CONFAMILIAL PREDATION IN NATICID GASTROPODS FROM NORTH CAROLINA THROUGH A PULSED EXTINCTION IN THE PLIO-PLEISTOCENE

A two-pulsed extinction occurred in the western Atlantic Ocean in the Plio-Pleistocene. Turnover related to the extinctions has been studied, but accompanying ecological effects have been largely neglected. This study examined the effects of the turnover event on confamilial predation in shell-drilling naticids, a well studied and easily quantified predator-prey interaction in the record. Frequencies of confamilial and alternative prey were compared across the extinctions with the goal of elucidating changes in the ecological interaction within the community and specifically in naticids. Recent experiments have shown low frequencies of cannibalism result from high competition (Gould et al, 2010). Thus, drilling frequency on naticids is expected to increase after the extinctions, corresponding to a less competitive environment (Dietl et al, 2004).

Naticid specimens were collected in the Pliocene Duplin Formation and the Pleistocene lower and upper Waccamaw Formation of southeast North Carolina to test this hypothesis. Field and museum collections examined by Christie et al. (2009) were also used, yielding a database including eighteen localities and 2228 specimens. Specimens were examined for drill holes, and frequencies of predation were determined for each formation. Drilling frequency is significantly different (p<0.01) between formations. The Duplin Formation is characterized by a drilling frequency of 0.04 (n=340). In contrast, both lower (n=1628) and upper (n=260) Waccamaw Formation are characterized by a drilling frequency of 0.14. The data support the conclusion that the competitive environment following the extinctions was less severe than pre-extinction conditions.

Preliminary data on alternative prey species from the Duplin and lower Waccamaw formations indicate a high abundance of alternative prey items. The Lumber River locality in the Duplin Formation exhibits a drilling frequency of 0.13 on naticids, ranking them as the ninth most preyed upon species. The Register Quarry locality in the lower Waccamaw Formation has a higher frequency of drilling at 0.16, but the naticids drop in rank to 14th most drilled. These results support the conclusion that increased predation was not caused by a decrease in alternative prey species, but was due to release of competitive pressure.