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