The extinction of the dinosaurs is intricately linked to an asteroid impact that marked the end of the Cretaceous period. However, the specifics surrounding their demise have sparked ongoing debates since the discovery of the impact crater. While there is substantial evidence suggesting that the asteroid's impact alone could have led to their extinction, it is crucial to consider that the asteroid struck during a time of significant volcanic eruptions associated with previous mass extinctions. Furthermore, fossils dating back to the period just before the impact indicate that dinosaur-dominated ecosystems had experienced a decline in diversity, making them more susceptible to collapse.
A groundbreaking study has unveiled that fossils previously studied originated within the last few hundred thousand years prior to the cataclysmic event that wiped out all non-avian dinosaurs, except for birds. The findings suggest that species richness was not likely a contributing factor to their extinction, at least not in the immediate vicinity of the impact site.
Much of what we understand about the final days of the non-avian dinosaurs derives from the Hell Creek Formation, a rich fossil bed located in present-day Wyoming. This formation not only dates back to a few hundred thousand years before the impact but may also contain deposits that reflect the immediate aftermath of the event. However, the Hell Creek ecosystem, representing the northern Great Plains, limits our understanding of global dinosaur diversity.
It remains uncertain whether the diversity of species observed at Hell Creek mirrors global conditions or if regional differences in ecosystems played a significant role. This distinction is vital for addressing the ongoing debate regarding the state of dinosaurs just before the impact. If the communities reflected in Hell Creek were representative of all dinosaurs, it would strengthen the argument that they lacked the diversity necessary to survive the catastrophic consequences of the K/T impact.
To contextualize the fossils in Hell Creek, we require comparative data from other regions. Unfortunately, known fossil assemblages from elsewhere predominantly originate from earlier in the Cretaceous, complicating our understanding of global dinosaur diversity. However, recent argon dating of strata from the Naashoibito Member in the San Juan Basin of present-day New Mexico has changed this narrative. This region, previously thought to date back nearly a million years before the mass extinction, now appears to share a contemporary timeline with Hell Creek, dating to the last few hundred thousand years before the mass extinction.
The fossils discovered at Naashoibito have unveiled a distinct ecosystem now referred to as the “Alamo Wash local fauna.” Despite being only 1,500 kilometers south of Wyoming, the species present in this ecosystem differ significantly from those found in Hell Creek. The researchers' analysis indicates that late Cretaceous dinosaurs existed in two separate bioprovinces, suggesting distinct ecosystems in northern and southern regions. This divergence does not seem to be an artifact of the fossil sites; mammalian fossils indicate a unified community across both areas during the mass extinction period, yet showcase distinct ecologies before and after.
The researchers hypothesize that temperature differences were key factors contributing to this ecological distinction. This phenomenon may have impacted dinosaurs more significantly than mammals, which have a greater ability to regulate their body temperatures. Overall, the findings suggest that rather than being dominated by a limited number of major species, “dinosaurs were thriving in New Mexico until the end of the Cretaceous.”
This revelation has direct implications for the ongoing discussion about limited diversity potentially priming dinosaurs for extinction. Furthermore, it raises questions about the impact of contemporaneous eruptions in the Deccan Traps. If these eruptions had a substantial global effect, it seems unlikely that dinosaurs could have thrived in any region.
Despite these new insights, our understanding remains confined to ecosystems present on the North American continent. While fossils from other areas exist, their dating is often ambiguous. There are indications of dinosaur presence in late Cretaceous Europe and South America; however, a clear picture of the ecosystems in which they were discovered is still lacking. Thus, while these findings enhance our understanding of dinosaur diversity leading up to their extinction, much remains to be uncovered.
For further reading, refer to the study published in Science in 2025 (DOI: 10.1126/science.adw3282).
