BURIED ALIVE: DECIPHERING EDRIOASTEROID PALEOECOLOGY USING A CATASTROPHICALLY ENTOMBED BIVALVE SHELL PAVEMENT FROM THE UPPER ORDOVICIAN (MAYSVILLIAN) MIAMITOWN SHALE, SHARONVILLE, OHIO

Occurrences of articulated multi-element skeletons of edrioasteroids, an extinct echinoderm clade, provide evidence of sudden burial of the living assemblage and an opportunity for detailed paleoecologic analysis. This study examines a catastrophically buried community including 82 live Isorophus edrioasteroids residing on dead shells of Ambonychia and Modiolopsus bivalves in the Upper Ordovician Miamitown Shale at the Trammel Industrial Park in Sharonville, Ohio. Other taxa found on the pavement include Cornulites worm tubes and Zygospira brachiopods, thought to be alive at the time of burial, along with skeletal debris of dead cephalopods. It is ambiguous whether gastropods and encrusting bryozoans on the pavement were alive at time of burial.

The in situ bivalve shell pavement was examined to assess edrioasteroid life mode and community dynamics. We examined age structure, thecal orientation, spatial utilization, and degree of post-mortem disarticulation. Thecal diameters, used as a proxy for age, allow examination of population structure at the time of burial. Size frequency analysis shows a right-skewed distribution with mean diameter of 11.13 mm, suggesting high juvenile mortality. Of the 82 edrioasteroid specimens, 29 could be measured for orientation as related to present-day north. Vector analysis gives a mean azimuth of 175.32 degrees, magnitude of 0.10 and p-value of 0.74, indicating no preferred orientation. Edrioasteroids cover a total surface area of 7.8 m², yielding a density of 10.5 edrioasteroids per m². Two spatial distribution analyses were performed. Nearest Neighbor Analysis, which compares the mean distance between nearest neighbors to the expected mean distance between randomly distributed edrioasteroids, indicated a weakly clumped distribution. Ripley’s K analysis, which examines the spatial distribution at many distance scales, shows the L(d)-d line lying above the upper 95% confidence interval for complete spatial randomness, again indicating clustering. Of the 82 specimens, 70 showed high levels of disarticulation, suggesting extended residence time on the seafloor without protection of sediment cover. Although no interactions were noted between edrioasteroids and other faunal elements, a strong epibiotic relationship between Cornulites and brachiopods exists.