Range shifts and extinctions in the Blue Ocean under past and future climate changes

Understanding how marine biodiversity responds to climate change is essential for designing effective conservation strategies in areas beyond national jurisdiction. Planktonic foraminifera offer one of the most continuous archives of open-ocean biotic change, providing an opportunity to link past range dynamics with future extinction risk. Here we combine two complementary lines of evidence; (1) Late Quaternary range shifts that shaped the bimodal latitudinal diversity gradient (LDG) of planktonic foraminifera, and (2) long-term extinction selectivity inferred from Cenozoic thermal niches; to develop a time-extended perspective on biodiversity change in the Blue Ocean.Using paired fossil and modern assemblages, we tested whether the emergence of the characteristic bimodal LDG was driven by deglacial equatorial extirpations. The data show that equatorial species generally persisted through deglaciation, with trailing-edge contractions being uncommon. Instead, mid-latitude richness gains were dominated by coordinated poleward expansions, particularly in the Atlantic. In the Pacific, richness changes were spatially heterogeneous: localized extirpation clusters characterized the western Pacific, whereas eastern and southern regions experienced notable colonisations. Species’ thermal preferences only weakly accounted for these local patterns, underscoring substantial basin-scale restructuring that is not captured by coarse biodiversity metrics.To evaluate longer-term extinction vulnerability, we estimated species’ thermal niches from fossil occurrences spanning the past ~66 million years. These niche characteristics explained the majority of observed climate-driven losses in the fossil record, accounting for 87–99% of local extirpations and 92–100% of global extinctions. Forward projections based on these empirically derived niches reveal concentrated extinction risks under anthropogenic warming, with particularly high vulnerability in tropical ocean regions.Together, these studies demonstrate that past climate change produced complex, scale-dependent range dynamics while long-term thermal niches strongly constrained extinction selectivity. Integrating these insights provides a fossil-informed baseline for anticipating future biodiversity loss and supports the identification of priority areas for conserving genetic, taxonomic, and functional diversity in the Blue Ocean, in alignment with the goals of the Global Biodiversity Framework.