CHANGES IN GUILD COMPOSITION AND ENERGETIC DEMANDS THROUGHOUT THE PLIOCENE-PLEISTOCENE EXTINCTION EVENT

Marine productivity may potentially play an important role in controlling the relationships between energy use, body size, abundance, and species richness. Identifying changes in those relationships can improve our understanding of how energetic requirements influence community structure. Here, we explore how the decline in marine productivity linked to the Plio-Pleistocene Extinction Event (PPEE) modified the guild structure of bivalves and gastropods. Through bulk sampling, we estimated species richness, abundance and body size of bivalves and gastropods for several PPEE sections that span the upper Pliocene Tamiami Formation and the lower Pleistocene Caloosahatchee Formation (Sarasota, Florida). Specimen-level morphometric measurements were used to estimate biovolume patterns for each species and each sample. Rarefaction techniques and Bray-Curtis index combined with SIMPER analysis were used to estimate taxic richness and compositional turnover throughout the PPEE interval. Species body weights and energy were estimated from biovolume using allometric equations. Energy use was estimated as the product of species abundance and species energy use. A total of 376 bivalves species and 212 gastropods species were recorded. A total of ~46% of bivalves and 65% of gastropods recorded their last occurrence in the Tamiami Formation, indicating a high species turnover during the PPEE interval. Infaunal and epifaunal bivalves recorded mean dissimilarities of ~88% and ~91%, respectively, while carnivorous and herbivorous gastropods show compositional turnover of ~85%. Following the extinction event, body size and energy use increased in all bivalves, whereas both decreased significantly in all gastropod guilds. We suggest that the post-extinction increase in body size and energetic demands observed for bivalves indicate a rapid ecological release due to the lower ecological packing and lower predation pressure by gastropods. In contrast, gastropods experienced higher extinction rate, drastic size reduction, and slower post-extinction recovery, possibly reflecting their higher energetic requirements. Results suggest that during the PPEE, taxon specific energetic demands associated with drops in ocean productivity generated predictable recovery patterns in molluscan ecological guilds.