DRILLING PREDATION IN THE BIVALVE GENUS ASTARTE FROM THE PLIOCENE OF THE ATLANTIC COASTAL PLAIN

Gastropod drill holes found in the fossil record are an important tool for studying the ecological patterns of the past, as they are relatively common, easily preserved, and represent a direct biological interaction. Despite certain difficulties in identifying the species of the drilling predator from drill hole morphology, these trace fossils can still be used to study how predation trends and intensity have affected prey organisms. Drilling predation on the infaunal bivalve genus Astarte from the Pliocene of the Atlantic Coastal Plain was studied in terms of drill hole location and drilling frequency. A total of 1,066 valves from the Florida Museum of Natural History were analyzed. A comparison of prey size, measured as shell height, and drill hole diameter, a common proxy for predator size, was used to evaluate the presence of predator size selectivity. Nearest neighbor analysis was conducted on Bookstein-transformed geometric morphometric landmarks of drill hole placement to look for stereotypy in drill hole placement. Both analyses were conducted at the species and location level. Drilling frequency ranged between 0% and 48% for the 15 species evaluated. No species nor location yielded a significant correlation between prey size and drill hole diameter, as has been frequently demonstrated for naticid-only assemblages. Nearest neighbor analyses consistently showed random or overly dispersed distributions of drill hole placements. Again, this is contrary to the significantly clumped drill hole distributions typical of naticid-only assemblages. Both the prey size to drill hole diameter comparison and the nearest neighbor analysis showed a lack of stereotypy that indicates a mixed assemblage of drilling predators that most likely included both naticid and muricid drilling snails. Drilling frequency by species shows that there may be some preference to drill the species Astarte floridana and Astarte undulata over other extremely abundant species such as Astarte concentrica. Future work will include examining these prey preferences in the context of ongoing work to quantify the morphological variability within and across Astarte species. In particular, correlations between external shell sculpture or other morphological characteristics and drilling frequency will be evaluated.