The late Cenozoic Ampullariidae (mollusca, gastropoda) of the Albertine Rift Valley (Uganda-Zaire)

vanDamme D., Pickford M.
State University of Ghent, Lab Palaeontology, Krijgslaan 281-S8,B-9000 Ghent, Belgium

ABSTRACT

The Albertine Rift Valley (Uganda-Zaire) contains vast sedimentary sequences of late Cenozoic age. They were deposited in an extensive riftlake, Lake Obweruka, which existed from c. 8 Ma to 2.5 Ma and was comparable in size and depth to the present L. Tanganyika.
Many freshwater molluscs that occur in these lacustrine deposits are characterised by their aberrant shell morphology, their extreme ornamentation and general form, making them resemble marine species. This convergence, rare in freshwater molluscs, is called thalassoidism and extreme ornamentation in marine as well as in freshwater molluscs is considered to be the result of a gradual process of prey/predator coevolution.
In the present paper the Albertine representants of the ampullariid genera Lanistes and Pila, most of which are new to science, are taxonomically described and their phylogenetic relation, based upon apomorphic characters, is given. In addition the evolutionary history of these freshwater snails in the basin has been reconstructed.
In the pre-riftlake environment 3 species of Lanistes occurred, with no special shell adaptations against predation. After the formation of a riftlake, 2 of these, colonising the new lacustrine ecospace, changed morphologically and radiated. The 3 derived lines show minor adaptations against predation. After the extinction of the dominant Lanistes species group around 6 Ma, the sole surviving lacustrine Lanistes suddenly radiates, the ancestral line persisting next to the 3 new daughter lines. This second morphological shift is spectacular as it produces shells with distinct thalassoid features. All the Lanistes species of L. Obweruka became extinct during a cataclysmic event around 4.5 Ma. Populations of the genus Pila colonised lacustrine habitats after this event, the derived form also showing striking thalassoid characters. There is no doubt that the intense morphological change occurred during a brief period, geologically speaking. The degree of morphological change in molluscs appears hence not to be linked with time. After the sudden radiation all lineages remain morphologically stable until they became extinct c. 1 Ma later. This pattern corresponds to the punctuated equilibrium model. Other groups (viviparids, thiarids) show more gradual changes.

Hydrobiologia, 316 (1): 1-32. 1995.

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