Early Eocene Cerithioidean Gastropods from a Subtropical Coast Environment (South–Central Pyrenees, Spain)

Abstract

The biodiversity of Eocene Cerithioidean gastropods from a subtropical marine littoral environment, including mangroves with Nypa palms, is documented from the Figols Group (FG) and the overlying Castigaleu Group (CG) of the Ager and the Tremp–Graus basins (south–central Pyrenees, Spain). The stratigraphic interval includes the middle and upper Ypresian, encompassing the early Eocene climatic optimum (EECO). The malacofauna comprises a rich association of possibly intertidal species, dominated by Potamididae, Batillariidae, Pachychilidae, and Cerithiidae, with subordinate Melanopsidae, Thiaridae, Diastomidae, and Pachymelaniidae. The subtidal fauna is dominated by Turritellidae. The Cerithioidea fossil distribution is interpreted according to their local palaeoenvironmental gradients and through their genus-level stratigraphic distributions and according to known molecular phylogeny, ecology, and geographical range of Recent forms. The Cerithioidean record during the Palaeogene is revised on a global scale to obtain insights into the evolution across the EECO of some of the most widespread intertidal tropical organisms. The results suggest that Potamididae were adapted to mangrove trees since their first important diversification in Western Tethys and that some early Eocene genera survive in Recent mangrove-fringed coasts. In addition, the Batillariidae are very abundant and widespread in intertidal assemblages, undergoing rapid early Eocene turnovers, but within genera unrelated to modern forms. EECO Pachychilidae lived in brackish water habitats, contrary to purely freshwater Recent species. Turritellidae display an environmental distribution in the early Eocene similar to their modern distribution.