Coordinated Disruption across Molluscan Megaguilds during a Late Neogene Regional Mass Extinction in Florida

Declines in nutrient levels and associated planktonic productivity have been proposed as a likely cause of regional mass extinction in the Late Neogene of Florida. Such changes, however, should favor benthic producers, such as some seagrasses, epiphytes, and diatoms, that thrive in mesotrophic conditions with increased water clarity. Although the fossil record of primary producers is lacking, this hypothesis can still be tested indirectly by investigating whether extinction and ecological changes were selective among primary consumer megaguilds.

To test this hypothesis, we examined genus-level richness, evenness, and abundance patterns of gastropod megaguilds using new data from standardized bulk sampling of Plio-Pleistocene shell beds in Florida. Our results, which are based on a sample size of over 15,000 fossils, indicate that the richness declines of nearly 50-60% were proportionately equivalent across all three megaguilds (suspension feeders, herbivores, carnivores) at the Pliocene-Pleistocene boundary in contrast to our predictions of selectivity. Changes in the abundance structure of gastropods, or evenness, show a similar trend.

Preliminary abundance data reveal a dramatic rise in herbivore dominance at one Early Pleistocene site, consistent with expectations, but the increase appears to be local and not representative of Pleistocene samples overall. Thus, at present, we find little support for the hypothesis that consumers of benthic productivity fared any better than consumers of planktonic productivity as predicted of nutrient decline.