CLIMATE FORCING, SPECIES TRAIT ADAPTATION, AND MARINE EXTINCTION PATTERNS OF THE PALEOCENE-EOCENE THERMAL MAXIMUM

Paleoclimate intervals of rapid warming are potential analogues of the future but yield dramatically different prognoses for marine biodiversity. The end-Permian produced the largest mass extinction in Earth history while extinctions at the Paleocene-Eocene Thermal Maximum (PETM) barely register, except in the deep ocean, a striking difference that lacks quantitative explanation. Here we use climate simulations and an ecophysiological model that reproduces end-Permian extinction patterns and accounts for evolutionary adaptation of physiological traits to test three hypotheses for why extinctions were muted in the PETM: 1) Species traits adapted to a warm Paleocene climate, 2) Shallow tropical oxygenation, and 3) Weaker temperature rise compared to the end-Permian. Adaptation of species hypoxia traits to a warm Paleocene background climate ameliorates upper ocean extinctions across the PETM even without a recently hypothesized tropical O₂ rise. By contrast, O₂ loss in the abyss increases extinctions with habitat depth, reproducing observed trends in foraminifera. Despite trait adaptation and tropical oxygenation, PETM extinctions can rival end-Permian levels under a similar degree of temperature rise, highlighting the role of warming magnitude in extinction severity.