For much of the past century, scientists believed dinosaurs were already in decline when the asteroid struck 66 million years ago — but a 2025 study published in Science found fossils in New Mexico showing they were flourishing in diverse, thriving ecosystems right up until the moment of impact

For much of the past century, many scientists believed the dinosaurs were already in decline when the asteroid struck 66 million years ago, a group past its prime that a cosmic accident merely finished off. A 2025 study published in the journal Science, built on fossils from New Mexico, argues almost the opposite. Right up to the end, it finds, dinosaurs were flourishing in diverse, thriving ecosystems.

It is a striking result, and a genuine challenge to the older view. It is also, as its own authors and critics agree, one powerful data point in a debate that is not yet fully settled.

The long-running argument

The question sounds simple: were dinosaurs slowly fading before the impact, or doing well until the sky fell in? It has been hard to answer for a practical reason. To know how dinosaurs were faring in their final chapter, you need fossils in rock that can be reliably dated to just before the extinction, and such rock is rare.

Much of what we thought we knew came from the well-studied Hell Creek Formation, in what is now Montana and the Dakotas. It preserves famous animals like Tyrannosaurus and Triceratops in rocks dated to the very end of the Cretaceous, but it contained no long-necked, giant sauropods, which led some researchers to suspect those had already died out. A picture of thinning diversity took hold.

A window in New Mexico

The new work, led by Andrew Flynn of New Mexico State University with an international team, opens a different window. Its site is the Naashoibito Member of the San Juan Basin in northwestern New Mexico, and the crucial achievement was pinning down its age.

The team dated the layers two ways: by reading the direction of Earth’s magnetic field frozen into the rock, which flipped in a known pattern over geological time, and by radiometric dating of grains within the sandstone. Together these placed the fossils within roughly the final 380,000 years before the extinction. That work took more than a decade, which is exactly why dated fossils this close to the boundary are so scarce.

What they found

The community preserved there was rich. Alongside Tyrannosaurus and the horned Torosaurus and duck-billed dinosaurs, the site held Alamosaurus, one of the largest long-necked dinosaurs that ever lived, stretching around 30 metres and weighing more than 30 tonnes.

That single animal undercuts the old story neatly. Far from having vanished, giant sauropods were flourishing in the south, in what was then a warm, humid, tropical forest not unlike modern Panama. The study paints a continent split into two thriving but different dinosaur worlds: the cooler coastal plains of the north, with their own mix of species, and the lush southern forests where the great sauropods still walked. This was not a fauna running out of ideas. It was one still dividing up niches and varying in size, shape and diet, as dinosaurs had done for over 150 million years.

Why it challenges the decline story

To the study’s authors, a diverse, well-dated ecosystem sitting right at the boundary is hard to square with the idea of a slow slide toward oblivion. The apparent decline seen elsewhere, they argue, may say more about where fossils happen to have been found and dated than about the animals themselves.

Flynn put it plainly, saying 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. On this reading, extinction was not the last push for a failing group. It was a sudden blow to a successful one.

But the debate is not over

Here honesty matters, because the new study strengthens one side of a live argument rather than ending it. It is a detailed snapshot of one region, not a global census. Michael Benton of the University of Bristol, whose own work has pointed to a decline, noted that this is just one location and cannot capture the state of dinosaur life across the whole world.

He also pointed out that the new paper itself records an overall drop in dinosaur species in western North America across the last several million years of the Cretaceous, from around 43 to about 30. His reading is that there was a broad decline, dotted with rich local communities wherever the climate was especially favourable, which New Mexico appears to have been. Others, such as Darla Zelenitsky of the University of Calgary, see the findings as strong support for the view that dinosaurs stayed vigorous to the very end. In other words, the same fossils are being read in more than one reasonable way, which is how healthy science usually looks.

Why it matters

Underneath the dispute is a real stake. If dinosaurs were thriving when the asteroid arrived, their extinction becomes a story about a sudden, external catastrophe overwhelming a flourishing group, rather than the tidy ending of a lineage already on the way down. The very fact that a 30-tonne animal was shaking the ground one moment and gone the next makes the point vividly.

What to watch now is whether other sites, dated with the same care, tell the same story. A single well-studied basin has reopened the question. It will take more windows like it, spread across the globe, to say for certain whether the dinosaurs met their end at the peak of their success or somewhere on a long, slow slope down. For now, the best-dated evidence from New Mexico says they were doing just fine until the sky fell.