TESTS OF THE ESCALATION HYPOTHESIS: THE ROLE OF MULTIPLE PREDATORS

In this study, we build on earlier efforts to understand whether and how morphological evolution in the Plio-Pleistocene members of a prey clade, the bivalve genus Chione in the Southeastern Coastal Plain of North America, influenced the evolution of behavioral stereotypy of drilling predators of the gastropod family Naticidae.

Previously, it was assumed that the Chione-naticid interaction could be isolated by analyzing only beveled drillholes, which are characteristic of naticids. Thus, observed changes in stereotypy of prey size selection, drillhole site selection, and variance of site location of these drillholes were interpreted as reflecting behavioral evolution of naticid predators. However, literature reports and field observations indicate that beveled drillholes may also be produced by muricid gastropod predators. If this is the case here, and if muricids exhibit different patterns of site selectivity than naticids, then trends reported to depict naticid behavioral evolution may, instead, be an artifact of changing the relative proportions of Chione sampled from naticid- versus muricid-dominated assemblages at each time interval.

We tested this hypothesis experiementally using drillholes produced by living naticids (Neverita duplicata) and muricids (Chicoreus dilectus, Phyllonotus pomum) in Chione elevata prey to develop a Recent baseline for each predator that could be compared to drilling traces in the fossil record. Naticid and muricid traces were indistinguishable in the ranges of outer drillhole diameter and drillhole profile angles observed but differed significantly in drillhole site selection; naticids preferred the dorso-posterior region, and muricids, particularly Phyllonotus, preferred the posterior-adductor region. Drilling patterns in fossil Chione reported previously are consistent with shifts between preservation and/or sampling of naticid-only, muricid-only, and combined-predator assemblages. Our data suggest that previous results derived from the record of drilling traces on Chione are likely biased by predator misidentifications.