ANTIPREDATORY MORPHOLOGY AND INTENSITY OF SUBLETHAL PREDATION IN MESOZOIC AMMONOIDS

Studies concerning many benthic taxa from Mesozoic faunas show the diversification episodes that constitute the Mesozoic Marine Revolution to result from escalation in which interactions between individuals and their enemies are the primary drivers of natural selection. Shelled cephalopods are less represented in such work; predation on shelled cephalopods in the fossil record has not been studied as extensively and predation traces on shelled cephalopods are often difficult to identify. Lethal predation usually results in destruction of the shell or in damage that is difficult to distinguish from post-mortem taphonomic damage. However, sub-lethal predation attempts leave repair scars on the shells that can potentially be used as a proxy for predation. Because ornamentation in ammonites has been considered an antipredatory adaptation, we hypothesized that more highly ornamented shells should exhibit more repairs because of increased survivorship of attacks. To test this hypothesis, ornamentation and repair scar frequency (RF) data were collected from nine ammonite genera of Jurassic to Late Cretaceous age from various sites in North America, the Caribbean, and Europe. The metric used to gauge ornamentation was ratio of rib thickness to shell diameter; RF = % individuals with scars. Preliminary data on 335 specimens revealed a logarithmic relationship between RF and shell ornament, with higher repair scar frequency corresponding to taxa with lower degrees of shell ornament, contrary to our hypothesis. If exterior shell ornament is antipredatory in nature, then the less ornamented shells may have experienced more intense predation either because more predators were present in their environments (e.g., deeper waters occupied by Lytoceras) or because predators could detect and preferentially attack less armored prey. Preliminary analysis of repair scar occurrence with respect to body size in Scaphites shows that larger specimens are more likely to exhibit repair scars, though the difference is not statistically significant. Future work will include size standardization of repair data, comparison of suture complexity to repair scar frequency to evaluate the potential antipredatory function of complex sutures, and attribution of different types of repair scars to different causes of sublethal injuries.