REGENERATION FREQUENCY OF PIRASOCRINID CLADID CRINOID SPINES IN THE UPPER PENNSYLVANIAN WANN AND BARNSDALL FORMATIONS OF NORTHEASTERN OKLAHOMA

Predator-prey interactions are increasingly recognized as major drivers of macroevolutionary trends through the Phanerozoic. One such predation-generated trend in the increase in spinosity among crinoids from the Silurian to the Pennsylvanian, culminating in evolution of the extremely spinose pirasocrinid cladids. Pirasocrinid crowns contain five prominent primibrachial spines, smaller spines at multiple positions along the arms, and a hypertrophied tegmen capped by a radiating array of long, slender spines. This spinosity has been interpreted as a response to heavy targeting by predators, which is supported by the common occurrence of pirasocrinid primibrachial and tegmen spines showing evidence of regeneration following breakage, reflecting unsuccessful predation attempts. However, many details regarding the detailed aspects of geographic, temporal, and paleoecological patterns regarding this predator-prey interaction remain poorly understood. A relatively large sample of pirasocrinid spines, including both primibrachial and tegmen spines, from the Upper Pennsylvanian (Missourian) Wann and Barnsdall formations of northeastern Oklahoma, USA, was analyzed for predation-generated damage. 940 spines were studied (854 tegmen spines, 86 primibrachial spines in total; 852 from the Wann Formation, 88 from the Barnsdall Formation), yielding an overall regeneration frequency of 20.3%. The Wann Formation collection has an overall regeneration frequency of 20.4% (20.0% for tegmen spines, 24.4% for brachial spines) and the Barnsdall Formation collection, which consisted entirely of tegmen spines, has an overall regeneration frequency of 19.3%. All of these are higher than the typical regeneration frequencies reported for pirasocrinids in Pennsylvanian deposits of the North American midcontinent (5-15%) and exceed the highest reported regeneration frequency for primibrachial spines among non-pirasocrinid cladids (18% for the catacrinid Delocrinus). These data indicate intense predation upon pirasocrinids, which is further supported by the presence of 14 spines that were regenerated twice. The reason for such elevated predation intensity involving pirasocrinid targets remains enigmatic, as these crinoids are characterized by morphologies suggestive of being poor prey items.