Brachiopods and forams reduced calcification costs through morphological simplification during mass extinction events.

Environmental stressors have exacerbated the collapse of marine ecosystems during mass extinctions. However, the survival strategies of marine species during mass extinctions remain unclear. Here, we investigated morphological evolution of brachiopods across the Permian-Triassic mass extinction (PTME) using a database of 3,225 specimens representing 1,061 species and foraminifera across the PTME and early Toarcian oceanic anoxic event (T-OAE) using a database of 757 specimens representing 12 species. We found a significant reduction in the number and proportion (plicae length/shell length) of shell plicae of brachiopods (36.4% and 60.0%, respectively) across the PTME and a significant decrease in the shell thickness of foraminifera (18.9% and 42.4% across the PTME and 36.9-61.8% across the T-OAE). We calculated that these adaptive strategies could reduce the energetic costs of calcification by more than half for brachiopods across the PTME, and by ~20-62% for foraminifera across the PTME and T-OAE, to compensate for the elevated cost of calcification due to environmental and ecological pressures. We propose that simplification of morphological features, such as reduced shell ornamentation and shell thinning, serves as a potential economic strategy for calcifying organisms to cope with extinction events by reducing energy demands, but further studies with a broader range of taxa and extinction events are needed to confirm the generality of this bioenergetic strategy.