PREDATION AND ESCALATION OF MESOZOIC AMMONITES: EVIDENCE FROM REPAIR SCARS AND ANTIPREDATORY SHELL MORPHOLOGY

Escalation is evolutionary change in which interactions between individuals and their enemies are the primary drivers of natural selection. We examined escalation in ammonites using repair scars indicative of sub-lethal predation. Because ammonite ornamentation is considered an antipredatory adaptation, we hypothesized that 1) more highly ornamented shells would exhibit more repairs due to increased survivorship of attacks; 2) different ornament types (ribs, nodes, keels) would vary in success as defensive strategies; 3) scars on taxa with larger ribs would be less able to propagate and thus have smaller longitudinal extents; 4) body size would have a limited effect on repair scar occurrence; 5) taxa with more complex sutures would exhibit more repairs; 6) repair scars would be concentrated on the body chamber of the shell; and 7) predatory attacks would exhibit no right-left selectivity. To test these hypotheses, ornamentation, suture complexity, and repair scar frequency (RF) data were collected from 26 Jurassic and Cretaceous ammonite genera from North America, the Caribbean, and Europe. Ornamentation was gauged as ratio of rib thickness to shell diameter; a suture complexity factor (CF) was calculated as a weighted count of suture features. RF was measured as % individuals with scars and as average number of scars per individual. Data on 1180 specimens revealed a significant inverse correlation between RF and shell ornament, contrary to hypothesis 1. If exterior shell ornament is antipredatory, then less ornamented shells may have experienced greater predation because more predators were present in their environments, predators preferentially attacked less armored prey, or heavily armored prey sustained fewer permanent injuries. Taxa exhibiting ribs, nodes, and a keel had fewer repair scars than expected, and taxa with ribs and nodes but no keel had more repair scars than expected. Rib size had a slightly significant positive correlation with longitudinal scar size. No relationship was found between CF and RF. We found a significant effect of body size on repair scar occurrence in Scaphites but not in Jeletzkytes or Hoploscaphites. Scars were concentrated on the first 90° of the shell, implying that predators selectively attacked the body chamber. No right-left selectivity was found.