INTER- AND INTRASPECIES COMPETITION AMONG EDRIOASTEROIDS (ECHINODERMATA) EXHIBITED ON A NEW HARDGROUND FROM THE UPPER ORDOVICIAN (KATIAN) OF THE CINCINNATIAN

An obrution deposit from the upper Corryville/lower Mount Auburn members of the Upper Ordovician Grant Lake Formation in Maysville, Kentucky, USA preserves a hardground community dominated by the isorophid edrioasteroid Isorophus cincinnatiensis locally in very dense placement; one 23cm by 27cm slab contains 130 individuals. At least two distinct generations of the Isorophus are present based upon size-frequency studies. The hardground itself is rather thin, a 3 to 4cm cemented veneer of calcisiltite atop a fine-grained grainstone with a hummocky upper surface, as well as many centimeter-scale dimples in the surface and collapsed, cracked, and deformed depressions, some of which contain fossils that were distorted or damaged as well. Other encrusters include trepostome bryozoans and the rare edrioasteroid Streptaster sp. Instances of edrioasteroids becoming encrusted by the bryozoans to which they were attached are notable, as they demonstrate contemporary growth of these organisms. There are also several examples of individual Isorophus growing into each other, causing distortion and displacement of the peripheral rims. Multiple edrioasteroid juveniles abut other adult individuals, precluding intergrowth. These interactions were potentially fatal for the edrioasteroids and further support the suggestion that they could not move from a surface once encrustation was established, even when their lives depended on it. In addition to the encrusting organisms of the hardground upper surface, there are smaller encrusters (Cornulites sp., Petrocrania sp., and thin trepostome bryozoans) in life position attached to the ceilings of shallow cavities and overhangs scoured under the hardground surface by removal of less lithified sediments. Notably rare or absent are larger brachiopods, crinoid material, ramose bryozoans, and Trypanites borings, which are much more common in other edrioasteroid communities from the Cincinnatian. This evidence suggests that Isorophus cincinnatiensis was an r-selected species (pioneering, opportunistic), among the first arrivals to the recently colonizable hardground.