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For decades, many scientists believed dinosaurs were already dwindling in number and variety long before an asteroid strike sealed their fate 66 million years ago.

However, new in the journal Science from Baylor University, New Mexico State University, the Smithsonian Institution and an international team is rewriting that story.

The dinosaurs, it turns out, were not fading away. They were flourishing.

A final flourish in the San Juan Basin

In northwestern New Mexico, layers of rock preserve a hidden chapter of Earth's history. In the Naashoibito Member of the Kirtland Formation, researchers uncovered evidence of vibrant dinosaur ecosystems that thrived until just before the asteroid impact.

High-precision dating techniques revealed that fossils from these rocks are between 66.4 and 66 million years old—placing them in the catastrophic Cretaceous–Paleogene boundary.

"The Naashoibito dinosaurs lived at the same time as the famous Hell Creek species in Montana and the Dakotas," said Daniel Peppe, Ph.D., associate professor of geosciences at Baylor University. "They were not in decline; these were vibrant, diverse communities."

Dinosaurs in their prime

The New Mexico fossils tell a different story than originally thought. Far from being uniform and weakened, dinosaur communities across North America were regionally distinct and thriving. Using ecological and biogeographic analyses, the researchers discovered that dinosaurs in western North America lived in separate "bioprovinces," divided not by mountains or rivers, but by temperature differences across regions.

"What our new research shows is that dinosaurs are not on their way out going into the mass extinction," said first author Andrew Flynn, Ph.D., assistant professor of geological sciences at New Mexico State University. "They're doing great, they're thriving and the asteroid impact seems to knock them out. This counters a long-held idea that there was this long-term decline in dinosaur diversity leading up to the mass extinction, making them more prone to extinction."

Life after impact

The asteroid impact ended the age of dinosaurs in an instant—but the ecosystems they left behind set the stage for what came next, the researchers said. Within 300,000 years of their extinction, mammals began to diversify rapidly, exploring new diets, body sizes and ecological roles.

The same temperature-driven patterns that shaped dinosaur communities continued into the Paleocene, showing how climate guided life's rebound after catastrophe.

"The surviving mammals still retain the same north and south bio provinces," Flynn said. "Mammals in the north and the south are very different from each other, which is different than other mass extinctions where it seems to be much more uniform."

Why the discovery matters today

The discovery is more than a window into the past—it's a reminder of the resilience and vulnerability of life on Earth. Conducted on public lands managed by the U.S. Bureau of Land Management, the research highlights how carefully protected landscapes can yield profound insights into how ecosystems respond to sudden global change.

With a clearer understanding of the timeline of the dinosaurs' final days, the study reveals not a slow fade into extinction but a dramatic ending to a story of flourishing diversity cut short by cosmic chance.