New 2025 Study Finds Dinosaurs Were Flourishing Before Asteroid Impact

For a century, the prevailing narrative in paleontology suggested dinosaurs were a group past their prime. The theory posited that a cosmic accident merely finished off a population already sliding toward oblivion. This “slow decline” hypothesis often pointed to a perceived drop in species diversity in the fossil record leading up to the Cretaceous-Paleogene (K-Pg) boundary. However, new evidence from the San Juan Basin in northwestern New Mexico indicates a far more resilient prehistoric world.

The Naashoibito Member fossils in New Mexico

The shift in understanding stems from a site known as the Naashoibito Member. To prove dinosaurs were thriving, researchers needed fossils in rock layers that could be dated with extreme precision to the moments preceding the extinction event. Such layers are rare, which is why the “decline” theory persisted for so long; many fossil sites lack the volcanic ash or specific mineral markers required for high-resolution dating.

A team led by Andrew Flynn of New Mexico State University spent over a decade pinning down the age of these layers. They utilized two distinct methods to ensure the timeline was accurate:

• Magnetostratigraphy: Reading the direction of Earth’s magnetic field frozen in the rock, which allows scientists to track polarity reversals over geological time.
• Radiometric Dating: Analyzing grains within the sandstone to determine the absolute age of the sediment.

These techniques placed the fossils within the final 380,000 years before the extinction, providing a narrow window that captures the state of the ecosystem almost exactly as the asteroid struck.

Evidence of a thriving, diverse ecosystem

The findings at the San Juan Basin site reveal a community that was far from depleted. Researchers found a variety of species, including Tyrannosaurus, the horned Torosaurus, and various duck-billed dinosaurs. Most significantly, the site contained Alamosaurus, one of the largest long-necked dinosaurs to ever exist.

Alamosaurus reached lengths of approximately 30 metres and weighed more than 30 tonnes. Its presence is a critical data point because previous studies of the Hell Creek Formation in Montana and the Dakotas—which lacked giant sauropods—led some to believe those massive herbivores had already vanished from North America long before the impact.

The data suggests a continent divided into two distinct, thriving worlds, illustrating that dinosaur health varied significantly by latitude:

Why this disrupts the “slow decline” theory

The existence of a rich, well-dated ecosystem at the very boundary of extinction makes the “slow slide” hypothesis difficult to maintain. The research suggests that the perceived decline seen in other regions may be a result of sampling bias—where fossils were found and dated—rather than a reflection of the actual health of the dinosaur populations across the entire continent.

“the dinosaurs were not on their way out but doing great and thriving, and that the asteroid seems to have knocked them down while they were strong.”

Andrew Flynn, New Mexico State University, via SpaceDaily

This implies that the extinction was not the final push for a failing group, but a sudden termination of a population that was still actively dividing niches and varying in size, shape, and diet—a strategy they had employed for over 150 million years. By demonstrating that giant sauropods like Alamosaurus were still present and thriving in the south, the study removes one of the primary pieces of evidence used to argue that dinosaurs were already fading.

While the results are striking, the authors and critics acknowledge that this remains one powerful data point in a debate that is not yet fully settled. The discovery highlights the importance of geographic diversity in fossil records; focusing only on northern sites like Hell Creek provided an incomplete picture of the Cretaceous period’s end. This suggests that the “extinction” was not a gradual biological failure, but a geologically instantaneous event that caught a robust and diverse biological community by surprise.

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