LIFE MODES OF PLEISTOCENE BIVALVES FROM LA BELLE, FLORIDA: TAPHONOMIC IMPLICATIONS AND RELATIONSHIP TO DRILLING PREDATION

Students in a graduate paleoecology class at University of North Carolina Wilmington analyzed Pleistocene bulk samples on loan from the American Museum of Natural History. The samples were collected from spoil banks 3.5 km west of La Belle, Florida, by Squires and Heaslip in 1955. Although museum records assign the samples to the Caloosahatchee Formation, some faunal characteristics indicate the assemblage at least in part represents the Bermont Formation, which had not been distinguished from the underlying Caloosahatchee at the time of collection.

Several UNCW classes have worked on this fauna; the 2014 class completed identification of bivalves >5mm, increasing the number of identified specimens from ~8000 to ~26,000. Life modes were assigned to taxa using information from the Paleobiology Database and compared to data on taphonomic damage and drilling predation.

To investigate the taphonomy of the assemblage, taphonomic grades (based on the Flessa et al. 1993 system) were determined for selected taxa to test the hypothesis that taphonomic grade differed with species’ relation to substrate. Mean taphonomic grade ranged from 2.26 – 2.48 for three infaunal and two epifaunal species (median grade = 2 for all species). Mann-Whitney and Kruskal-Wallis tests yielded no significant differences between epifaunal and infaunal species, or among infauna based on burrowing depth. These results suggest that little transportation outside the habitat occurred, but that shells were repeatedly reworked, so that life mode did not affect degree of taphonomic alteration.

All specimens were examined for complete and incomplete drillholes and drilling frequencies (DF = % mortality due to drilling) and prey effectiveness (PE = % drillholes that were incomplete) were calculated. We tested the hypothesis that drilling predation should vary with life mode. For the total assemblage, DF and PE were 5.4% and 9.3% respectively. Suspension feeders showed the lowest DF (5.1%) and greatest PE (9.9%). Stationary taxa were drilled significantly more than mobile taxa (11.8% vs 5.2%) and exhibited significantly more failed drilling (PE = 35.7% vs 7.3%). Epifauna were drilled significantly more frequently (18.3%) than infaunal prey (5.2%) and had significantly more incomplete drillholes (41% vs 7%).