New Dates from New Mexico: The Last Dinosaurs in North America

A multi-institution team led by Andrew Flynn has published new dates for the Naashoibito Member in the San Juan Basin of northwestern New Mexico. Using high-precision U-Pb and sanidine dating together with paleomagnetic sampling, the researchers constrained the age of the fossil-bearing rocks. The team dated 1,046 single sanidine grains; ten yielded ages between 66.4 and 66.8 million years, which implies the fossils are no older than 66.4 million years. These results were published in the journal Science and place the Naashoibito fossils within the latest Cretaceous interval.

The dated fossils are recorded as living between 66.4 and 66 million years old, situating them very near the Cretaceous–Paleogene boundary. Among the remains are large sauropods, notably Alamosaurus, together with a diverse assemblage that contrasts with the Triceratops-dominated faunas farther north. Researchers argue that western North America was partitioned into regional 'bioprovinces'—zones of distinct communities shaped largely by temperature gradients—so that different basins harbored distinct dinosaur lineages up to the extinction.

While the new chronology strengthens the case that many dinosaurs were flourishing until the end of the Cretaceous, it derives from a single well-dated area and does not by itself resolve continental or global trends. Nevertheless, the study shows that ecological diversity persisted until a few hundred thousand years before the impact, and that mammals began to diversify within about 300,000 years after the extinction. Consequently, the findings emphasize the abruptness of the mass extinction and the rapid ecological turnover that followed. Future work will test patterns across other basins and continents.