This appears to have changed with new argon dating of the Naashoibito Member in the San Juan Basin in present-day New Mexico. Many dinosaur fossils have been discovered in this region, and we know that this site is different from the ecosystem found at Hell Creek. But it was previously thought that it arose about a million years before the mass extinction. The new dates, as well as the alignment of magnetic field reversals, tell us that this ecosystem was contemporary with the Hell Creek ecosystem and dates back to the last few hundred thousand years before the mass extinction.
Diverse Ecosystems
Fossils at Naashoibito revealed an ecosystem we now call the “Alamo Wash native fauna.” And they are quite different from those found in Wyoming, despite being only 1,500 kilometers to the south. By analyzing extant species using ecological metrics, the researchers found that in the Late Cretaceous, dinosaurs formed two “bioprovinces”—essentially, separate ecosystems in northern and southern regions.
This does not appear to be an artifact of these sites, as the mammal fossils appear to reflect the same community in both areas near the mass extinction, but had different ecologies both before and after. The researchers speculate that the key factors in the difference were temperature differences, which may have had less of an impact in mammals, which tend to have better control over their own temperatures.
Overall, the researchers conclude that rather than being dominated by a small number of core species, “dinosaurs thrived in New Mexico until the end of the Cretaceous.”
While this directly suggests that limited diversity may have led to the extinction of the dinosaurs, it may also have implications for the impact of contemporaneous eruptions in the Deccan Traps. If they had had a major global impact, it is unlikely that dinosaurs would have flourished anywhere.
However, even with new data, our picture is still limited to the ecosystems present on the North American continent. We have fossils from other places, but they are not dated. There are some indications of the presence of dinosaurs in the Late Cretaceous in Europe and South America, but we do not have a clear idea of the ecosystems in which they were found. So, while these discoveries help clarify the diversity of dinosaurs in the period leading up to their extinction, there is still much to be learned.
Science, 2025. DOI: 10.1126/science.adw3282 (About DOI).
