MAPPING PALEOECOLOGICAL GRADIENTS OF BRACHIOPODS ACROSS THE LATE DEVONIAN EXTINCTION IN THE APPALACHIAN BASIN

Quantitative analyses of ecological change across major mass extinction events are vital to understanding of ecosystem evolution in deep time, and may yield predictions about future biotic responses to environmental change. Ecological ordination techniques like NMDS (non-metric multidimensional scaling) place samples along gradients in species composition that can correspond with environmental factors that controlled the distribution of species, e.g., depth, lithology, inferred system energy, etc. These scores can be used to track environmental change through stratigraphic sections and to calculate each species’ preferred environmental placement.

We applied NMDS to bulk samples of brachiopods taken from a paleoenvironmental transect of Upper Devonian strata from the Appalachian Foreland Basin. We focused on the Lower Kellwasser Event (LKW), the first pulse of the Frasnian-Famennian extinction, during which ~50% of brachiopod species went extinct. NMDS was applied separately to samples from the Wiscoy Formation, which predates the LKW, and Canaseraga Formation, which post-dates it. Analyses indicate that the distribution of species in the two formations is similarly structured by two primary gradients: onshore-offshore position and degree of environmental disturbance.

Here, we use the NMDS results to track paleoenvironmental change through stratigraphic sections that cross the LKW. However, differences in faunal composition and in the paleoenvironmental distribution of samples preclude the simple combination of pre- and post-extinction NMDS analyses. Therefore, we normalized the NMDS results between the two formations with regression equations fit to each axis using the “preferred environment” values (i.e. NMDS coordinates) of survivor species. This method is applicable to other instances of faunal turnover; however, some number of species must survive the extinction without dramatically changing their paleoenvironmental distribution.