SPPAT: Spatial point pattern analysis of traces

Trace fossils left by predators on skeletons of their prey, including drillholes, repair scars, fractures, and tooth marks, are the most powerful direct indicators on predator-prey interactions available in the fossil record. For instance, predatory drillholes can provide valuable behavioral information regarding selectivity of predatory attacks in terms of prey species, prey size, or drilling location on the prey skeleton (site-selectivity). Although predatory drillholes are spatially explicit traces that represent a uniform record of a single behavior and are frequent enough to make statistical analyses, current approaches used to evaluate site-selectivity ignore some spatial information. In this paper, we introduce the spatial point pattern analysis of traces (SPPAT), an approach for visualizing and quantifying the distribution of predation traces on shelled invertebrate prey, which includes improved collection of spatial information inherent to drillhole location, improved visualization of spatial trends, and distance-based statistics for hypothesis testing. SPPAT provides information on spatiotemporal changes in site-selectivity patterns of drilling predators, including variation in alternative shell-drilling behaviors, useful for understanding predator behavior and anti-predatory responses of prey. The SPPAT approach is transferrable to a wide spectrum of paleoecologic and taphonomic data such as encrustation and bioerosion, allowing for standardized investigation of a wide range of biotic interactions.

Site-selectivity in drilling predation

Naticids are a group of predators in soft-bottom marine communities. They drill a hole through the wall of the prey’s shell in order to feed. Laboratory-feeding trials, beach and museum samples were used to evaluate site-selectivity in the behavior of Notocochlis unifasciata on the bivalves Protothaca  asperrima and Iliochione subrugosa. Despite that death assemblages could mix records from multiple predators, the distribution of predatory traces on samples of I. subrugosa is reminiscent of the pattern observed on the feeding trials. Currently, I’m developing a spatially explicit approach to study site-selectivity using point process modeling that may improve our ability to identify drilling predators in the fossil record and to explore more rigorously spatial and temporal changes in site selectivity by drilling predators.

The post-Palaeozoic fossil record of drilling predation 

Drilling predation on shelled marine invertebrates represents a unique opportunity to quantifying a biological interaction in the fossil record. Predatory drillholes provide information on intensity and selectivity of this predator-prey interaction. Based on an extensive compilation of literature sources and museum samples, the macroevolutionary history of the drilling predation on the lingulide brachiopods was assessed.  Several examples of this biological interaction, ranging from Eocene to Recent in age, were documented. The Mesozoic-to-Cenozoic increase in drilling frequencies on lingulides is similar to the trends observed in other marine benthic invertebrates and consistent with the hypothesis that predation pressures increased through time in marine ecosystems.