ANTICIPATED VERSUS REAL TAPHONOMIC BIASES ASSOCIATED WITH GASTROPOD BORE HOLE-WEAKENED BIVALVES: KEEPING DR. PANGLOSS AWAY FROM EXPERIMENTAL TAPHONOMY

Functional and evolutionary morphology has long recognized that plausibility is not sufficient to demonstrate a trait is an aptation. Taphonomy is similarly encumbered by the burden of proof: experimental or theoretical biomechanics might suggest a taphonomic bias, but such bias must be documented as genuine in nature before interpretations are made. Gastropod predatory drillholes have been demonstrated experimentally to weaken bivalve shells, creating a potential for selective post-mortem loss. However, if the forces required to break a weakened shell are rare in nature, the effect lacks significance. Results of two studies are presented; one from modern wave-swept environments supports the importance of bias, and another from fossil shelf assemblages dismisses it. Susceptibility to breakage of modern drilled and non-drilled Chione elevata shells was tested experimentally. While forces due to drag, lift, and acceleration reaction in beach environments rarely exceed the threshold strength of drilled shells, impact forces are often great enough to generate taphonomic bias. Patterns in predation intensity in modern beach and estuarine environments along the Florida Gulf Coast support this interpretation. The hypothesis that taphonomic processes preferentially destroy drilled shells was also tested for the Miocene Choptank Fm (MD) and Plio-Pleistocene Caloosahatchee Fm (FL). If drillholes weaken shells, drilled shells should be more pristine than undrilled shells because they do not survive exposure. Most taphonomic measures showed no significant difference between drilled and undrilled shells. In the Choptank, only 1 of 16 cases (shell sculpture, Boston Cliffs Mbr) suggested bias against drilled shells, and many shell fragments bore intact drillholes. Breaks passed through holes in only 17% of drilled fragments, suggesting drillholes did not weaken shells significantly. Only 1 of 24 Caloosahatchee cases (nonpredatory bioerosion in Chione elevata) supported taphonomic loss of drilled shells. Additionally, typical flow-induced forces on modern shelves do not exceed drilled Chione strength. The opposing interpretations provided by these Recent and fossil studies demonstrate that “taphonomic aptation” is circumstance specific and must be evaluated for each environment of occurrence.