VARIATION IN GASTROPOD BOREHOLE FREQUENCY WITHIN THE SAME PREY SPECIES FROM SIMILAR PALEOENVIRONMENTAL SETTINGS IN THE FORT THOMPSON AND BERMONT FORMATIONS OF FLORIDA

Gastropod boreholes are a popular source of data for paleoecologic and evolutionary analyses of predator/prey interactions preserved in the fossil record. Thus it is important to understand the sources of variation in borehole frequencies (e.g., evolutionary, paleoenvironmental, paleoecologic or taphonomic differences). To examine this variation, borehole data from 8 abundant bivalve species were analyzed from two paleoenvironmentally and temporally similar Pleistocene formations (the Fort Thompson and Bermont Formations) from the same locality (Caloosa Shell Quarry) in Florida.

Each species was represented by a minimum of 30 complete valve specimens per formation. All species were more abundant in one formation than the other. They varied in size from tiny Parvilucina multilineata (average shell length 0.42 cm) to relatively large Carditamera floridana (average shell size 1.54 cm). All species were either shallow burrowing suspension feeders or lucinids.

Over 800 out of approximately 10,000 specimens of the eight bivalve species were bored. When borehole frequencies for all species were compared, there was no significant difference in overall borehole frequency between formations. In the majority of species, borehole frequency was highest in samples from the formation in which the bivalve species was relatively less abundant, which is contrary to a prediction of optimal foraging strategy theory. The taphonomic and paleoecologic implications of these results will be discussed.