EFFECT OF LITHIFICATION ON DRILLING PATTERN: AN EXPERIMENTAL APPROACH

The study of past biotic interactions is important to understand the paleoecological history of a community and the evolutionary role of such interactions. Drill holes in invertebrate exoskeletons provide one of the very few scenarios where biotic interaction could be studied directly and the related hypotheses could be tested with statistical rigor. Hence, the documentation and interpretation of the spatio-temporal patterns of drill hole frequencies has been the subject of extensive paleontological research. However, taphonomic biases should be taken into account before interpreting a temporal pattern of interaction. We tried to explore a potential source of bias: lithification bias. Lithification bias is the phenomenon that results in a different drilling frequency in a lithified specimen compared to shells in unconsolidated matrix. The conceptual idea behind such bias is the fact that a drilled shell could be misdiagnosed as an undrilled shell in a lithified specimen due to partial exposure of the shell; however, an undrilled shell would never be identified as a drilled one. Hence this bias would always result in a lower observed drilling frequency compared to the original.

Using fresh bivalve shells of Donax scortum, we have calculated the original drilling frequency. Using industrial grade cement, we have lithified the shells and recalculated the drilling frequency based on the exposed surface of the shells. We have always found a significant decrease in drilling frequency from the original one due to lithification. Parameters such as size, ornamentation, sampling area do not seem to affect this bias. However, population of articulated shells are less affected by such bias compared to population of disarticulated shells.

This bias should be considered while studying the long temporal pattern of drilling predation. The Phanerozoic trend shows an overall low but variable predation intensity in Paleozoic and Mesozoic, followed by a significant increase in Cenozoic. Such observed low drilling frequency in Paleozoic and Mesozoic could have been affected by such bias due to the dominantly lithified nature of pre-Cenozoic deposits.