RELATIVE EFFECTS OF ABIOTIC ENVIRONMENTAL CHANGE AND FAUNAL TURNOVER ON OBLIGATE COMMENSALS OF THE APPALACHIAN BASIN DURING THE LOWER KELLWASSER (LATE DEVONIAN) EXTINCTION PULSE

Mass extinctions expose species to changes both in the abiotic environment and in community composition. Here, we examine the dynamics of a mass extinction from the perspective of both biotic interactions and changing abiotic environments. Specifically, we examine changes in patterns of skeletobiosis (encrustation and boring of shells) on brachiopods in the Catskill delta complex (Appalachian foreland basin) during the Frasnian-Famennian (F-F) mass extinction. Skeletobionts preserve an unambiguous record of interaction, in that host shells are required for settling, and there is turnover of brachiopod host taxa in the extinction. This work focuses on the first pulse of the F-F extinction, the Lower Kellwasser (LK) event, because it is well represented in the surveyed locations in western New York State and northern Pennsylvania and because it is the more severe of the two Kellwasser events in this region. Previous work with this system has shown that skeletobionts of the Catskill delta complex experienced a change in relative abundance across the LK event. This work goes on to examine potential causes of this change. Paleoenvironmental gradients were identified using non-metric multidimensional scaling (NMDS) of the brachiopod assemblages, and the environmental distributions of skeletobionts living on the brachiopods are here examined along these gradients. Host shell sculpture, body size, and fate during the extinction event (victim vs. survivor) were also evaluated for their influence on skeletobiosis. Multivariable logistic regressions indicate that body size and to a lesser degree shell sculpture were important predictors of skeletobiosis prior to the LK event but that proximity to shoreline was the sole significant predictor following the LK event. Survivorship of hosts was not found to be a significant predictor of skeletobiosis. These results suggest that steepening onshore-offshore environmental gradients at the LK event had a much greater influence on skeletobionts than overturn in the host assemblages.