New example of Devonian predatory boreholes and the influence of brachiopod spines on predator success

Three abundant Devonian (Frasnian) brachiopod species from the Lime Creek Fm. of Iowa, USA (Douvillina arcuata, Devonoproductus walcotti and Pseudoatrypa devoniana), contain boreholes (pooled drilling frequency is 10.6%). The boreholes are small (usually <1.0 mm diameter), cylindrical, and drilled perpendicular to the shell surface. The borings appear to be Type A boreholes, as described by Ausich and Gurrola (1979, J. Paleontol. 53, 335–344). Based on multiple criteria, including statistically significant stereotypy with respect to valve preference, site on valve and size class of the prey, the boreholes are inferred to be predatory or parasitic in origin. For each species, the predator preferred to attack the valve closest to the substrate, regardless of whether that was the pedicle or brachial valve. Boreholes were concentrated over the muscle field on each of the three species. The success percentage (percent of complete holes) was statistically lower (χ2, p=0.04) for the spinose Devonoproductus walcotti (33.33%) than for the non-spinose, but otherwise morphologically similar, Douvillina arcuata (54.22%), in spite of the fact that D. walcotti has a thinner shell. This is the first quantitative evidence for an anti-predatory function for brachiopod spines.