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 predation pressure drove an evolutionary trend toward easily-regenerated arm branching patterns. Here we test this hypothesis, using a metric for resilience to predation (ALC, "arm-loss coefficient") based on the average proportion of arm lost in an encounter with an arm-cropping predator. 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 subclasses. Camerates, especially monobathrids, display a pattern of increasingly convergent and predation-resistant arm morphologies from the Ordovician through the Devonian, followed by a period of relaxation during the Mississippian radiation. In contrast, the mean ALC for advanced cladids remains relatively constant over the Paleozoic, with periods of less predation-resistant mean morphology in the late Devonian and early Permian, while poteriocrines . We conclude that camerates may have been the primary targets of Paleozoic durophagous grazing predators.