THE DYNAMICS OF RAPID, ASYNCHRONOUS BIOTIC TURNOVER IN THE MIDDLE DEVONIAN APPALACHIAN BASIN OF NEW YORK

The study of ancient biotic transitions on a regional scale has typically revealed prolonged periods of relative compositional stability punctuated by intervals of rapid reorganization. Many previous studies have focused on delineating periods of stability, but few studies have dissected in detail the intervals of change. To better understand the nature and variation of regional biotic transition at high resolution, a detailed analysis was conducted of the well known faunal turnover during the interval ranging from the so called “Hamilton fauna” of the Hamilton Group through the biota of the Tully Formation, in the Middle Devonian Appalachian Basin of New York. Until recently, it was difficult to study this interval in detail because in most areas the Taghanic Unconformity truncates the transitional beds . However, newly discovered localities display a nearly continuous stratigraphic record and are the focus of this study.

Faunal sampling of localities along a 220 km east-to-west transect across the basin was conducted to compare the turnover dynamics of biotas in different paleoenvironments. In particular, it might be expected that shallow water communities were replaced at a different rate than communities in deeper settings because of depth-related differences in the responses to perturbation. Multivariate analysis of faunal data collected at the species and genus level for several shallowing up cycles within the upper Windom Shale of the Hamilton Group shows that community membership in various biofacies remained virtually unchanged until the onset of the Tully Limestone, with a fairly profound transition thereafter in low diversity, dysoxic biotas. Surprisingly, preliminary analyses suggest that most elements of the Hamilton fauna persisted into the upper Tully Limestone within diverse, oxygenated communities. Taken together, these results suggest that the transition between the Hamilton and Tully biotas was fairly abrupt, but was asynchronous among depth related biofacies. Hamilton communities persisted longer in shallow water, while deeper, dysoxic communities were replaced earlier. More broadly, this investigation suggests that, in the study of regional biotic turnover, the paleoenvironmental context of biotic transitions may be important.