UNSUCCESSFUL PREDATION (CANNIBALISM) DOMINATES OICHNUS TRACES ON EXTANT NATICIDS ALONG THE U.S. NORTHEAST COAST

The predator-prey relationships involving the drilling naticids have persisted since Mesozoic. The distinct round boreholes (often beveled) have been attributed to Oichnus isp. and are found on bivalve and gastropod shells. This study focused on drillholes on northern moonsnail (Lunatia heros) along the coast of Massachusetts and New Jersey. Whereas different moonsnail species exist along U.S. Mid-Atlantic Coast, L. heros dominant naticid species inhabiting the nearshore zone of Nahant Beach, MA, suggesting cannibalism. This gastropod comprised >60% of mollusk shells, with nearly 10% containing Oichnus, along with occasional irregular openings likely produced by bird or crab predation. The Oichnus diameter ranged from 4.0-7.5 mm and its ratio to shell height clustered at 0.16 and 0.36 (small shells). More than 80% of the drillholes did not penetrate the shells. Due to breakage of the outer lip, the clockwise angle was measured between the apertural inner margin/outer lip junction and the borehole center. This value ranged from 15°-100°, with modes at 52° and 70°. Within a landmark triangulation diagram, all Oichnus traces occur along the umbilicus axis, with center-hole distances to the umbilical tangent of 6.8±4.0 mm. These findings provide quantitative data on likely cannibalistic naticid traces. Further investigation will focus on quantitative analysis of diagnostic criteria of predation success and will complement other shell bioerosion research that addresses predation and cannibalism among ancient naticids.