Comparing the Last Interglacial (MIS 5e) with the present interglacial period (MIS 1) using a multidimensional functional diversity analysis: The marine molluscs from Santa Maria Island (Azores Archipelago, central Atlantic) as a case study

ABSTRACT Marine fossil records hold outstanding importance for ecological, evolutionary and biogeographical studies. Santa Maria Island in the Azores Archipelago (central Atlantic) features a remarkable marine fossil record spanning from the Pliocene to recent times. This record includes late Pleistocene outcrops with fossiliferous deposits dated from the last interglacial period (MIS 5e). In this study, we describe a newly discovered MIS 5e outcrop (Ponta do Cedro) and investigate its marine mollusc assemblages. Fifteen taxa (11 gastropods and four bivalves) constitute new records for the MIS 5e highstand deposits of the Azores, increasing the total number of MIS 5e mollusc taxa to 160 (132 gastropods and 28 bivalves). Four genera/species are reported herein for the first time in the global/world fossil record, further emphasising the exceptional palaeobiodiversity of the MIS 5e mollusc assemblages on Santa Maria. Using a functional diversity‐based approach, we compared, for the first time, the MIS 5e fossil record with the modern molluscs from the Azores. To achieve this, we compiled two data sets with information covering 35 functional traits from a total of 420 mollusc taxa: 160 from the MIS 5e sedimentary deposits of Santa Maria and 365 modern shallow‐water molluscs reported from the Azores. After excluding taxa lacking complete functional trait information, the final data set included 385 mollusc taxa: 132 from the MIS 5e deposits and 320 modern species. All functional alpha‐diversity indices were higher for the modern assemblage compared to the MIS 5e, except for Functional Divergence (FDiv), which was greater in the MIS 5e assemblage. This suggests higher productivity, along with the use of a larger number of ecological niches and thus increased niche specialisation for the modern communities compared to the MIS 5e ones. Conversely, the MIS 5e communities appear to have been less resilient to invasions and climate change than their modern counterparts. However, the higher FDiv observed in the MIS 5e communities indicates more efficient use of available resources by these communities, when compared with the modern ones. The results of the multidimensional functional beta‐diversity reveal that the nestedness‐resultant value is significantly higher than the turnover component. This indicates that the MIS 5e assemblage is a subset of the modern assemblage. Finally, this study provides the first baseline for comparing MIS 5e mollusc assemblages with others, highlighting the unique palaeobiodiversity and functional characteristics of the MIS 5e fossil record on Santa Maria Island.