Crinoid Anal Sac Spines with Multiple Planes of Regeneration: Predation-Generated Features in the Upper Pennsylvanian of Eastern Ohio, USA

Primibrachial spines of pirasocrinid cladid crinoids that contain two discrete regeneration planes were recently described from the Upper Pennsylvanian Ames Member of the Glenshaw Formation in eastern Ohio, USA. This occurrence constitutes the first report of isolated crinoid ossicles showing evidence for repeated breakage and regeneration, most likely reflecting multiple predation attempts throughout the lifespan of single crinoid individuals. Herein we report specimens of pirasocrinid anal sac spines bearing multiple regeneration planes from the same stratigraphic interval as the previously described brachial spines. These specimens represent the first documentation of tegmen spines that were broken and began regeneration multiple times during the lifetime of an individual. The spines with multiple regeneration planes occur in an assemblage of spines that has the highest regeneration frequency of the entire Paleozoic, suggesting that pirasocrinid crinoids in eastern Ohio during deposition of the Ames Member were subjected to anomalously high (attempted) predation intensities. Additional examples of similar specimens are needed to generate an explanatory model for the unusual frequency of breakage and regeneration, but relationships between the morphology of pirasocrinid crowns and interactions with associated non-predatory organisms may be the most important factor in explaining the high regeneration frequency of crinoid spines belonging to this group during the Pennsylvanian.