Ecology and "Life History" of Mid-Devonan Brachiopod 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. A previous paleoecological study of the most abundant brachiopod, Ambocoelia umbonata (Conrad), (Bray, 1969) revealed that aggregations were in-situ 'life' associations rather than mechanical accumulations of shell debris. This conclusion suggested that fossil clusters represent short-lived ecological successions or seres and the present study was undertaken to examine the origin, development, and extinction of clusters .</p> <p>Three clusters, sampled in 10 x 10 x 0.5 cm blocks, were subjected to an exhaustive analysis. The absence of sedimentary structures, lack of internal lamination, minor amounts of silt-size quartz, pelleted thin section textures, numerous burrow traces and presence of 3 species of deposit feeding nuculanid bivalves strongly suggested a bioturbated, fluid substratum. Under these conditions fossil clusters were initiated by attaclunent of spat to scattered shell surfaces (i.e. trilobite carapaces, cephalopod shells and rooted crinoids). Successful growth of sessile invertebrates (largely brachiopods) produced continuous addition of surface area for future spat attaclunent and clusters expanded laterally while the soft bottom prevented vertical buildup faster than sedimentation rates. Fossil density distributions within samples indicated that several species had a patchy arrangement within clusters and that the dispersion pattern of two taxa, .Ambocoelia and Styliolina, altered during cluster development. Analysis of associations by presence-absence criteria revealed that specimens larger than 2 mm had few taxonomic associations near the base of clusters and many associations near the top. The number of associated taxa among specimens smaller than 0.75 mm remained constant, but there was a decrease in the number of shared associations by some taxa toward the top of clusters. Species-abundance distributions indicated no temporal change in diversity and the succession within clusters apparently did not proceed by colonization of different taxa at different times. The lack of bottom currents and the reduction of bioturbation in shell laden patches resulted in fecal concentration that created a substrate which was locally lethal. This ultimately caused failure of spat recruitment and clusters were terminated over a short period of time. </p>