A BORING HISTORY OF DRILLING PREDATION ON THE PALEOZOIC BRACHIOPOD COMPOSITA

The history of drilling predation is at best full of holes and at worst full of gaps. Nevertheless, changes in drilling patterns through time offer one of the best records of long-term biotic interactions and can provide rigorous ways for evaluating the hypothesis of escalation for predator-prey systems. One way to test for escalation is to look at one specific lower taxon throughout its geologic range. We examined all specimens of the brachiopod genus Composita (geologic range Late Devonian through Permian) in the biologic collection of the Smithsonian Institution (n=20,566) for evidence of drilling predation. Brachiopods were chosen because of their low mobility (i.e., morphologically cryptic prey escalation via increased metabolism or mobility can be safely ruled out in this case). Composita has a smooth shell that shows no outward signs of classic morphologic escalation (spines, ridges, etc.). There is no change in the shell morphology or body size of Composita through its geologic range. Our data show that Composita was subject to drilling predation, but at extremely low levels. Overall drilling intensity per geological period ranges from a low of 0.55% in the Pennsylvanian to a high of 0.71% in the Mississippian. However, at some localities the drilling intensity is as high as 10%. The following explanations can be proposed for the absence of change in the morphology and size of Composita: (1) predation rates were too low to result in the escalation of prey morphology; (2) holes were made by parasites and negative effects for host were insufficient to force an evolutionary response; and (3) other factors than predation or parasitism were more important in controlling the morphological evolution of Composita. On the other hand, the continuous presence of drill holes in Composita through its entire range suggests that the drilling behavior was a successful strategy for some predators/parasites of Composita.