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

During late Cenozoic pre-rift times the viviparid genus Bellamya was probably confined to eastern Africa while in the Congo Basin occurred the genera Neothauma and Kaya (a new genus described herein). During the Pliocene, Kaya became extinct and Neothauma became a relict in Lake Tanganyika. All African rift lakes formed during or after the Pliocene were colonised by populations of Bellamya, which evolved into lacustrine endemics. The changes in shell morphology in Bellamya, such as ornamentation functioning as protection 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 Palaeolake Obweruka, Neothauma instead of Bellamya was the coloniser. Initially morphological inertia also occurred in this genus both in Tanganyika and, during the first four million years of its existence, in Palaeolake Obweruka, although from the outset this lake contained abundant highly specialised 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 species 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 occurred, the surviving lineage changing into a strongly ornamented thalassoid form. The dramatic morphological change is deemed to have been due to focussed selection by the predators on isolated and small prey populations. A radiative event immediately followed, producing still more strongly ornamented forms. The morphological changes that occurred in the Obwerukan Neothauma, in comparison to the modest and repeated patterns of evolution that can be observed in viviparids from other lakes, provides an example of true or quantum evolutionary change and yields evidence as to how it may have occurred. The fossil record of the fresh water molluscs of the Albertine Basin, thanks to its duration (ca. 12 million years), its relatively fine resolution (0.5–1.0 million years) and its sound chronostratigraphic framework, is a unique resource for understanding the tempo and mode of macroevolution.