Role of bioerosion in taphonomy: effect of predatory drillholes on preservation of mollusc shells

Although bioerosion is recognized as causing significant destruction of hard substrates, few studies have assessed loss of hard substrates by taphonomic bias against shells with predatory borings (the trace fossil Oichnus). Results of point-load compression experiments suggest preferential loss of Oichnus-bearing shells; however, studies of drilling predation often assume lack of taphonomic bias against drilled shells.This project tested the hypothesis that taphonomic processes preferentially destroyed shells with predatory borings for the Miocene Choptank Formation of Maryland and the Plio-Pleistocene Caloosahatchee Formation of Florida. If bias against drilled shells occurred, drilled shells should be more pristine than undrilled shells because they do not survive exposure, and shells should break through drillholes. Drilled and undrilled shells from bulk samples of the Drumcliff and Boston Cliffs members of the Choptank Formation were analyzed for taphonomic condition, and shell fragments were examined for breakage through drillholes. Taphonomic condition was also determined for five species of Caloosahatchee bivalves that exhibited predatory bioerosion: Dosinia elegans, Chione latilirata, Chione elevata, Macrocallista nimbosa, and Stewartia anodonta. Most measures of taphonomic condition showed no statistically significant difference between drilled and undrilled shells for both formations. In the Choptank, condition of shell sculpture (Boston Cliffs Member) suggested taphonomic bias against drilled shells. However, distinctness of the pallial line (Drumcliff Member) supported the opposite conclusion. In addition, numerous shell fragments contained intact drillholes; breaks passed through drillholes in only 17% of Choptank Oichnus bearing fragments, suggesting that drillholes did not weaken shells significantly. In the Caloosahatchee, taphonomic condition did not vary significantly between drilled and undrilled shells for most of the 24 cases studied. One case, nonpredatory bioerosion in Chione elevata, supported the hypothesis that drillholes cause taphonomic loss of shells. In contrast, visibility of the pallial line and muscle scars and condition of sculpture in Stewartia anodonta indicate that specimens with Oichnus survived more taphonomic damage than undrilled specimens. The predominance of shells with breaks that did not pass through drillholes in the Choptank Formation, and the lack of difference between drilled and undrilled shells for most taphonomic measures in both formations, indicates that drillholes did not weaken shells significantly.