Significance The Toarcian oceanic anoxic event (OAE; ∼183 million y ago) is marked by one of the largest carbon cycle perturbations in Earth history, rapid climate change, widespread ocean oxygen deficiency, and strong changes in marine ecosystems. The temporal links between increasing atmospheric p CO ₂ , changes in ocean oxygen availability, and marine biotic response during this event are still poorly understood. Here we use isotopic analyses of calcite and organic matter from belemnites, marine predators of that time, to address their response to bottom water anoxia during the OAE. We infer that some belemnite taxa showed resilience to a strong reduction in ocean oxygen availability and occupied ecological niches in the Cleveland Basin (United Kingdom), enabling a strong evolutionary diversification after the event.