Investigating the response of <i>Glycymeris septentrionalis</i> (Bivalvia) and <i>Terebratalia transversa</i> (Brachiopoda) to euxinia: Implications for mass extinctions

Abstract In the wake of the end-Permian mass extinction, bivalves replaced articulate brachiopods as the dominant benthic marine filter-feeders, despite morphological and ecological similarities. Several factors have been suggested as drivers of the extinction's selectivity, including competition, ocean acidity, CO2 poisoning, and differing reproductive potential. The spread of sulfidic water (euxinia) has been proposed as a general kill mechanism, but its physiological selectivity has not been investigated. Survival studies of the co-occurring bivalve Glycymeris septentrionalis and articulate brachiopod Terebratalia transversa from San Juan Island, Washington, USA, were carried out in anoxic and euxinic environments at 13.7, 16, and 22 °C. In colder anoxic conditions without sulfide, the brachiopod exhibited greater survival than the bivalve, but at higher temperatures, the bivalve generally outlasted the brachiopod, and by a greater margin in euxinia. The alignment between physiological tolerance in experimental settings and survival patterns in the paleontological record indicates that euxinia may have played a role in end-Permian extinction severity and selectivity, serving as an additional stressor in already warm, oxygen-depleted oceans.