EFFECTS OF CLIMATE CHANGE ON NATICID CANNIBALISM IN THE EOCENE LA MESETA FORMATION, SEYMOUR ISLAND, ANTARCTICA

Climatic cooling during the Eocene resulted in the disappearance of high-latitude shell-crushing predators from Antarctica due to physiological constraints, restructuring the shallow-water bottom fauna to one dominated by epifaunal suspension feeders and slow-moving, non-durophagous invertebrate predators. This study uses the frequency of naticid cannibalism in the Eocene La Meseta fauna from Seymour Island, Antarctica to examine changes in the food web as climate cooled and diversity of shell-crushing predators in the benthic community declined. We hypothesized that as climate cooled shell-drilling naticid gastropods faced lower predation pressure and that the less risky environment should have resulted in higher drilling frequencies. Examination of almost 2000 individuals of Polinices sp. from over 100 localities showed no change in drilling frequency over the course of the Eocene. Although drilling frequency varied from 1.6 to 6.1% among individual stratigraphic units, no pairwise comparisons were significantly different (Chi-square, p=0.999). Grouping data into bins before and after the cool-down resulted in a similar lack of directional change in drilling frequency (average drilling frequency = 4.7 and 5.4%, respectively; Man Whitney U, p =0.817). This counterintuitive result may be explained by the effect cooling had on metabolism of naticids and thus feeding rates. Lower metabolism due to cold temperatures might have replaced risk from shell-crushing predators as a constraint on naticid predation, effectively dampening expected increases in drilling frequency.