That appears to be changing from now on, with a new argon dating of strata from the Naashoibito Member in the San Juan Basin in present-day New Mexico. Many dinosaur fossils have been obtained from this area and we know that the site differs from the type of ecosystem found at Hell Creek. But it was previously thought to date back a million years before the mass extinction. The new dates, along with the alignment of magnetic field reversals, tell us that the ecosystem was contemporary with that of Hell Creek and dated hundreds of years before the mass extinction.
Diverse ecosystems
The Naashoibito fossils revealed an ecosystem that we now refer to as the “Alamo Wash local fauna.” And they’re quite distinct from those in Wyoming, despite being just 1,500 miles further south. By analyzing the species present using ecological measures, the researchers discovered that dinosaurs formed two “bioprovinces” at the end of the Cretaceous: essentially, there were distinct ecosystems present in the northern and southern areas.
This does not appear to be an artifact of the sites, as the mammal fossils appear to reflect a single community in the two areas near the mass extinction, but had distinct ecologies before and after. The researchers propose that temperature differences are the main drivers of this distinction, which may have had less impact on mammals, which are generally better able to control their own temperature.
Overall, the researchers conclude that, rather than being dominated by a small number of major species, “dinosaurs thrived in New Mexico until the end of the Cretaceous.”
While this speaks directly to the idea that limited diversity may have prepared dinosaurs for extinction, it could also have implications for the impact of contemporary eruptions in the Deccan Traps. If these changes had a major global impact, then it is unlikely that dinosaurs would thrive anywhere.
However, even with the new data, our picture remains limited to ecosystems present on the North American continent. We have fossils from elsewhere, but they are not exactly 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 findings help clarify the diversity of dinosaurs during the period before their extinction, there is still much to learn.
Science, 2025. DOI: 10.1126/science.adw3282 (About DOIs).
