A HIGH DENSITY EDRIOASTEROID COMMUNITY ON AN UPPER ORDOVICIAN (KATIAN) FROM NORTHERN KENTUCKY: IMPLICATIONS FOR POPULATION AND COMMUNITY PALEOECOLOGY

An obrution deposit from the uppermost Corryville Member, Grant Lake Formation (Upper Ordovician, Katian) from Maysville, Kentucky, USA preserves a hardground community dominated by the edrioasteroid Isorophus cincinnatiensis (Roemer, 1851). The hardground itself is rather thin, a 3 to 4 cm 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. A total of nearly 300 individuals from what appears to have been a living population edrioasteroids have been obtained on small slabs. Other encrusters include trepostome bryozoans and the rare edrioasteroid Streptaster (~10 specimens). Rare, articulated brachiopods (Vinlandostrophia ponderosa) occur in situ and some were host to small encrusting edrioasteroids. Trypanites borings are notably absent, in contrast to most hardgrounds. This low diversity, high dominance community suggests that Isorophus cincinnatiensis was a pioneering, opportunistic species, among the first arrivals to a recently exposed cemented pavement. The distribution of edrioasteroid individuals on the hardground is patchy, with some nearly bare areas, but locally very high density (~2100 per m 2 ). Larger (10 to 15 mm) individuals are rather regularly spaced suggesting spaced larval settling that avoided intraspecific competition on a new substrate. However, there are a few examples of individual Isorophus growing into each other, causing distortion and displacement of the peripheral rims, and multiple later settling edrioasteroid juveniles abut adult individuals. These interactions support the suggestion that these edrioasteroids could not move once encrustation was established, even when their lives depended on it. Instances of edrioasteroids becoming encrusted by the bryozoans to which they were attached are notable as they demonstrate contemporary growth of these organisms. In addition to the encrusting organisms of the hardground upper surface, there is a distinct assemblage of smaller encrusters (Cornulites sp., Petrocrania sp., and thin trepostome bryozoans) preserved in life position attached to the ceilings of narrow cavities and overhangs under the thin cemented layer by scouring and removal of less lithified sediments.