The late Cenozoic Viviparidae (Mollusca, Gastropoda) of the Albertine RiftValley (Uganda-Congo)

During late Cenozoic pre-rift times the viviparid genus Bellamya was probab ly confined to eastern Africa while in the Congo Basin occurred the genera Neothauma and Kaya (a new genus described herein). During the Pliocene, Kay a became extinct and Neothauma became a relict in Lake Tanganyika. All Afri can rift lakes formed during or after the Pliocene were colonised by popula tions of Bellamya, which evolved into lacustrine endemics. The changes in s hell morphology in Bellamya, such as ornamentation functioning as protectio n against predators, are modest and repetitive in time and space. After the initial adaptations stasis followed, and there is no indication of an arms race between prey and predator in this genus. In the lakes of the western rift that were formed prior to the Pliocene, namely Lake Tanganyika and Pal aeolake Obweruka, Neothauma instead of Bellamya was the coloniser. Initiall y morphological inertia also occurred in this genus both in Tanganyika and, during the first four million years of its existence, in Palaeolake Obweru ka, although from the outset this lake contained abundant highly specialise d molluscivorous fishes. About 4.5 Ma a major extinction event occurred in the Obweruka Basin which led to the extermination of 50% of the molluscan s pecies but none of the molluscivores. Among the viviparids, only one of the Neothauma species survived, its populations isolated and highly reduced in numbers. Immediately after this crisis a conchological quantum change occu rred, the surviving lineage changing into a strongly ornamented thalassoid form. The dramatic morphological change is deemed to have been due to focus sed selection by the predators on isolated and small prey populations. A ra diative event immediately followed, producing still more strongly ornamente d forms. The morphological changes that occurred in the Obwerukan Neothauma , in comparison to the modest and repeated patterns of evolution that can b e observed in viviparids from other lakes, provides an example of true or q uantum evolutionary change and yields evidence as to how it may have occurr ed. The fossil record of the fresh water molluscs of the Albertine Basin, t hanks to its duration (ca. 12 million years), its relatively fine resolutio n (0.5-1.0 million years) and its sound chronostratigraphic framework, is a unique resource for understanding the tempo and mode of macroevolution.