EVOLUTIONARY RESPONSE IN PALEOZOIC CRINOID ARM BRANCHING PATTERNS TO GRAZING PREDATORS

The rise of durophagous predators during the middle Paleozoic represents a new ecological constraint imposed on sessile marine fauna. In crinoids, it has been suggested that increasing predation pressure drove several evolutionary trends, such as increases in spinosity and thecal plate thickness, as protection against crushing predation. It has also been shown that for crinoids the average proportion of arm lost in a non-lethal arm-cropping predatory encounter varies as a function of arm branching pattern. Here, using a metric for resilience to predation (ALC, "arm- loss coefficient"), we test the hypothesis that the increase in predation led to more predation-resistant arm branching patterns among Paleozoic crinoids. ALC was computed for 198 genera in the Webster (1997) compendium of Paleozoic crinoids, using specimen illustrations from the Treatise of Invertebrate Paleontology, and analyzed with respect to taxonomy and temporal and geographic range. The resulting data show significant variability between taxa. Camerates, especially monobathrids, display a pattern of increasingly convergent and predation-resistant arm morphologies from the Ordovician through the Devonian, followed by an apparent relaxation in this constraint during the Mississippian. In contrast, the mean ALC for poteriocrines remains constant through the later Paleozoic, while that for advanced cladids shows periods of less predation-resistant mean morphology in the late Devonian and early Permian. These trends suggest that camerates may have had the most frequent non-lethal interactions with grazing durophagous predators; reports of arm regeneration frequencies indicate they were highest among camerates, a pattern largely consistent with the above interpretation.