FOSSIL GASTROPODS FROM THE LATE OLIGOCENE NSUNGWE FORMATION OF TANZANIA

Located in the Rukwa Rift Basin of southwestern Tanzania, the Nsungwe Formation represents the only well-characterized sub-equatorial Oligocene terrestrial sequence on the African continent. In recent years, these deposits have revealed a remarkable number of fossil localities spanning over +400m of exposed section. Multiple approaches have yielded a radiometric date of ~24.95MY for the top of the sequence. Ongoing sedimentological study of the Nsungwe Formation suggests that most fossiliferous localities reflect small, flashy discharge fluvial and floodplain deposits draining into a local lake system. This is consistent with faunal evidence of aquatic and semi-aquatic taxa (fish, crustaceans, and anurans) suggesting perennial availability of water with possible periodic or seasonal climate fluctuation. The Nsungwe Formation also contains a smaller number of localities reflecting low-energy shallow lacustrine deposition.

Fossils described herein were recovered from six localities within the Nsungwe Formation. The dominant mollusk is a novel species of Lanistes, the first reported from sub-equatorial Africa prior to the Miocene. Extant and fossil Lanistes are associated with tropical freshwater lakes and lagoons. Less well-represented in the fauna are a second species of Lanistes, the genus Cleopatra, and an as-yet undetermined taxon potentially assignable to the Achatinidae. Achatinids are not documented prior to the Pleistocene, although phylogenetic studies have postulated a much earlier origin.

Importantly, one Nsungwe Formation locality preserves a complete ontogenetic sequence for Lanistes. Understanding the ontogenetic trajectory of this taxon elucidates the early reproductive history of the family. For example, the genus has commonly been used as an outgroup for Ampullariid phylogenies, and to infer the mode of oviposition of fossil and modern Ampullariids. Micro-computed tomography and scanning electron microscopy were used to study the ontogenetic sequence of Lanistes. These non-destructive methods reveal that Oligocene Lanistes larvae appear non-planktonic. This result questions interpretations regarding the early reproductive history of Ampullariids, as well as the placement of Lanistes as an outgroup for the dispersal and diversification of New World forms.