The Ecological Consequences of Mass Extinctions: The Effects of Brachiopod Extinction on Encrusting Organisms across the Frasnian-Famennian Mass Extinction

The extinction of species has trickle-down effects on other organisms within ecosystems. Here, we examine the effects of brachiopod extinction across the Frasnian-Famennian mass extinction on encrusting organisms with regard to changes in brachiopod surface texture, the preferences of encrusting organisms for brachiopod surface textures, and taxonomic loss through the extinction using both high-resolution community and broad, regional- and global-scale diversity perspectives.

Prior to the Frasnian-Famennian mass extinction, costate and plicate brachiopods were more common than smooth and spinose brachiopods in benthic ecosystems of the upper Midwest US. Most epizoans significantly encrusted costate and plicate brachiopods, more often than smooth or spinose brachiopods, even when encrustation rate is corrected for brachiopod abundance. After the extinction, encrusting organisms declined in both abundance and diversity in benthic communities. Costate and plicate brachiopods decreased in abundance, whereas smooth and spinose brachiopods became common. Strongly plicate brachiopods continued to be encrusted, but smooth-shelled brachiopods bore the majority of encrusting organisms. These results suggest that the change in abundance of preferred substrates across the Frasnian-Famennian extinction greatly affected the encrusting portion of benthic ecosystems.

Across the extinction event, changes in the North American diversity of brachiopod textures correlates significantly with that of global diversity, but global and regional diversity does not correlate with ecological-scale abundances of brachiopod surface textures. Only the diversity and community abundance trends in spinose brachiopods were congruent: spinose brachiopods, at generic diversity levels and as individuals in ecosystems, were few prior to the Famennian, and abruptly increased following the extinction. Costate and plicate brachiopod diversity decreases moderately compared to abundance data, whereas smooth-shelled brachiopod diversity is highest in the Devonian. Therefore, the diversity of brachiopod textures does not reflect abundance patterns at the community level and cannot explain the detrimental effect of brachiopod mass extinction on encrusting organisms.