Dinosaurs may have flourished right up to when the asteroid hit

"Many dinosaur fossils have been obtained from this region, and we know the site differs from the sort of ecosystem found at Hell Creek. But it was previously thought to date back closer to a million years before the mass extinction. The new dates, plus the alignment of magnetic field reversals, tell us that the ecosystem was a contemporary of the one in Hell Creek, and dates to the last few hundred years prior to the mass extinction."

"The fossils at Naashoibito have revealed an ecosystem we now label the "Alamo Wash local fauna." And they're fairly distinct from the ones found in Wyoming, despite being just 1,500 kilometers further south. Analyzing the species present using ecological measures, the researchers found that dinosaurs formed two "bioprovinces" in the late Cretaceous-essentially, there were distinct ecosystems present in the northern and southern areas."

"This doesn't seem to be an artifact of the sites, as mammalian fossils seem to reflect a single community across both areas near the mass extinction, but had distinct ecologies both earlier and after. The researchers propose that temperature differences were the key drivers of the distinction, something that may have had less of an impact on mammals, which are generally better at controlling their own temperatures."

New argon dating of Naashoibito Member strata in New Mexico aligns with magnetic reversal data and places the Alamo Wash local fauna within the last few hundred years before the Cretaceous mass extinction, contemporaneous with Hell Creek. The Naashoibito assemblage shows distinct species compositions relative to Wyoming, indicating two dinosaur bioprovinces—northern and southern—across the region. Mammalian fossils indicate a single community near the extinction but show regional differences earlier and later. Temperature gradients likely drove the dinosaur provinciality, with mammals buffered by thermoregulation. Dinosaurs persisted and were locally diverse in New Mexico up to the end of the Cretaceous, affecting interpretations of Deccan Traps global impact.

Read at Ars Technica