The Venus-Ostrea assemblage in the Lower Pleistocene Arda River section: species-specific responses to climate change

The Early Pleistocene was a time interval characterized by several climatic oscillations linked to glacial/interglacial cycles and by a progressive climatic cooling which culminated with the onset of Middle-Late Pleistocene large continental glaciations in the Northern Hemisphere. After the Last Glacial Maximum the Earth underwent a progressive warming, up to nowadays conditions. The Mediterranean area was strongly affected by these climatic changes both in marine and continental settings. The Lower Pleistocene Emilian marine successions (northern Italy) represent excellent case studies where to study how mollusk fossil assemblages changed over time in response to climate modifications.

In particular, the Venus-Ostrea assemblage, reported in several Lower Pleistocene marine successions of northern Italy (Arda, Stirone and Chero sections), has never been studied in detail. Here, the Venus-Ostrea assemblage of the Arda River section has been analyzed from a systematic and a paleoecological point of view, resulting in the identification of 23 taxa belonging to bivalves and gastropods. As the majority of the retrieved taxa is represented by living species, a comparison between their fossil and present-day environment has been carried out to observe if there were changes in their habitat from the Early Pleistocene to nowadays.

Three different species-specific responses to climate changes were observed: 1) some species, namely Turritellinella tricarinata, Aporrhais uttingeriana and Nassarius semistriatus, were unable to adapt to the climatic cooling and became extinct at the end of the Pleistocene; 2) other species have changed their bathymetric distributions from cooler Pleistocene to warmer present-day conditions; for instance, Venus nux, which during the Early Pleistocene was abundant in the Adriatic Sea at shallow water depths (20-40 m), currently lives in restricted areas of the Mediterranean Sea at water depths of 75-100 m, where seawater temperatures are cooler and thus more favorable for the proliferation of the species; 3) finally, other species do not show variations in their geographic or bathymetric distribution.

This study reveals the occurrence in the Lower Pleistocene marine successions of northern Italy of an association of taxa which is no longer present in the modern Adriatic Sea, stressing the importance that environmental and climatic conditions have on the distribution of organisms.