COMPARING GASTROPOD GUILDS PRIOR TO AND FOLLOWING A REGIONAL EXTINCTION: EXAMPLES FROM THE PLIO-PLEISTOCENE OF FLORIDA

A guild analysis of gastropod feeding habits from the lower Pleistocene Bermont "Formation" (LL 3C, 6B) and the Upper Pliocene Pinecrest (QA 2, 4) beds of Florida was undertaken to study temporal changes in community structure during an interval of accelerated faunal turnover. Bulk paleontologic samples (5 gallon buckets) were collected from a measured section of the Pinecrest beds from the Quality Aggregates shell pits, Sarasota, FL (QA), and Bermont sediments at Longan Lakes Quarry, Collier County, FL (LL). Bulk material was processed at two different sieve sizes (3mm and 5.5mm) to investigate differences between these two size fractions. Although the dominant analysis was undertaken stratigraphically, the guild structure was also examined laterally for one Bermont bed (LL 3C), in order to investigate the small-scale variability in feeding habits, related to issues such as initial community 'patchiness' or taphonomic differentiation. Smaller 1 gallon samples were collected within LL 3C approximately every 15.25 m, for 91.5 m across the stratigraphic unit. The guild structure of gastropod families was established according to trophic groups of predatory carnivores, herbivores, and suspension feeders.

Two different guild structures were detected in the Pinecrest samples, which pre-date the Plio-Pleistocene regional extinction. In sample QA 2, carnivores dominated 56% of the assemblage (n = 944), whereas in QA 4, which represents a limited gastropod population, suspension feeders represent 53% (n = 169). Following the Plio-Pleistocene extinction, within the Bermont sample LL 3C, predatory gastropods and suspension feeders significantly declined, whereas the relative percentage of herbivores dominate an average of 57% of the transect assemblage (n = 971) and 67% of the bulk material (n =1554). Sample LL 6B contains evidence that carnivores dominate 60% of the assemblage (n = 918), similar to the dominance of carnivores in QA 2. Overall, prior to and following the Plio-Pleistocene mass extinction, feeding habits of gastropod families between the two beds show considerable variability and point to the need for detailed stratigraphic analysis to differentiate between evolutionary and ecologic trends.