For much of the past century, scientists thought dinosaurs were already in decline long before the asteroid impact that ended their reign 66 million years ago. However, a new study published in Science by researchers from Baylor University, New Mexico State University, The Smithsonian Institution, and several international partners challenges that long-standing belief.
The findings reveal that dinosaurs were not fading away at all -- they were thriving.
A final flourish in the San Juan Basin
In northwestern New Mexico, layers of ancient rock hold clues to a lively, previously overlooked chapter of Earth's history. Within the Naashoibito Member of the Kirtland Formation, scientists found evidence of rich dinosaur ecosystems that continued to flourish until just before the asteroid struck.
High-precision dating determined that fossils from these rocks are between 66.4 and 66 million years old, placing them right at the boundary between the Cretaceous and Paleogene periods, when the global extinction event occurred.
"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 fossil evidence from New Mexico tells a strikingly different story from what many had assumed. Instead of dwindling, dinosaurs across North America were thriving in distinct regional communities. By analyzing ecological and geographic patterns, researchers found that dinosaur populations in western North America were divided into separate "bioprovinces" shaped primarily by regional temperature differences rather than by mountains or rivers.
"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. '20, assistant professor of geological sciences at New Mexico State University. "They're doing great, they're thriving and that 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 brought the age of dinosaurs to an abrupt end, but the ecosystems they left behind became the foundation for a new evolutionary chapter. Within just 300,000 years, mammals began rapidly diversifying, developing new diets, sizes, and ecological roles.
The same temperature-related patterns that once defined dinosaur ecosystems continued into the Paleocene epoch, guiding how life recovered after the disaster.
"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 this discovery matters
This discovery offers more than just a look into the distant past. It underscores both the resilience and fragility of life on Earth. Conducted on public lands managed by the U.S. Bureau of Land Management, the research highlights how protected landscapes can unlock vital insights into how ecosystems respond to global upheaval.
By refining the timeline of the dinosaurs' final days, the study reveals that their extinction was not a slow decline but an abrupt, catastrophic end to a flourishing era of life -- cut short by chance from beyond the sky.
About the authors
In addition to Peppe and Flynn, the research team included scientists from Baylor University, New Mexico State University, the Smithsonian Institution, the University of Edinburgh, University College London and multiple U.S. and international institutions.
- Stephen L. Brusatte, Ph.D., The University of Edinburgh
- Alfio Alessandro Chiarenza, Ph.D., Royal Society Newton International Fellow, University College London
- Jorge Garcia-Giron, Ph.D., University of Leon
- Adam J. Davis, Ph.D., WSP USA Inc.
- C. Will Fenley, Ph.D., Valle Exploration
- Caitlin E. Leslie, Ph.D., ExxonMobil
- Ross Secord, Ph.D., University of Nebraska-Lincoln
- Sarah Shelley, Ph.D., Carnegie Museum of Natural History
- Anne Weil, Ph.D., Oklahoma State University
- Matthew T. Heizler, Ph.D., New Mexico Institute of Mining and Technology
- Thomas E. Williamson, Ph.D., New Mexico Museum of Natural History and Science
Funding
This research was supported by the National Science Foundation, European Research Council, Royal Newton International Fellowship, Geologic Society of America Graduate Research Grant, Baylor University James Dixon Undergraduate Fieldwork Fellowship (AGF), the European Union Next Generation, the British Ecological Society and the American Chemical Society -- Petroleum Research Fund.
The researchers would like to thank the Bureau of Land Managementfor providing collecting permits and supporting the research.
