FUNCTIONAL GROUPS, PREDATOR IDENTITY, AND DRILLING GASTROPODS IN THE FOSSIL RECORD

Ecologists and paleoecologists commonly group organisms into functional groups. This practice assumes that organisms within these groups “act and respond” in a similar manner (Chalcraft and Resetarits, 2003). However, ecological studies have shown that such simplification can be misleading. For instance, predators within the same functional group may elicit different responses in their shared prey.

Shell-drilling gastropods, which leave a small hole in their prey, are an important functional group within the fossil record. Traces left by these predators have played a central role in testing evolutionary hypotheses such as escalation or co-evolution. However, even within the drilling predator-prey system, where the end result is always a hole in the shell of the prey, the question of predator identity becomes important due to the potentially different selective pressures exerted by different drillers.

We examined the question of predator identity in the Moore House Mbr. of the Pliocene Yorktown Formation, which yields a large number of drilled adult bivalves of the genus Mercenaria and potential drilling predators - in particular members of the naticid genera Euspira and Neverita and the muricid genus Ecphora. It is not widely acknowledged that naticids and muricids often share prey within a similar environment. In this case, naticids have been assumed to be the sole drillers of Mercenaria even though Ecphora was also present.

We examined prey size and drill-hole position, diameter, depth and angle in fossil Mercenaria shells, and in Mercenaria shells drilled by living Neverita and Euspira in the lab. Preliminary results suggest that naticids were not solely responsible for the drilled fossil Mercenaria. Further comparative analysis of other stratigraphic units (e.g. Rushmere Member of the Yorktown Fm.) is required to determine whether different proportions of muricids and naticids influenced prey morphology. If they didn't, then grouping these drilling predators into a functional group would be valid in this particular case. On the other hand, if there were meaningful differences, then any analysis must begin with an explicit demonstration of predator identity.