Comparative Spatial Variability in Faunal Composition Along Two Middle Devonian Paleoenvironmental Gradients

Previous studies of spatial patterns among fossil assemblages have demonstrated the utility of gradient analysis as a n investigative tool inpaleobiology. However, because lateral samples in these studies have routinely been widely spaced (commonly tens of kilometers apart), there is little sense of the degree to which faunas vary locally, relative to variation observed a t larger scales along hypothesized transects. To address this concern, the present study assesses preserved macrofaunal spatial variability in composition among samples collected a t both outcrop (small) and basinal (large) scales, along two onshore-offshore transects in the Middle Devonian Hamilton Group within the Appalachian Basin. The horizons sampled are stratigraphically discrete and temporally constrained. Quantitative analyses of faunal composition among samples demonstrate that: 1) Transitions in composition, associated with the onshore-offshore transect, are readily discernible a t the basinal scale in both horizons. Along the Browns Creek bed transect, faunal associations vary from a n onshore rugose coral-dominated assemblage in a wackestone to a n offshore ambocoeliid assemblage in a calcareous mudstone. I n the Hills Gulch bed, the faunas range from a rugose coral-meristellid assemblage in a crinoidal packstonelwackestone to a diverse brachiopodl infaunal bivalve assemblage in a calcareous mudstone. 2) I n most instances, compositional variation from locality to locality exceeds the degree of variation recognized among samples a t individual outcrops. However, there are exceptions, suggesting that it is important to collect multzple samples from each locality. 3) Possible non-random variability a t the outcrop-scale is apparent among samples a t two of the localities (one in each bed), suggesting that outcrop-scale uariability is worth exploring in more detail in future analyses. INTRODUCTION