INCREASING GROWTH RATES IN HERBIVORES FOLLOWING AN EXTINCTION EVENT IN THE LATE NEOGENE OF FLORIDA: SIGNATURE OF PLANKTONIC PRODUCTIVITY DECLINE?

A Late Neogene extinction event in Florida wiped out nearly 70% of marine mollusk taxa, although the cause of this event and the magnitude of biotic disruption are still being debated. One proposed cause is the decline of nutrient levels, leading to a sudden drop in planktonic productivity, causing starvation and population collapse among suspension feeders and associated species. A corollary to this hypothesis is that net productivity did not decline but merely shifted from planktonic to benthic producers (e.g., seagrasses, benthic algae, benthic diatoms, etc.), which would have benefitted from lower nutrients and increased light penetration to the seafloor.

To test this hypothesis, we use stable isotope sclerochronology to examine growth rates of the herbivorous gastropod Strombus spp. from the Plio-Pleistocene shell beds of Florida. This taxon was chosen because of its relatively large size and ecological abundance in these units. Any environmental changes affecting benthic productivity should be seen most clearly in Strombus . We analyzed approximately 25 samples from each shell collected along the growth axis and plotted these against size to determine age and estimate growth rates through time. Preliminary data reveal that growth rates and body size of this dominant herbivore increased in step-wise fashion through time, although the largest increases occurred before and after the extinction event with only an increase in the minimum size across the extinction boundary. These results are preliminary but generally consistent with a long-term shift in productivity regimes (planktonic to benthic) rather than a complete collapse of productivity per se.