A 30-tonne animal was shaking the ground one moment and gone the next
Sixty-six million years ago, an asteroid ended the age of dinosaurs — but whether it struck a dynasty in full flourish or one already dimming has long shaped how we understand catastrophe and decline. A 2025 study led by Andrew Flynn, drawing on fossils from New Mexico's San Juan Basin, now challenges the prevailing view: the evidence suggests dinosaurs were not fading but thriving, their communities rich and varied, right up until the moment of impact. The finding does not close the question so much as reopen it, reminding us that the stories we tell about endings are often shaped as much by where we look as by what actually happened.
- A century-old scientific consensus — that dinosaurs were already in decline before the asteroid struck — has been directly challenged by fossils dated to within 380,000 years of the mass extinction event.
- The old narrative rested heavily on the Hell Creek Formation, where the absence of giant sauropods was read as evidence of vanishing diversity, but that absence may have reflected geography more than biology.
- Flynn's team spent over a decade precisely dating the San Juan Basin site, finding not a thinning fauna but a thriving one — including Alamosaurus, a 30-metre, 30-tonne giant still walking a lush, tropical southern landscape.
- The discovery reframes extinction itself: rather than a final push against a weakening lineage, the asteroid may have been a sudden catastrophic blow to a group still at the height of its success.
- Critics like Michael Benton argue the site represents a locally favorable pocket within a broader continental decline, while others see it as decisive evidence of global dinosaur vigor — the same fossils, two reasonable readings.
- The debate now turns on whether other carefully dated sites around the world will tell the same story, making this less a conclusion than an urgent invitation to look more widely.
For most of the twentieth century, the scientific story of dinosaurs ended with a slow fade: a lineage past its prime, worn down by time, finished off by cosmic misfortune. That narrative held for decades. Then in 2025, a team led by Andrew Flynn at New Mexico State University published findings in Science that challenged it directly. Using fossils from the San Juan Basin in northwestern New Mexico, they argued that dinosaurs were not declining at all — they were thriving, right up until the moment they weren't.
The difficulty has always been finding the right rock. Fossils reliably dated to the final moments of the Cretaceous are rare, and for decades scientists leaned heavily on the Hell Creek Formation in Montana and the Dakotas. Hell Creek preserved Tyrannosaurus and Triceratops but held no long-necked sauropods, and some researchers took that absence as evidence the giants had already disappeared. A picture of thinning diversity became conventional wisdom.
Flynn's team opened a different window. Using magnetic field signatures frozen into the rock and radiometric dating of sandstone grains — work that took more than a decade — they placed their New Mexico site within roughly the final 380,000 years before extinction. What they found was a rich, varied community: not only Tyrannosaurus, horned Torosaurus, and duck-billed dinosaurs, but also Alamosaurus, one of the largest animals ever to walk the Earth, stretching 30 metres and weighing more than 30 tonnes. The continent, the study suggested, was home to two distinct but equally flourishing dinosaur worlds — cooler northern plains and lush southern forests where the great sauropods still roamed.
Flynn's conclusion was direct: the dinosaurs were doing great, and the asteroid struck them while they were strong. Extinction, on this reading, was not the last push for a failing group but a sudden blow to a successful one.
Not everyone agrees. Michael Benton of the University of Bristol noted that the new paper itself records a drop in western North American dinosaur species over the final few million years of the Cretaceous — from around 43 to about 30 — and argued that rich local communities like New Mexico's may have been favorable exceptions within a broader decline. Others, like Darla Zelenitsky of the University of Calgary, read the same fossils as strong evidence that dinosaurs remained vigorous to the very end. The disagreement is genuine, and it is the kind that healthy science produces.
What remains clear is the stakes of the question. If dinosaurs were flourishing when the asteroid arrived, their extinction becomes a story about sudden catastrophe overwhelming a thriving dynasty — not the tidy conclusion of a lineage already fading. A single well-studied basin has reopened that question. It will take more sites, dated with equal care and spread across the globe, to say with confidence whether the dinosaurs met their end at the peak of their power or somewhere on a long slope down.
For most of the twentieth century, the story scientists told about dinosaurs went like this: they were already fading when the asteroid came. A lineage past its prime, worn down by time, finished off by cosmic bad luck. It was a tidy narrative, and it held. Then in 2025, a team led by Andrew Flynn at New Mexico State University published something in Science that upended it. Using fossils from the San Juan Basin in northwestern New Mexico, they argued that dinosaurs were not declining at all. They were thriving, right up until the moment they weren't.
The question itself sounds straightforward enough: were dinosaurs in trouble before the impact, or were they doing well? But answering it has always been harder than it sounds. You need fossils from rock you can reliably date to the final moments of the Cretaceous, and such rock is rare. For decades, scientists relied heavily on the Hell Creek Formation, stretching across what is now Montana and the Dakotas. It preserved Tyrannosaurus and Triceratops in rocks from the very end, but it held no long-necked sauropods. Some researchers took that absence as evidence the giants had already vanished. A picture of thinning diversity took hold and became conventional wisdom.
Flynn's team opened a different window. Their site was the Naashoibito Member of the San Juan Basin, and the crucial work was dating it precisely. They used two methods: reading the direction of Earth's magnetic field frozen into the rock, which flipped in a known pattern over geological time, and radiometric dating of grains within the sandstone itself. Together, these placed the fossils within roughly the final 380,000 years before extinction. That work took more than a decade, which explains why fossils this close to the boundary are so scarce.
What they found was a rich community. Alongside Tyrannosaurus and horned Torosaurus and duck-billed dinosaurs, the site held Alamosaurus, one of the largest long-necked dinosaurs that ever lived. It stretched around 30 metres and weighed more than 30 tonnes. That single animal undermined 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 painted 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.
Flynn put it plainly: the dinosaurs were not on their way out but doing great and thriving, and 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. 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.
But the debate is not over. Michael Benton of the University of Bristol, whose own work has pointed to 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.
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 matters 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.
Citações Notáveis
The dinosaurs were not on their way out but doing great and thriving, and the asteroid seems to have knocked them down while they were strong.— Andrew Flynn, New Mexico State University
This is just one location and cannot capture the state of dinosaur life across the whole world.— Michael Benton, University of Bristol
A Conversa do Hearth Outra perspectiva sobre a história
Why did scientists believe dinosaurs were already fading before the asteroid?
The Hell Creek Formation in Montana and the Dakotas was the best-dated fossil site from the end of the Cretaceous, and it had no giant sauropods. Researchers assumed the giants had already died out, so the picture that emerged was of a group in decline.
And this New Mexico site changes that?
It does, because it's dated to within 380,000 years of the extinction and it has everything—Tyrannosaurus, horned dinosaurs, duck-billed dinosaurs, and Alamosaurus, a 30-tonne sauropod. A thriving, diverse ecosystem right at the boundary.
But other scientists disagree?
They do. Michael Benton argues that New Mexico was just one favorable pocket in a broader decline. The same fossils can be read different ways depending on what you think the overall pattern was.
So which is it?
That's the honest answer: we don't know yet. One well-dated site has reopened the question, but you'd need similar sites across the globe to settle it. The debate is live.
What's at stake in getting this right?
Whether extinction was a sudden catastrophe that struck a thriving group, or the final chapter of a long decline. It changes how we understand what happened.