MORPHOMETRIC ANALYSIS OF PREDATORY DRILLHOLES

Quantitative studies of post-Jurassic predatory drillholes assume that drillholes were made by either naticid or muricid gastropods. However, other potential predators can leave similar traces, and the traditional method of discrimination (based on differences in vertical shape of drillholes: cylindrical vs. countersunk) is not always applicable. Poor preservation or thin shells may prevent identification. A multivariate morphometric approach, including principal component analysis (PCA), canonical variate analysis (CVA) and size-free discriminant analysis, is suggested here as a supplementary method. I test the approach by analyzing 98 drillholes made in the valves of the thin-shelled bivalve Mulinia coloradoensis collected from subfossil deposits in northeastern Baja California. Using traditional criteria 10 (10.2%) of 98 drillholes were not correctly identified. Morphometric analyses indicate that in planar view muricid and naticid drillholes do not differ in shape. However, they perfectly discriminate in size. The cylindrical drillholes (Oichnus simplex) attributed to muricid predators are significantly smaller than countersunk ones (Oichnus paraboloides) made most likely by naticid gastropods. The results do not necessarily undermine the validity of the traditional ichnotaxonomy: muricids tend to produce smaller drillholes than naticids. Nevertheless, the vertical shape of a drillhole may be a function of both the type of predator and the size of hole drilled. More than half of the unidentified drillholes (6 out of 10) form an independent group characterized by a more oval shape (octopod drillings?). The traditional method, supported by the morphometric methods employed here, is a powerful tool for analysis of drillholes. It may be valuable not only as a tool for analysis of muricid and naticid drillholes but also as a method for comparative analysis of drillholes of unknown source found throughout the Phanerozoic fossil record.