New dates for a long-debated New Mexico fossil site reveal that dinosaurs were thriving and regionally diverse until the end-Cretaceous asteroid strike 66 million years ago, according to a new study. The disappearance of non-avian dinosaurs 66 million years ago ended the final chapter of the Cretaceous Period, yet scientists still debate whether their extinction was sudden – triggered by the asteroid impact – or the culmination of a gradual decline that left them vulnerable to catastrophe. According to Andrew Flynn and colleagues, this uncertainty stems from gaps and biases in the fossil record from this period. To date, the best dated paleontological evidence of faunas straddling the Cretaceous-Paleogene (K-Pg) boundary comes from the Hell Creek and Fort Union Formations in the northern Great Plains of North America. However, investigations into these and other nearby fossil assemblages have produced conflicting conclusions about whether the fossil record reflects a sudden dinosaur extinction at the end of the Cretaceous or a more gradual, Late Cretaceous decline. Here, Flynn et al. present new geochronological data that precisely date the Naashoibito Member – a fossil-rich rock layer of the Kirtland Formation of northern New Mexico – to ~66.4 to 66 million years ago, making its dinosaur fossils contemporaneous with those of the Hell Creek Formation farther north. These dinosaurs, diverse in size, diet, and species, lived within roughly 340,000 years of the Cretaceous–Paleogene boundary, showing no signs of ecological decline before the mass extinction. Ecological analyses incorporating these new findings indicate that, contrary to the long-held idea of uniform Late Cretaceous ecosystems, distinct regional differentiation of dinosaur species persisted across western North America until the very end of the Cretaceous. Evidence from other continents, though less precisely dated, suggests similarly robust and diverse dinosaur faunas surviving until the final moments of the Cretaceous. According to the authors, taken together, these findings suggest that non-avian dinosaurs were not in long-term decline but were instead abruptly wiped out by the end-Cretaceous asteroid impact. In an accompanying perspective, Lindsay Zanno discusses the study’s findings in greater detail.