REPAIR SCARS AS INDICATORS OF PREDATION AND ANTIPREDATORY RESPONSE TO PREDATION IN MESOZOIC AMMONOIDS

Studies concerning benthic Mesozoic taxa 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 because predation traces on shelled cephalopods are often difficult to identify. Sub-lethal predation attempts leave repair scars on the shells that can be used as indicators of predation. Because ornamentation in ammonites is considered an antipredatory adaptation, we hypothesized that more highly ornamented shells should exhibit more repairs because of increased survivorship of attacks. Complex sutures may also have developed as an antipredatory adaptation and we hypothesized that taxa with more complex sutures would also exhibit more repair scars. To test these hypotheses, ornamentation, suture complexity, 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 or because predators could detect and preferentially attack less armored prey. Suture complexity was measured using the suture complexity factor (CF) calculated as a weighted count of the features of a suture. No relationship was found between CF and RF. Preliminary analysis of repair scar occurrence with respect to body size in Scaphites shows that there is no significant effect of body size on repair scar occurrence. Additional data will include right versus left occurrence of scars to test for predator preference, the effects of multiple scars to test which individuals are more likely to have multiple scars, and the types of scars present.