A STANDARDIZED TEST OF THE INVERSE RELATIONSHIP BETWEEN DRILLING AND REPAIR FREQUENCIES

Chattopadhyay and Baumiller (2010) recently presented fossil evidence that ecological factors (e.g., activity of shell-crushing predators) indirectly affected how drilling predators interacted with their prey. Using assemblage-level data, they found an inverse correlation between drilling and repair frequencies. Because of the potential for bias, due to the sensitivity of repair and drilling frequency values to taxonomic, prey size, and sample size variation, we tested the robustness of their result using a series of standardization methods.

Our preliminary results are derived from samples collected from the late Miocene Cercado (n=3) and early Pliocene Gurabo (n=2) formations exposed in the Rio Gurabo, Cibao Valley, Dominican Republic. The Cercado Fm. represents a shallower water environment, whereas the Gurabo Fm. is a deeper water facies. From these samples we collected shell size and drill hole and repair scar frequency data from over 1,100 shells assigned to six species of Conus (cone snails), which vary substantially in their development of shell ornamentation.

We first analyzed our dataset without standardizing for either prey species, shell size or sample size. The complete dataset shows a significant negative correlation between the drill hole and repair frequencies, with the shallow water Cercado Fm. samples showing much greater drill hole and lower repair frequencies relative to the deeper water Gurabo Fm. samples. When the unstandardized dataset was analyzed with respect to taxonomy, we found that one shallow water species--C. cercadensis, which shows drilling frequencies over 50%--was responsible for driving the higher drill hole frequency patterns in the Cercado Fm. samples. Next, we analyzed the dataset by partitioning species by their occurrences in individual samples; in this case, we found that there was no association between drilling and repair scar frequencies.

Standardizing these partitions by shell size and inclusion of samples with >15 specimens, however, caused the negative correlation between drilling and repair scar frequency to reemerge. Further, this pattern is consistent in one species, C. symmetricus, which occurs in both of the formations. These standardized results support the generality of the indirect effect of crab activity levels on drilling predators.