The late Cenozoic Thiaridae (Mollusca, Gastropoda, Cerithioidea) of the Albertine Rift Valley (Uganda-Congo) and their bearing on the origin and evolution of the Tanganyikan thalassoid malacofauna

The recent cerithioid malacofauna of meromictic Lake Tanganyika is unique in its degree of thalassoid convergence with marine molluscs. This is generally considered the result of a long-lasting intensive escalation and cladogenesis caused by a coevolutionary prey-predator interaction in a freshwater ecosystem with sea-like characteristics, i.e. exceptional longevity (ca. 7-12 Ma) and vast dimensions (present surface area: 32900 km^sup 2^, present maximum depth: 1470 m). In the Albertine Basin, ca 300 km north of the Tanganyika Trough, Palaeolake Obweruka existed during Mio-Pliocene times. In many aspects, it can be considered as a 'sister lake' of Lake Tanganyika, being also long-lived (from ca 7.5 to 2.5 Ma), extensive (surface: 27000 km^sup 2^) and meromictic. Like Lake Tanganyika it belonged to the Congo catchment. Although thalassoid molluscs have been known from the Albertine deposits since the beginning of the 20th Century, previous researchers recognised only four polymorphic thiarid species, two of which were thalassoid. Detailed in situ collecting shows that the perceived low species diversity and high variability of the Obwerukan thalassoids is a phenomenon of the museum drawer. In the field the thiarid species and other molluscs are not extremely polymorphic, as formerly thought, but tend to occur in discrete morphometric packages according to stratigraphic level and geographic area. The species richness of the Palaeolake Obweruka thiarids has thus been severely underestimated, as is also the case for the Recent Tanganyikan thiarids. The present authors discern 35 species and 7 genera in the Albertine deposits (2 of which are new to science), and consider this to be a conservative estimate. In their degree of diversification and morphological escalation, the Obwerukan thiarids are thus comparable to the Tanganyika thalassoids. Arguments are presented that the thalassoids of these lakes are polyphyletic, that they are derived from the same genera (mainly Potadoma, Potadomoides and Pseudocleopatra) and that the tempo and mode of their intralacustrine evolution must have been largely similar. As for the tempo of evolution, the Albertine fossil record shows that escalation in shell morphology and ornamentation in the Thiaridae was extremely rapid, as in the Ampullariidae and Viviparidae (Van Damme & Pickford, 1995, 1999). In the Thiaridae heavily ornate shells evolved from an unornamented ancestor in two consecutive bursts, each lasting less than 0.