Editor’s summary
Since researchers first began to understand that the mass nonavian dinosaur extinction occurring at the end of the Cretaceous was due to an asteroid, many have also debated whether these groups were already on the decline. This debate has been ongoing for decades, in large part because there are few fossil sites dated to this precise time period outside of those in the northern plains of North America, which may only tell part of the story. Flynn et al. now date a previously problematic site in New Mexico and model diversity and endemism around the asteroid impact. They found that nonavian dinosaurs were not in decline before this event, and it really was the asteroid that did them in (see the Perspective by Zanno). —Sacha Vignieri
Abstract
It has long been debated whether non-avian dinosaurs went extinct abruptly or gradually at the end-Cretaceous (66 million years ago), because their fossil record at this time is mostly limited to northern North America. We constrain a dinosaur-rich unit to the south, the Naashoibito Member in New Mexico, to the very latest Cretaceous (~66.4 to 66.0 million years), preserving some of the last-known non-avian dinosaurs. Ecological modeling shows that North American terrestrial vertebrates maintained high diversity and endemism in the latest Cretaceous and early Paleogene, with bioprovinces shaped by temperature and geography. This counters the notion of a low-diversity cross-continental fauna and suggests that dinosaurs were diverse and partitioned into regionally distinct assemblages during the final few hundred thousand years before the end-Cretaceous asteroid impact.
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