Dinosaurs hung on in New Mexico right up to the asteroid apocalypse.
New dating of fossil-rich rocks in the San Juan Basin pins some of the last non-bird dinosaurs to within a few hundred thousand years of the mass extinction 66 million years ago, researchers report in Science.
These creatures, including horned plant-eaters, duck-billed dinosaurs, and massive long-necked sauropods, roamed what is now the southwestern United States in the final stretch of the Cretaceous period. Until now, scientists weren’t sure exactly when these southern dinosaurs lived, with estimates placing them millions of years before the end. This study shows they persisted alongside their northern cousins, like those in Montana’s Hell Creek Formation, until nearly the bitter end.
The finding comes from a fresh look at the Naashoibito Member, a layer of river-deposited sediments in New Mexico packed with dinosaur bones. By analyzing radioactive minerals in the rocks and magnetic signals preserved in the layers, the team narrowed the unit’s age to about 66.4 to 66 million years old — putting it squarely in the latest Maastrichtian stage, the Cretaceous’s closing act.
This timeline means the New Mexico dinosaurs lived through the massive Deccan volcanic eruptions in India and were still around when the asteroid slammed into what is now Mexico’s Yucatán Peninsula, triggering the extinction that wiped out three-quarters of Earth’s species.
Debate has swirled for decades over whether dinosaurs faded gradually or vanished in a flash. Much of the evidence comes from northern sites, where the fossil record shows an abrupt cutoff. But farther south, the picture was blurrier, leaving open the possibility that dinosaurs lingered longer or dwindled earlier in different regions.
The new study fills that gap. It reveals a thriving southern ecosystem, with a mix of predators and prey that doesn’t look rundown compared to older faunas. “The Naashoibito dinosaurs include a variety of species spanning major groups, sizes, and dietary categories and do not appear abnormal or depauperate compared to earlier faunas,” the researchers write, suggesting dinosaurs were “thriving in New Mexico until the end of the Cretaceous.”
Beyond the dating, the team used ecological modeling to map out animal distributions across ancient North America, then called Laramidia. Drawing on fossil data from dozens of sites, they found that vertebrates like dinosaurs, mammals, and others, clustered into two main “bioprovinces” during the late Cretaceous: one in the cooler north and another in the warmer south.
This regional split held steady from about 84 million years ago through the extinction and even into the early Paleogene, the dawn of the mammal era. Temperature differences, more than just north-south geography, seem to have driven the divide. Warmer spots suited giants like sauropods, while cooler areas favored certain duck-billed types.
The results push back against the idea that dinosaurs were in a continent-wide slump, with low diversity making them vulnerable. Instead, “we recognize the final ~1 million years of dinosaur evolution as a time of biogeographic diversity and partitioning among terrestrial faunas in western North America,” the team notes.
After the extinction, mammals exploded in variety, filling niches left empty by the dinosaurs. In the San Juan Basin, diverse new mammal groups appeared within 300,000 years, some on the path to modern lineages. The quick rebound contrasts with slower recoveries after other mass die-offs, like the one 252 million years ago that nearly ended life on Earth.
Journal Reference: A.G. Flynn et al. Late-surviving New Mexican dinosaurs illuminate high end-Cretaceous diversity and provinciality. Science. doi: 10.1126/science.adw3282
