SKELETAL CRYSTALLOGRAPHY OF ORIGINAL AND REGENERATING CRINOID SPINES

Crinoid skeletal elements are single crystals of calcite that in some instances bear spines or other protuberances. We examined a variety of spine-bearing elements to determine whether a consistent relationship exists between spine orientation and crystallographic axis orientations in crinoids. Such a relationship is known for echinoid spines, in which the c axis parallels the long axis of the spine, but most crinoid spines differ from echinoid spines by forming as extensions of thecal plates rather than as independently articulated skeletal elements.

Calcite decoration and optical goniometry of Delocrinus, Plaxocrinus, and Dorycrinus reveal that primibrachials and tegmen plates with elongated spines resemble echinoids in having c axes that parallel the long dimension of the spine. In contrast, specimens with less pronounced protuberances are more variable: whereas Barycrinus has a protuberance that extends parallel to the c axis direction, Amphoracrinus and Gilbertsocrinus have protuberances oriented perpendicular to their c axes. Spinose primibrachials are atypical because most crinoid primibrachials have c axes that parallel the plate surface rather than extend perpendicular to it. Furthermore, Delocrinus and Plaxocrinus primibrachial and tegmen plate a axes are strongly aligned, a condition rarely observed in echinoderm plates. These observations indicate a strong relationship between skeletal crystallography and morphology in at least some plates with elongated spines. Smaller protuberances do not show as strong a correlation between crystallography and morphology.

The sample of Delocrinus primibrachials includes five spines that began regenerating following spine damage, permitting evaluation of whether the regeneration process left a distinct crystallographic signature. Although documented differences in microstructure exist between original and regenerating portions of spines there is no difference in the orientations of syntaxial calcite crystal faces across the contact between these regions, indicating that despite the change in microstructural fabric the regenerated portion developed in crystallographic continuity with the original plate.