The Mesozoic terminated in boreal spring

The Cretaceous-Paleogene (K-Pg) mass extinction ~66 million years ago (Ma) was triggered by the Chicxulub impact on the present-day Yucatán Peninsula. This event caused the highly selective extinction that eliminated ~76% of species, including all non-avian dinosaurs, pterosaurs, ammonites, rudists and most marine reptiles. The timing of the impact and its aftermath have mainly been studied on millennial timescales, leaving the season of the impact unconstrained. By studying fishes that died on the day the Mesozoic ended, we here demonstrate that the impact that caused the K-Pg mass extinction took place during boreal spring. Osteohistology together with stable isotope records of exceptionally preserved perichondral and dermal bones in acipenseriform fishes from the Tanis impact-induced seiche deposits reveal annual cyclicity across the final years of the Cretaceous. Annual life cycles, involving seasonal timing and duration of reproduction, feeding, hibernation, and aestivation, vary strongly across latest Cretaceous biotic clades. We postulate that the timing of the Chicxulub impact in boreal spring and austral autumn significantly influenced selective biotic survival across the K-Pg boundary.