TAPHONOMY OF THE PLIO-PLEISTOCENE CALOOSAHATCHEE FORMATION OF FLORIDA: PALEOENVIRONMENTAL IMPLICATIONS BASED ON BIVALVE AND GASTROPOD SHELL DAMAGE

This study examines taphonomic condition of a mollusc assemblage from the Plio-Pleistocene Caloosahatchee Formation, collected 3.5 km west of La Belle, Florida, by Squires and Heaslip in 1955, and housed at the American Museum of Natural History. In all, 1,381 disarticulated bivalve shells and 880 gastropods, representing 15 bivalve and 19 gastropod species, were examined and scored taphonomically using the following categories: interior/exterior encrustation, interior/exterior bioerosion, interior/exterior fine-scale surface alteration (FSA), fragmentation, and edge modification. Occurrence of complete and incomplete predatory drill holes was also noted.

We expected damage for epifaunal gastropods to be greater than for infaunal bivalves. To test this hypothesis, three taphonomic scores were selected for comparison: the percent of specimens with internal encrustation, bioerosion, and FSA. These scores were chosen because damage to the interior surface of the organism's shell would only have occurred post-mortem. Encrustation proved to be extremely rare for both bivalves (<1%) and gastropods (0%). Bivalves display a moderate degree of both internal and external bioerosion (33.5% and 36.9%, respectively). Conversely, gastropods rarely exhibit interior or exterior bioerosion (<6%). Both bivalves and gastropods display a high degree of exterior and interior FSA (73.3% and 79.2% of bivalves and 68.0% and 63.8% of gastropods). Overall, only 3.1% of bivalves received a total taphonomic score of zero, indicating lack of damage to the shell. In comparison, 20.3% of gastropods received a score of zero.

Gastropods received a higher taphonomic score than bivalves in only one category; gastropod shell fragmentation occurred in 38.6% of shells compared to 33.9% for bivalves. This statistically significant difference may be attributed to the shape and fragility of the gastropod shells coupled with post-mortem transportation prior to burial. Edge modification was common for bivalves and gastropods, with 72.4% of bivalves and 51.9% of gastropods having chipped edges. Edge rounding was rare (<5% of both bivalves and gastropods). Predatory drilling was low for both bivalves and gastropods (4.8% and 0.1% respectively). These results indicate a moderate energy environment with repeated reworking.