The Microstructural Record of Predation: A New Approach for Identifying Predatory Drill Holes

Drill holes in prey skeletons are the most common source of data for quantifying predator-prey interactions in the fossil record. However, to be useful, such drill holes need to be identified correctly. Field emission scanning electron microscopy (FE-SEM) and environmental scanning electron microscopy (ESEM) were applied to describe and quantify microstructural characteristics of drill holes. Various specimens, including modern limpets and mussels drilled by muricid snails in laboratory experiments, subfossil limpets collected from a tidal flat (San Juan Islands, WA, USA), and various fossil bivalves collected from marine Miocene bioprovinces of Europe (Boreal province, Paratethys, and southeast North Atlantic), were examined for microstructural features via FE-SEM and ESEM. In total, all of the laboratory-drilled specimens and approximately 32% of the subfossil and fossil specimens displayed physical microstructures interpreted here as radulichnus-like rasping marks made by the radula of drilling gastropod predators. The mean adjacent spacing of traces (ranging from ~5.1–11.2 μm) is notably denser than the spacing of radular teeth in literature-assessed muricid gastropods (ranging from ~11.5–21.4 μm). Given the fact that the observed radular rasping marks typically overlie or cross-cut each other, the denser spacing of microstructural marks likely reflects superimposition of scratches that resulted from repeated passes of the radula. In addition, one incomplete drill hole showed a clear chemical dissolution signature around its outer margin, while a number of other specimens showed possible dissolution traces. The range of organisms examined illustrates the utility of SEM imaging for identifying microstructures associated with predatory drill holes. Moreover, the presence of rasp marks on subfossil and fossil materials demonstrates that SEM-detectable microstructures can be preserved in the fossil record. The distinct microstructural marks, which can be identified and quantitatively evaluated using high-resolution SEM imaging, offer a promise for augmenting our ability to identify drill holes in fossil specimens.