formed.

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.