INCREASING SIZE OF DRILLING PREDATORS THROUGHOUT THE HISTORY OF METAZOAN ECOSYSTEMS

The fossil record can provide powerful quantitative insights into the history of ecological interactions. Predatory and parasitic drill holes, common trace fossils preserved in skeletal remains of prey or hosts, provide information on both predator and prey body size. We combined a literature compilation of fossil species occurrences with drill hole sizes (918 occurrences, ~32000 drilled specimens) and present-day observations of drilling organisms (11 predator families) to investigate macroevolutionary trends in drill hole size. Analysis of recent drillers demonstrates that predator size is strongly correlated with drill hole size, irrespective of the ecology and phylogeny of drilling organisms. Analysis of the fossil record indicates that drill holes increased in size from the Neoproterozoic through the Cenozoic. In contrast, mean prey size has remained relatively stable since the mid-Paleozoic. Consequently, the mean ratio between prey size and predator size has decreased through time (i.e., drilling predators have shifted to smaller prey relative their body size). This trend is not easily explained by secular trends in lithification or other long-term taphonomic biases. The Phanerozoic decline in mean prey/predator ratio suggests a long-term change in the nature of trophic interactions.