SPATIAL DISTRIBUTION, ORIENTATION, AND TAPHONOMY OF THE FLORENCE EDRIOASTEROID PAVEMENT, UPPER ORDOVICIAN, KENTUCKY, USA
Of the 322 edrioasteroids, 309 are found attached to the shells of 165 Rafinesquina brachiopods. Edrioasteroid interspecific Nearest Neighbor Analysis (NNA) shows a clustered distribution on the substrate. However, NNA shows a random distribution for the edrioasteroid-encrusted brachiopods. Frequent occurrences of multiple edrioasteroid specimens attached to individual brachiopod shells are noted and helps explain the clustered edrioasteroid distributional pattern. Some edrioasteroids display margin deformation as a result of crowding and provides evidence that edrioasteroids were unable to move once attached.
Population structure analysis for Isorophus cincinnatiensis shows a strong right-skewed distribution (skewness = 1.92) with a mean diameter of 9.5 mm. The paucity of mature specimens is attributed to high juvenile mortality. Specimens of Streptaster vorticellatus show a normal distribution with a mean diameter of 11.96 mm, while Carneyella pilea had too few specimens for meaningful analysis.
Of the three edrioasteroid species on this pavement, only Isorophus cincinnatiensis occurs in sufficient numbers for orientation analysis. Vector analysis gives a magnitude of 0.04, indicating that no preferential orientation exists. The magnitude increases to 0.13 with a p-value of 0.054 when the angles are doubled, indicating slight bipolar orientation. This is likely the result of edrioasteroid thecal orientation to current flow.
Edrioasteroids show wide variation in taphonomic grade with 83% of the specimens either fully articulated or missing portions of the peripheral rim. We interpret this pattern of thecal disruption to be the result of postmortem decay, which caused bloating and rupture of the edrioasteroid theca.