TAPHONOMY AND TIME-AVERAGING OF DRILL HOLES: ASSESSING THE QUALITY OF THE FOSSIL RECORD LEFT BY HUNTING GASTROPODS

Drill holes left by predatory gastropods in their victims offer direct evidence of biotic interactions accessible in the fossil record. However, multiple assumptions must be invoked before interpreting quantitative data provided by drill holes. These include: (1) biological assumptions: (i) drilling behavior is obligatory; (ii) mistaken drilling in dead prey never happens; (iii) complete holes always mark successful kills; and (iv) other predators do not destroy prey shells; (2) ichnological assumptions: (i) biotic and abiotic holes can always be differentiated; and (ii) a reliable behavioral and/or taxonomic interpretation of all holes is possible; and (3) taphonomic assumptions: (i) drilling does not affect the prey’s preservational potential; and (ii) processes being interpreted from drill holes operate over time scales exceeding typical levels of time-averaging. Additional assumptions may apply depending on research methods and goals. We focus here on taphonomic assumptions. To assess the effect of drill holes on the prey’s fossilization potential, we test if drilling frequencies are independent of the extent of post-mortem alteration of prey shells. In both test systems, Holocene beach ridges of Baja California and Miocene fossil assemblages of Europe, there is no statistical relation between drilling frequencies and shell taphonomy, suggesting that the pre-burial survival of mollusk shells is not affected by drill holes. To evaluate time-averaging, we compared drilling patterns across three generations of beach ridges that span the last ~5kyr. None of the parameters obtained from drill holes displayed a significant variation across the ridges, suggesting that patterns obtained from individual samples (time-averaged over multi-centennial scales) do not change notably over multi-millennial time scales. Pre-burial taphonomic processes are unlikely to have been a major biasing factor for either of the studied systems and drilling patterns appear to be stable over time scales exceeding typical levels of time-averaging. Whereas it would be premature to postulate any far-reaching generalizations, this study showcases explicit strategies for evaluating the impact of taphonomy and time-averaging on the quality of the fossil record left by drilling gastropods.