Mediterranean onshore–offshore gradient in the composition and temporal turnover of benthic molluscs across the middle Piacenzian Warm Period

The mid-Piacenzian Warm Period (mPWP: 3.3–3.0 Ma) is the most recent geological analogue of ongoing climate change and has been the subject of considerable interest for numerical models of the climate system. To verify the effect of the mPWP on diversity and temporal turnover of marine benthic communities, we evaluated changes in species-level abundance and the composition of Mediterranean Pliocene molluscs prior, during and after the mPWP. The Pliocene onshore–offshore gradient in species composition did not change during the mPWP (and continued basically unchanged up to the present day in the Mediterranean Sea), with most dominant species occupying the same rank in a given environment. During the mPWP, species evenness generally increased towards offshore environments. Within the three time intervals, temporal similarity is greater in offshore environments, except during the mPWP when offshore communities also exhibited greater dissimilarity. The temporal turnover in composition decreased again with depth as global temperatures decreased after the mPWP. The structure of mPWP communities suggests that warming and sea-level rise contributed to the expansion of vegetated bottoms (onshore) and shelly and coralligenous bottoms (offshore). Although the effects of mPWP warming did not change the onshore–offshore gradient in the long term, its effect disproportionately affected deeper environments, in contrast to colder climate regimes.