PALEOECOLOGIC ANALYSIS OF EDRIOASTEROID (ECHINODERMATA) ENCRUSTED SLABS FROM THE CHESTERIAN (UPPER MISSISSIPPIAN) KINKAID LIMESTONE OF SOUTHERN ILLINOIS

Edrioasteroids, an extinct clade of echinoderm, are rarely preserved in the fossil record due to rapid post-mortem disarticulation of their multi-element body. Here we investigate two well-preserved slabs with 243 specimens of Neoisorophusella edrioasteroids attached to internal molds of Promytilus bivalves. To assess edrioasteroid paleoecology we examined edrioasteroid age structure, thecal orientation, spatial utilization, and degree of post-mortem disarticulation.

Thecal diameter could be measured for 112 of the 127 specimens on slab one and 111 of the 116 specimens on slab two. Size frequency analysis, used as a proxy for age, gives a mean thecal diameter of 2.61 mm for edrioasteroids on slab one and 2.62 mm for edrioasteroids on slab two. Right-skewed distribution of thecal diameter suggests high juvenile mortality. Because the slabs were collected from float, an arbitrarily assigned “North” direction was selected as a frame of reference to determine if preferential orientation of the anterior ray of the theca exists. Thecal orientation was determined for 120 of the 127 specimens on slab one and 105 of the 116 specimens on slab two. Vector analysis shows a magnitude of 0.07 and p-value of 0.11 for the first slab and a magnitude of 0.12 and p-value of 0.78 for the second, suggesting no preferential orientation exists. The 243 edrioasteroids cover a surface area of 99.9 cm², yielding an overall density of 24,324 edrioasteroids per m². Ripley’s K analysis, which examines the spatial distribution at many distance scales, was performed on XY coordinate data for all specimens on each slab. The edrioasteroids on slab one are dispersed when examined at a small scale (< 2.1 cm), randomly distributed between 2.1 cm and 3.3 cm, and clustered at distances > 3.1 cm. Slab two results are similar with edrioasteroids dispersed at distances from 0.8 cm to 1.8 cm and clustered at distances > 1.8 cm. Dispersal at short distances results as a consequence of intraspecific competition for limited resources where clustering at large distances reflects the patchy substrate. Of the 243 edrioasteroids, 219 could be measured for degree of disarticulation. No thecal disarticulation occurs in 94% of the population suggesting that the edrioasteroids were alive when catastrophically buried.