EDGE DRILLING ON THE PALEOZOIC BRACHIOPOD PERDITOCARDINIA

Predator-prey and parasite-host interactions are important ecologically and, as they may lead to co-evolution, evolutionarily. The fossil record provides a source of information about the nature and temporal distribution of such interactions and data for assessing their evolutionary consequences. Drillholes represent an important source of fossil evidence of such interactions and in this study we present data on drillholes in the Mississippian brachiopod, Perditocardinia.

We examined specimens of the Mississippian brachiopod Perditocardinia dubia from the Salem Limestone of St. Louis County, Missouri. Our data revealed that 26.3% (n=259) of the specimens (length 1.47 mm to 5.33 mm) contained boreholes. The boreholes are small (0.13 mm to 0.8 mm in diameter) and round, and each brachiopod has been drilled only once. There appears to be little evidence for valve selectivity, but the brachiopods are uniformly drilled on the edge of the shell, some directly on the commissure.

The high frequency of borings is more similar to the post-Paleozoic record of drilling, generally associated with predation by muricid and naticid gastropods, than with previously reported Paleozoic examples. Reports of edge drilling are rare throughout the Phanerozoic. The Perditocardinia example may thus represent a rare, geologically short-lived driller and behavior analogous to modern capulids that parasitize pectinid bivalves by positioning themselves along their edges and sometimes drilling the shell.