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

New research has re-examined rock layers in the San Juan Basin of northwestern New Mexico. Scientists used high-precision U-Pb and sanidine dating and paleomagnetic sampling to place the fossils in time. These methods have produced robust ages for the Naashoibito Member. As a result, the age of the bones can be compared directly with other late Cretaceous sites.

The fossils are dated between 66.4 and 66 million years old, placing them very close to the Cretaceous–Paleogene boundary. The Naashoibito Member contains large sauropods such as Alamosaurus and a mix of other dinosaurs. While the north was dominated by Triceratops and duck-billed dinosaurs, the New Mexico fauna is different. Therefore, researchers describe regional 'bioprovinces' that were shaped by temperature differences.

These results suggest dinosaurs were flourishing in some regions just before the asteroid strike. However, the pattern is known from one well-dated area and may not reflect global trends. Further work is needed to compare other late Cretaceous basins. Nevertheless, the study strengthens the view that the asteroid triggered a sudden mass extinction.