EFFECT OF PREDATORY DRILLHOLES ON TAPHONOMY OF MOLLUSC SHELLS FROM THE MIOCENE CHOPTANK FORMATION OF MARYLAND

Frequencies of drillholes in bivalve and gastropod shells are often assumed to represent accurately the proportion of mortality caused by drilling predation. Such an assumption would be invalidated if drilled shells are destroyed preferentially by taphonomic processes. We tested the hypothesis that taphonomic processes preferentially destroyed drilled shells in the middle Miocene Choptank Formation of Maryland.

Taphonomic condition of drilled and undrilled Choptank shells was studied by teams of student researchers in the context of an invertebrate paleontology course at University of North Carolina Wilmington. Bulk samples were collected from the Drumcliff and Boston Cliffs members of the Choptank at Matoaka Cottages, Calvert Co., Maryland. Samples were wet sieved through 2-mm screens and all whole pelecypod valves and gastropods, as well as all shell fragments bearing drillholes, were picked and identified. Taphonomic condition and occurrence of predatory drillholes were recorded for whole specimens. For shell fragments with drillholes, we determined if breaks passed through drillholes.

Drilling frequencies ranged from 0–32% and 0-16% on abundant species in the Drumcliff and Boston Cliffs members respectively. Numerous shell fragments contained intact drillholes; breaks passed through drillholes in only 17% of Choptank shell fragments, suggesting that drillholes did not weaken shells significantly. Most of the eight measures of taphonomic condition showed no statistically significant difference between drilled and undrilled shells. In both members, shell sculpture was preserved better on drilled shells than on undrilled shells, suggesting that drilled shells experienced less exposure on the seafloor than did undrilled shells. Distinctness of the pallial line (Drumcliff Member) and presence of nonpredatory boring (Boston Cliffs) also indicated that drilled shells did not survive post-mortem exposure as well. However, results for encrustation in the Drumcliff supported the opposite conclusion. Although results are mixed, the predominance of shells with breaks that did not pass through drillholes, and the lack of difference between drilled and undrilled shells for most taphonomic measures, indicates that drillholes did not weaken shells significantly in the Choptank Formation.