COMPARATIVE ANALYSIS OF FACTORS AFFECTING PREDATORY AND NONPREDATORY BIOEROSION ON STEWARTIA AND DOSINIA FROM THE PLIO-PLEISTOCENE CALOOSAHATCHEE FORMATION OF FLORIDA

In order to understand the factors affecting predatory and nonpredatory bioerosion on infaunal bivalves, students in an invertebrate paleontology class at University of North Carolina Wilmington used a comparative approach. Two species that were superficially similar in external morphology, with large, disc-shaped, unornamented valves, but with differing life modes were compared. Stewartia anodonta was a slow burrowing lucinid bivalve that acquired much of its nutrition from bacterial chemosymbionts, whereas Dosinia elegans is a rapidly burrowing suspension feeder. Both species are from bulk samples of the Plio-Pleistocene Caloosahatchee Formation near La Belle, Florida (on loan from the American Museum of Natural History). Shell length and thickness were measured and occurrence of predatory drillholes and extent and distribution of nonpredatory bioerosion were recorded.

Drilling predators (primarily naticid gastropods, although one octopod drillhole was found) preferred Stewartia; drilling frequency (DF) of Stewartia was 26% compared to 15% for Dosinia, though the difference is not statistically significant due to low sample sizes. This pattern is similar to that seen in the Miocene of the mid-Atlantic Coastal Plain, in which DF for Stewartia anodonta ranged from 34-50% and for Dosinia acetabulum from 0 – 19% for samples from the Calvert, Choptank, St. Marys and Eastover formations; most differences statistically significant. This preference occurs despite the significantly thicker shells of Stewartia (regressions of thickness on length yield a thickness of 2.3 mm for Stewartia and 1.1 mm for Dosinia at a length of 40 mm). Dosinia shells were larger than Stewartia (average length 54.3 vs 32.7 mm) and drilling on Dosinia focused on smaller sizes but was distributed evenly across Stewartia size classes. Thus size and perhaps burrowing speed was a greater deterrent to predation than shell thickness for these taxa. The hypothesis that unpalatability or toxicity of chemosynthetic lucinids deters drilling is not supported by these results. Nonpredatory bioerosion occurred on 63% and 71% of Dosinia and Stewartia respectively. For both taxa, bioerosion was most common on shell interiors and least common on the exterior dorsal region of the shells, indicating similar taphonomic histories of the two species.