Morphological plasticity and environmental heterogeneity drove resilience of high latitude marine calcifying taxa across the Cretaceous-Paleogene mass extinction

The Cretaceous-Paleogene (K-Pg) boundary 66 Ma coincides with the most recent of the ‘Big Five’ Phanerozoic mass extinctions. The extinction was driven by rapid and extreme environmental changes, which included global cooling, surface ocean acidification, and productivity decline, following an extra-terrestrial impact at Chicxulub in the Gulf of Mexico. The excellent global fossil record of the K-Pg extinction and recovery allows examination into how severe environmental perturbations affect calcifying organisms across different oceanographic settings. Combining samples from the expanded shelf section on Seymour Island, Antarctica, and IODP sites 690 and 1135 (paleolatitude 65°S), we present new quantitative morphometric and μ-computed tomography data from large collections of well-preserved macro- and microfossil taxa (benthic molluscs, planktonic and benthic foraminifera) across the K-Pg boundary in the southern high latitudes. Planktonic foraminifera show large extinctions and a size decrease in survivors at the K-Pg boundary itself, but the magnitude of size change is smaller than at lower latitude localities. Surviving shallow marine bivalves from Seymour Island show increased variability of body size and volume at the K-Pg boundary persisting for ~300 kyrs into the Paleogene. Surviving deep-sea benthic foraminifera from Site 690 in the same ocean basin and at the same paleolatitude, respond to both events at the boundary itself and the initial re-stabilisation of the global carbon cycle some 300 kyrs after the K-Pg, with shifts in size, growth rate, and surface:volume ratio. These data reveal the importance of morphological plasticity for promoting resilience and survival in calcifiers across this mass extinction event. They also suggest that trait changes across the K-Pg event occur at different times in different environmental settings and groups, supporting the hypothesis that environmental heterogeneity plays an important role in modulating resilience.