DRILLING PREDATION ON BIVALVES ACROSS THE PLIO-PLEISTOCENE EXTINCTION IN THE U.S. MIDDLE ATLANTIC COASTAL PLAIN

We investigated geographic differences and temporal patterns in drilling predation on bivalves across the multiphased Plio-Pleistocene extinction in Virginia and the Carolinas. Samples north of the Neuse River include the Yorktown, Chowan River, and lower and upper James City formations. We collected samples from the Duplin (equivalent to Yorktown), lower and upper Waccamaw (James City correlatives), and Canepatch formations in SE North Carolina and in South Carolina. Samples from 23 localities yielded ~70,000 whole valves, from which we tallied drillhole occurrence. We also examined the relationship between drilling frequency (DF) and prey life mode (determined for each genus from the NMITA database).

DF for southern samples exceeded those in the north (0.161 vs 0.097 when samples were combined in each area), though a Mann-Whitney U test was marginally significant (p=0.06). Northern and southern samples showed the same pattern crossing the extinction: Yorktown DF (0.227) significantly exceeded post-Yorktown drilling (0.05) and Duplin DF (0.174) was greater than in younger samples (~0.15). In the south, differences between intervals were not significant based on a Kruskal-Wallis test, but χ² tests comparing Duplin and younger intervals were significant. Ten of 16 common genera also decreased in DF from the Duplin to lower Waccamaw. Assemblage DFs did not differ significantly between lower Waccamaw and younger samples, but 13 of 16 genera increased in DF between the lower and upper Waccamaw.

Predation pressure may have changed across the extinction, but differences in assemblage DFs could occur if different life modes varied in DF and their representation within units. In most cases, the relationship of DF to life mode is not straightforward. Chemosymbiotic and suspension-feeding bivalves were drilled more than deposit feeders in the Duplin but not the Waccamaw; bivalves with chemosymbionts were drilled more than suspension feeders in the lower Waccamaw but not in other units. Epifaunal DF exceeded infaunal DF in the upper Waccamaw but not elsewhere. Prey mobility also yielded conflicting results. The only consistent results were in attachment; unattached taxa were drilled more than bysally attached taxa in all units, though unattached taxa were not drilled consistently more than cemented taxa.