MICROSTRUCTURAL CRITERIA FOR IDENTIFYING DRILL HOLES OF PREDATORY ORIGIN

Drill holes in prey skeletons are the most common source of data for quantifying predator-prey interactions in the fossil record. However, their utility requires (among other factors) that borings of predatory origin are correctly identified. Building on previous SEM-based studies, high-resolution field emission scanning electron microscopy (FE-SEM) and environmental scanning electron microscopy (ESEM) were applied here to describe and quantify microstructural characteristics of drill holes. Multiple specimens with drill holes, including limpets and mussels from laboratory feeding experiments, subfossil limpets collected from a modern tidal flat (San Juan Islands, WA, USA), and various Miocene bivalves (Winterswijk-Miste area, Netherlands, and Szabó Quarry, Várpalota basin, Hungary) were examined for microstructural features via FE-SEM and ESEM. In total, 6 of the 12 drilled limpets (2 laboratory and 4 field collected), all of the 6 drilled mussels (all laboratory), and 3 of the 14 Miocene bivalves displayed physical scratch marks consisting of parallel, straight-to-slightly-curvilinear, distinctly corrugated lines oriented in a laterally-sweeping pattern across the drill hole outline. These traces are markedly similar to those reported in previous literature and are interpreted here as rasping marks made by the radula of the predator. The mean spacing of lines (ranging from ~5–12 µm) is approximately 10–20 µm smaller than the spacing of radular teeth reported previously (and measured from published SEM photomicrographs) for muricid gastropods. In addition, in the case of one incomplete hole from a laboratory-drilled limpet, a chemical dissolution signature may be recognized toward the outer margin of the drill hole cavity. The distinct microstructural scratch marks that can be readily identified and numerically evaluated using FE-SEM and ESEM offer promise for augmenting our ability to identify drill holes in fossil specimens.