TROPHIC STRUCTURE OF SHELL BEDS FROM THE EARLY MIOCENE CHIPOLA FORMATION OF FLORIDA

The early Miocene Chipola Formation of northwestern Florida contains one of the most diverse molluscan ecosystems ever described in the fossil record. To test whether this ecosystem was supported by high nutrient levels and a food web based on primary production from phytoplankton, we analyzed the relative abundances of primary consumers, including herbivores and suspension feeders, from new bulk sampled collections of the Chipola Formation.

Samples were collected from two sites, Alum Bluff on the Apalachicola River in Liberty County and Cooter Bluff on the Chipola River in Calhoun County roughly 30 km away to the west, to examine spatial variability in trophic organization within the formation. Vertical and horizontal transects were bulk sampledling within the Chipola Formation at the Alum Bluff site was done undertaken to provide additional insight into temporal and smaller-scale spatial variation. Molluscan diets were determined using the NMITA database.

All samples were dominated by nearly equal, high abundances of herbivorous and carnivorous gastropods with only a minor component of suspension feeders. Herbivores contributed between 30 and 50% of the total specimen counts in contrast to much lower percentages for suspension feeders. The Chipola was clearly an algal- or detritus- based ecosystem and not heavily dependant on phytoplankton. While there was little variation between the horizontal and vertical transects at Alum Bluff, there were notable differences between Alum Bluff and Cooter Bluff sites. Suspension feeders comprised just 7% of the total specimen abundances for Alum Bluff, in contrast to 26% of total specimen abundances for Cooter Bluff. The Cooter Bluff site may have received more terrestrially-derived nutrients. This ecological complexity within the Chipola Formation should be considered in future work comparing this fauna to older and younger intervals.