ENVIRONMENTAL CONTROLS ON BIOTIC INTERACTION: AN UPDATED ANALYSIS OF SKELETOBIOSIS THROUGH THE LATE DEVONIAN MASS EXTINCTION IN THE APPALACHIAN BASIN, USA

Our understanding of how mass extinction influences epibiotic and endobiotic organisms (those that depend on other organisms for a substrate) is limited – in principle, they could be affected directly by environmental changes or indirectly by extinctions in the host community. Here, we examine the factors that controlled the distributions of skeletobionts (encrusters and borers of carbonate skeletal material) through the first pulse of the Late Devonian mass extinction (the Lower Kellwasser event) in the Appalachian Basin using an updated data set. Bulk samples of brachiopods were collected along an onshore-offshore paleoenvironmental transect in western New York State and northern Pennsylvania, representing times before, during, and after the extinction interval. About half the brachiopod species went extinct, but no skeletobiont group was lost. Nonmetric multidimensional scaling (nMDS) of the brachiopod assemblages placed the samples along axes representing onshore-offshore location (axis 1) and relative stability of substrate (axis 2). Logistic regression was used to evaluate the significance of multiple potential predictors of skeletobiosis, including formation (pre- vs. post-extinction), sculpture of host (ribbed vs non-ribbed), body size of host, onshore-offshore position (nMDS axis 1), stability of substrate (nMDS axis 2), and survivorship of host species. The most significant predictors of skeletobiosis were ribbed sculpture, larger host body size, and more onshore locations. Survivorship status of hosts failed to significantly predict skeletobiosis, suggesting that turnover in the host community had limited direct effects. Following the extinction, skeletobionts show a stronger affinity for shallower-water habitats, but, given the stratigraphic architecture immediately above the extinction interval, this pattern could instead reflect delayed recovery of skeletobiosis. Comparison of stratigraphic distributions of skeletobiosis among sections suggests that the former scenario (preference for shallow water) is more likely, however, because skeletobiosis does not display a lagged return in shallow-water settings. A decline in skeletobiont abundance across the extinction reflects in part this environmental shift.