ON THE ADAPTIVE FUNCTION OF SHELL ORNAMENTATION IN THE MURICID GASTROPOD ECPHORA

The muricid gastropod Ecphora is among the most recognizable fossil mollusks. Its striking, heavily-ribbed morphology has garnered attention from collectors and scientists alike, but few ecologically focused studies have been pursued with Ecphora. We know that Ecphora was a predator because it is the only snail large enough to have made certain drillholes in co-occurring bivalves in the Miocene Chesapeake Group of Maryland, but much of Ecphora’s ecology remains poorly understood.

One important evolutionary trend is Ecphora’s evolution of an additional rib. Most Early/Early Middle Miocene ecphoras have 3 primary ribs, but by Early Late Miocene time only 4-ribbed morphs occur. We hypothesized that the fourth rib was an adaptation against shell-crushing predators. To test this hypothesis we counted scars on 3 and 4-ribbed Ecphora from the Chesapeake Group in the collections of the Paleontological Research Institution in Ithaca, NY. Ecphora species identities are still contentious, so we binned our data by 3 and 4-ribbed morphotypes. Using repair scar frequency as a conservative index of selection pressure to enhance shell armor, we expected scar frequencies on Ecphora to increase between 3 and 4-ribbed morphologies and from Early to Late Miocene time.

Because the frequency of shell repair is often size dependent, we compared only specimens 15-40mm in shell width. The majority of Ecphora specimens had at least one scar (82% with 3 ribs (n=17) and 71% with 4 ribs (n=14)), so we calculated repair frequency as repair scars divided by the number of shells examined, to account for shells with multiple scars. Within our samples the 3-ribbed morphs averaged 1.4 scars/shell and the 4-ribbed morphs averaged 2.4 scars/shell. With such a low sample size, it is premature to draw firm conclusions, but our data are suggestive of increased scar frequency on 4-ribbed morphs. Cheseapeake Group bivalves, responding to shell-drilling predators, display similar trends (Kelley 1989, 1991).

Kelley’s and our preliminary results suggest an escalation of antipredatory morphology in response to increased predation pressure. Future work will increase sample sizes and explore the effects of rib shape, attack position, inter-rib distance, and partial shell uncoiling on scar accumulation.