Where it's worth it : frequency and spatial distribution of bioerosional drill holes in planktonic foraminifera reveal different strategies in site selectivity

Abstract

Despite advances in understanding planktonic foraminifera environmental interactions, their role as prey remains elusive, often inferred from indirect evidence such as drill holes. Bioerosional traces offer valuable insights into fossil assemblages, although knowledge for planktonic foraminifera remains limited compared with benthic species. This study addresses this gap by analyzing bioerosional site selectivity in late Quaternary planktonic foraminifera from the western South Atlantic. We examined 2588 specimens from eight species to map trace patterns using kernel density estimation, sector-based, and hotspot mapping approaches. Drilling traces were located, transposed to graphical representations, and transformed into x,y coordinates. We analyzed specimen frequency per trace quantity and trace frequency, sectoring them per chamber and test regions. Correspondence analysis and exact test of goodness of fit assessed groupings among the species and preferential regions. Frequencies revealed that spinose species had more multiple-drilled specimens than non-spinose ones. Bioerosional traces were prevalent in the final whorl, decreasing toward earlier chambers. However, when normalized by surface area, the penultimate whorl had higher trace frequencies, particularly for spinose species, while the ultimate whorl had higher trace density for some non-spinose ones. Spinose species are preferentially drilled in the early chambers, likely due to their thinner walls. Thus, bioeroders prioritize regions with a higher cost–benefit ratio, which is evident in the prevalence of successful–failed traces in early chambers of spinose species, but not in thicker-walled, non-spinose ones. Our study reveals distinct bioerosion patterns, highlighting strategic site selectivity and suggesting that morphological differences between spinose and non-spinose species contribute to varying vulnerability to bioerosion.