STEREOMIC MICROSTRUCTURE OF CRINOID SPINE REGENERATION: EXAMPLES FROM THE UPPER PENNSYLVANIAN OF EASTERN OHIO

Multiple crinoid groups evolved spines on various portions of the crown throughout the Paleozoic, most likely in response to increased predation pressure. Among the spiniest of Paleozoic crinoids are pirasocrinid cladids, which were abundant and geographically widespread during the Late Pennsylvanian. Pirasocrinid spines are common bioclasts within the Upper Pennsylvanian Ames Limestone member of the Glenshaw Formation (Conemaugh Group) of eastern Ohio, USA, where a large proportion of specimens—including both primibrachial and anal sac spines—display evidence of regeneration following traumatic breakage, presumably via attempted predation by fishes. The stereomic microstructural patterns associated with pirasocrinid spine regeneration have not hitherto been described in detail. Planes of regeneration display four main stereomic patterns, which are consistently observed in spines from all portions of the crown. (1) Pores in between stereomic rods are significantly larger than those present in the dense rectilinear stereom of spines away from regeneration planes. In some cases, these large pores can produce a “pseudogalleried” texture. (2) Large, triangular stereomic structures extend distally from regeneration planes, resulting in a “pseudocrystalline” texture. (3) The triangular extensions expanded laterally until eventually coalescing into a continuous layer. This is the major mechanism for regrowth along the regeneration plane. (4) Development of multiple concentrically arranged laminae of new stereom occurred concurrently, resulting in expansion of spine diameter to nearly full pre-breakage values along lobate regeneration fronts. Collectively, these observations provide greater information on the paleophysiology of cladid crinoids and the biological process of regeneration among Paleozoic macrofauna.