First report of heterobranch microgastropods from an early Eocene marginal marine succession of Cambay Basin, western India

Although gastropods are one of the most studied mollusk groups, fossil records of heterobranch microgastropods are scarce. Here, nine species of heterobranch microgastropods are described from the early Eocene Cambay Shale of the Cambay Basin, including 8 new species: Ringicula knolli n. sp., Cylichna (Cylichna) ypresiana n. sp., Aliculastrum suratense n. sp., Odostomia concavata n. sp., Megastomia canaliculata n. sp., Megastomia carinata n. sp., Costosyrnola taptiensis n. sp., and Cingulina eamesi n. sp. This new fauna is discussed here with respect to its associated lithology, faunal association, and habitat and feeding preferences. A low diversity microgastropod fauna composed predominantly of eurytopic genera in association with similar eurytopic macro-mollusks indicates a restricted marginal marine condition. Eight out of the nine species described here are endemic. On the other hand, apart from Cingulina and Costosyrnola, which have a poor fossil record, the reported genera were widespread during the early Eocene. The heterobranchs are characterized by planktotrophic larval development, believed to benefit large scale dispersal. As the strong endemism of the species does not support this hypothesis, it is suggested here that the isolated and restricted nature of the basin could have facilitated rapid speciation in the fauna.