The Paleoecology of Some Middle Devonian Fossil Clusters, Erie County, New York

Abstract

<p> Extensive bedding plane exposures in the Ludlowville shales along Cazenovia Creek near Spring Brook, New York display the spatial distribution of the skeletal remains from a marine faunal assemblage. Fossils typically occur in aggregates that are subcircular in plan view and plano-convex in cross-section with the convex side down. The clusters measure 1 meter in diameter and 2 centimeters thick at the center. This dispersion pattern has led to a general consideration of the different mechanisms responsible for creating fossil aggregations. Possible mechanisms, a spectrum from biological to geological, have been categorized into reproductive, ecological, postmortem redistributional, and preservational modes of formation. </p> <p> Quantitative sampling of the most abundant species, Ambocoelia umbonata, in four successive 5 millimeter layers within three clusters was carried out to determine which process is responsible for cluster formation. Between level variation in shell parameters demonstrates that fragmentation, distortion and valve ratios are independent of trends in position, density, and disarticulation. The trends are not controlled by geological agents, but rather result from ecological conditions. Furthermore, the size distributions of Arnbocoelia are bimodal and have to be explained on a biological basis. This has led to an interpretation of cluster development involving initiation by occasional spat survival on a somewhat "lethal" substrate, subsequent succession and regulation by ecological requirements, and final termination due to failure of spat recruitment probably because of fecal and/or decay toxin buildup. </p>