ABSTRACT

Edrioasteroids are an extinct clade of echinoderm that are rarely preserved in the fossil record due to rapid post-mortem disarticulation of their multi-element skeletons. Here we investigate two well-preserved limestone slabs from the Mississippian (Chesterian) Kinkaid Formation of southern Illinois bearing 242 specimens of the edrioasteroid Neoisorophusella lanei 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. Size-frequency analysis, used as a proxy for age, shows right-skewed, bimodal distributions of thecal diameter on both slabs suggesting high juvenile mortality and two spatfall events. Thecal orientation, using an arbitrarily assigned “North” direction, suggests that no preferential orientation exists. When examined at large scale using Nearest Neighbor Analysis and Ripley's K statistics, edrioasteroids on the slabs exhibit clustering nearing random distributions across the encrusted surfaces. Clusters of edrioasteroids examined at a smaller areal scale show evidence of dispersion. The 242 edrioasteroids cover a combined surface area of 104.41 cm2, yielding an overall density of 23,177 edrioasteroids per m2, an order of magnitude greater than the highest densities previously reported in other studies. No thecal disarticulation occurs in 85% of the population of 242 edrioasteroids suggesting that the edrioasteroids were alive when catastrophically buried. Thecal deformation occurs frequently as a result of competition for space. A striking example of edrioasteroids superimposed onto a larger edrioasteroid substrate may be due to (1) conspecific overgrowth or (2) transport, and subsequent dissolution, of a shell with edrioasteroid epigrowth.

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