Lower Pliocene gastropod assemblages from northwestern France: Palaeoceanographic and palaeobiogeographic implications

Upper Miocene to Pleistocene assemblages of northwestern France are commonly grouped as Redonian faunas. Although earlier authors recognized their diverse and heterochronic character, Redonian assemblages continued to be monographed as if they were homogeneous, hindering understanding. Here we critically review the lower Pliocene (Zanclean) Assemblage III gastropod faunas of the Nantes region (241 species within 145 genera) and consider their palaeoenvironmental and palaeobiogeographic implications. They occur in the wider context of a post-Miocene decrease in faunal diversity along the Atlantic seaboard of Europe. This decline has been linked to the loss of thermophilic faunal elements due to a cooling in Mean Monthly Sea Surface Temperature (MMSST). Based on Assemblage III gastropods, MMSSTs in the Ligerian Gulf of northwestern France were characterized by subtropical temperatures in Zanclean times (not cooler than approximately 16 degrees C in February to 22 degrees C in August and September, compared to present day values of 9.5 degrees C to 18.5 degrees C for this latitude). At generic level, the Assemblage III fauna lies at the northern limit of the subtropical French-Iberian molluscan province, which also included the Pliocene faunas of the Mondego Basin of Portugal. However, at species level, it is unlike other Redonian or French-Iberian assemblages of Pliocene age and justifies the creation of a distinct Ligerian molluscan subprovince. The origin and evolution of the assemblage were strongly dependent on the geography of northwestern France during the Pliocene; once the Ligerian Gulf vanished, so did the subprovince.