PALEOECOLOGY OF THE LATE OLIGOCENE LANISTES ( MOLLUSCA: GASTROPODA) FROM THE RUKWA RIFT BASIN, TANZANIA

Scarcity of paleontological data from the Cretaceous-Paleogene interval of continental sub-equatorial Africa (i.e. “African Gap”) has hindered comparative studies aimed at understanding paleobiological relationships between regions of Africa. One of the main reasons highlighted for this paucity is the lack of accessible outcrops with well-preserved fossils. However, the late Oligocene Nsungwe Formation of the Rukwa Rift Basin (RRB), a 300 km long 50 km wide segment of the Western Branch of the East African Rift System in southwestern Tanzania, preserves a diverse collection of vertebrate and invertebrate fossil fauna. Present study focuses on the poorly known but unique freshwater gastropods of Songwe Member of the Nsungwe Formation. The Songwe Member is a volcanic ash-rich wetland succession deposited between 26.0-24.5 mya. The main objectives of this study are to first describe the poorly known late Oligocene malacofauna and to investigate differences in diversity between the different study sites. We also tested for the presence of ecophenotypic plasticity by analyzing variations in shell shapes between localities.

We examined fossils collected by The Rukwa Rift Basin Project (RRBP) since 2002 from eight different study sites from the Songwe Member. All specimens were photographed in apertural view and shell measurements for descriptive purposes were taken using ImageJ. We used Geometric Morphometrics to analyze variations is shell shapes. To predict ecological roles, we used data on previous fossil finds of the Songwe Member and also incorporated ecological data of related recent gastropods.

Preliminary data show that the late Oligocene freshwater gastropods comprise of multiple morphospecies of Lanistes including several novel species. Furthermore, most depressed morphospecies are restricted to a subset of localities. In summary, differences in diversity and disparity of fossil gastropods provide additional information for reconstructing ecological and depositional environments within the Rukwa Rift Basin.