INTRASPECIFIC MORPHOLOGICAL DISPARITY OF CRINOID COLUMNALS IN RELATION TO WATER DEPTH IN THE TYPE CINCINNATIAN (UPPER ORDOVICIAN, 443-453 MYA)

Crinoid columnals are one of the major constituents in the fossil assemblage of the Upper Ordovician of the Cincinnati Arch Region. Several species of Cincinnatian crinoids have been shown to be identifiable based on columnal morphology alone. Disarticulated columnals of two species of crinoids were measured throughout a single 68-meter stratigraphic section of the Kope and lower Fairview formations (Upper Ordovician) to examine the relationship between columnal morphology and minor sea level fluctuations. The columnal diameter of the two crinoids, Cincinnaticrinus varibrachialus and Ectenocrinus simplex, both increased in size, 162 and 61 percent respectively, in the upward shallowing sequence. Detrended Correspondence Analysis (DCA) axis 1 scores computed using columnal measurements of the two crinoids correlated significantly with a faunal gradient curve that has been shown to be a proxy for depth. Therefore, crinoid columnals can provide a metric independent of lithology and faunal gradients for the study of small-scale sea level fluctuation in a depositional sequence. A larger scale study showed similar morphological shifts in Cincinnaticrinus morphology throughout the entire type Cincinnatian. Using measurements from complete crinoids an inverse relationship was established between columnal diameter and branch density in Cincinnaticrinus. Applying this relationship to the Cincinnatian showed a pattern of decreasing branch density in shallower water. This intraspecific pattern is inconsistent with the predictions of aerosol filtration theory that often govern interspecific distributions of crinoids. The morphologic shifts in the columnals are most likely due to differences in nutrient levels and flow regimes between depths.