QUANTITATIVE COMPARISON OF PALEOECOLOGICAL GRADIENTS OF BRACHIOPODS ACROSS A LATE DEVONIAN EXTINCTION EVENT

Quantitative analysis of ecological changes across major mass extinction events are important to understanding ecosystem evolution through deep time and may yield predictions about future biotic responses to environmental change. Ecological ordination techniques like NMDS (non-metric multidimensional scaling) organize samples along species composition gradients that can then be interpreted in the context of potential controlling environmental factors, e.g., depth, lithology, inferred system energy, oxygen availability, 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 brachiopod samples taken along a shallow-to-deep basin transect of the Appalachian Foreland Basin. These samples straddle the Lower Kellwasser Event (LKW), the first pulse of the Frasnian-Famennian Extinction Event. NMDS was performed separately on samples from pre-extinction (Wiscoy Formation) and post-extinction (Canaseraga Formation) strata. These analyses revealed similar compositional gradients in brachiopods in both formations, primarily structured by two environmental gradients: onshore-offshore position and degree of environmental disturbance.

Here, we revisit this method with a more refined brachiopod dataset. Using NMDS, we track paleoenvironmental change through the extinction interval and surrounding strata. Differences in the composition of the species pool before and after the extinction prohibit a combined analysis of the entire dataset. Instead, we normalized the NMDS results between the formations with regression equations fit to each axis using the NMDS coordinates (“preferred environment” values) of surviving species. These normalizations allow for more direct comparisons of species and samples along paleoenvironmental gradients. This method is broadly applicable, and can be used in other instances of partial faunal turnover where surviving species exhibit some degree of niche conservatism.