STEPWISE CHANGES IN ONSHORE-OFFSHORE BRACHIOPOD DIVERSITY GRADIENTS ACROSS THE LOWER AND UPPER KELLWASSER EXTINCTION EVENTS, UPPER DEVONIAN, APPALACHIAN BASIN, USA

Mass extinctions can affect regional biodiversity and its spatial and environmental distribution. We use non-metric multidimensional scaling to reconstruct onshore-offshore gradients in the species composition and diversity of rhynchonelliform brachiopods across the two pulses of the Late Devonian mass extinction, the Lower Kellwasser (late Frasnian) and Upper Kellwasser (Frasnian-Famennian boundary) events. The current dataset includes hundreds of samples and tens of thousands of specimens from numerous localities in western New York and north-central Pennsylvania. Our interpretations of paleoenvironmental gradients, derived from nMDS analyses of paleontological data, are corroborated by updated facies analyses of fossiliferous stratigraphic sections. We find that the Lower Kellwasser had only moderate effects on onshore-offshore diversity gradients, with the highest diversity (Shannon H) values near the middle of the gradient both above and below the extinction horizon, although species turnover was high. In comparison, we find larger changes in spatial distributions of biodiversity across the Upper Kellwasser horizon, with a shift to high-diversity, stereotyped faunal assemblages in deeper, muddier habitats contrasted against low-diversity assemblages in shallower, sandier habitats, even though overall species turnover was minimal across the boundary. These analyses shed new light on the complex ways that environmental changes may affect ecosystems and the spatial structure of diversity.