Site-selectivity analysis in drilling predation may provide useful behavioral information of a predator interacting with its prey. However, traditional approaches exclude some spatial information (i.e., oversimplified trace position) and are dependent on the scale of analysis (e.g., arbitrary grid system used to divide the prey skeleton into sectors). Here we introduce the spatial point pattern analysis of traces (SPPAT), an approach for visualizing and quantifying the distribution of traces on shelled invertebrate prey, which includes improved collection of spatial information inherent to drillhole location (morphometric-based estimation), improved visualization of spatial trends (Kernel density and hotspot mapping), and distance-based statistics for hypothesis testing (K-, L-, and pair correlation functions). We illustrate the SPPAT approach through case studies of fossil samples, modern beach-collected samples, and laboratory feeding trials of naticid gastropod predation on bivalve prey. Overall results show that Kernel density and hotspot maps enable visualization of subtle variations in regions of the shell with higher density of predation traces, which can be combined with the maximum clustering distance metric to generate hypotheses on predatory behavior and anti-predatory responses of prey across time and geographic space. Distance-based statistics also capture the major features in the distribution of traces across the prey skeleton, including aggregated and segregated clusters, likely associated with different combinations of two modes of drilling predation, edge- and wall-drilling. The SPPAT approach is transferrable to other paleoecologic and taphonomic data such as encrustation and bioerosion, allowing for standardized investigation of a wide range of biotic interactions.

Supplementary Table 1. Drilling data on L. latilirata and I. subrugosa specimens examined in this study including locality information, drillhole size and Bookstein shape coordinates. Abbreviations: Bx = Bookstein shape coordinate x, By = Bookstein shape coordinate y. Linear dimensions are in millimeters (mm).

Supplementary Table 2.  Occurrence of naticid gastropods across the fossil bulk samples examined in this study.

Figure S1. Pseudolandmark data collection. The following pseudolandmarks were identified on images of individual valves oriented such that the antero-posterior axis was horizontal (Kolbe et al. 2011): (1) point of maximum curvature of the ventral edge, (2) anterior end of the valve, (3) the beak on the outline, (4) posterior end of the valve, and (5) center of the predatory drillhole.