ARE BIFURCATED PIRASOCRINID CRINOID TEGMEN SPINES THE RESULT OF A NORMAL GROWTH PROCESS OR A PATHOLOGICAL RESPONSE TO PREDATION-INDUCED DAMAGE? PRELIMINARY STATISTICAL RESULTS

Pirasocrinid cladid crinoids are characterized by extreme spinosity best displayed by a prominent array of long, radiating spines capping a hypertrophied tegmen. This spinosity has traditionally been interpreted as an evolutionary response to intense predation intensity, an assertion that is supported by spines that commonly display planes of breakage and regeneration, indicating a non-fatal predatory attack. Tegmen spines are occasionally discovered with a bifurcated or “forked” morphology, in which the spine shaft terminates in a pair of sharp tips. This unusual morphology may be a pathology, reflecting abnormal regeneration following breakage of the spine during attempted predation. Alternatively, this may represent a normal ontogenetic process in which particularly wide spines developed a branch point along the shaft. The second hypothesis suggests that bifurcation should preferentially occur in relatively wide spines. A sample of 733 pirasocrinid tegmen spines was collected from the Upper Pennsylvanian Wann Formation near Bartlesville, northeastern Oklahoma, and neck width was measured for each specimen. This sample consisted of 7 bifurcated spines and 726 typical spines. A two-sample bootstrap test based on 1000 sub-samples of 7 spines from the non-bifurcated population found that a median width similar to that of the bifurcated spine population could be generated via random sampling less than 1% of the time. This result supports the ontogenetic hypothesis, which predicted an association between bifurcation and greater spine width. However, caution is warranted in ascribing bifurcation exclusively to a normal growth process: (1) the small sample size of bifurcated spines prevented reliable results from being obtained through several other statistical tests; (2) the spines are only identifiable to family, so differences in growth or regeneration patterns among pirasocrinid genera cannot be determined; and (3) at least one spine has been discovered that displays two spine tips regenerating from a plane of breakage in an otherwise non-bifurcating spine. Hence, the conclusion that a “forked” morphology in pirasocrinid tegmen spines is the result of a normal growth process for particularly wide spines is not unequivocal, and more data must be gathered before this interpretation can be confirmed.