PREDATION OF MESOZOIC AMMONITES THROUGH TIME AND SPACE: COMPARISON OF REPAIR SCAR FREQUENCY AND ANTIPREDATORY SHELL MORPHOLOGY IN NORTH AMERICA AND EUROPE

Predatory selection pressure for any given taxon varies in time and space. We compared predation intensity and antipredatory morphology in ammonites collected from multiple locations in North America and Europe. We used repair scars indicative of sub-lethal predation to infer predation intensity. Our previous work found that shells with smaller ribs tend to exhibit more repair scars. We hypothesized that 1) this relationship between repair scars and rib size would hold for multiple times and locations; 2) taxa with more tightly spaced ribs would exhibit more repairs regardless of geographic location; 3) different ornament types (ribs, nodes, keels) would vary in success as defensive strategies between geographic regions; 4) scars on taxa with larger ribs would be less able to propagate and thus have smaller longitudinal extents; and 5) predatory attacks would exhibit no right-left-medial selectivity for either North America or Europe. To test these hypotheses, ornamentation and repair scar frequency (RF) data were collected from 26 Jurassic and Cretaceous ammonite genera from North America and Europe. Ornamentation was gauged as ratio of rib thickness to shell diameter. Extent of scars and their position on the shell (left, right, or medial) were recorded. RF was measured as % individuals with scars and as mean number of scars per individual. Data on 1180 specimens revealed a significant inverse correlation between RF and rib size for both North American and European taxa. 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 shell breakages. Taxa with tightly spaced ribs were found to exhibit more repair scars, either because such shells were more prone to breakage or fewer injuries were lethal. Predatory selection of ammonites with different types of ornament varied between North America and Europe. In the European shells, more scars were found on taxa with ribs as their only ornamental feature. In the North American shells, more scars were found on taxa with nodes and ribs. No relationship was found between rib size and scar size for either geographic area. No right-left-medial selectivity was found.