More than a century of fossils pulled from the ground told a straightforward story: dinosaurs thrived, then declined, then vanished in a fiery cataclysm 66 million years ago. The idea is that non-avian dinosaurs were doomed regardless and that killer asteroid was the final nail in their coffin that sped up their inevitable demise.
But what if that story is wrong? What if the dinosaurs didn’t decline at all?
According to a new study, we’ve been misreading the fossil record. Led by Christopher Dean of University College London, the research team used a powerful tool from ecology, Bayesian occupancy modeling, to peer through the patchiness of fossil data.
Their findings suggest that if the mass extinction impact never occurred at Chicxulub off the coast of Mexico, the dinosaurs would have probably still ruled supreme over this planet.
“It’s been a subject of debate for more than 30 years — were dinosaurs doomed and already on their way out before the asteroid hit?” Dean said in a statement.
“We analysed the fossil record and found that the quality of the record of four groups of dinosaur (clades) gets worse during the final six million years prior to the asteroid. The probability of finding dinosaur fossils decreases, while the likelihood of dinosaurs having lived in these areas at the time is stable. This shows we can’t take the fossil record at face value.”
A Record Built on Rock — and Gaps
Dean and his colleagues analyzed around 8,000 fossil specimens from North America, focusing on four key dinosaur families: the club-tailed Ankylosauridae, the horned Ceratopsidae (which includes Triceratops), the duck-billed Hadrosauridae, and the fearsome Tyrannosauridae.
When they looked at the raw numbers, it seemed to confirm the prevailing view: dinosaur diversity peaked around 76 million years ago, then steadily fell. By the time the asteroid arrived, only a trickle of fossils remained.
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But numbers alone can deceive. Did this apparent drop reflect a real biological decline or just a gap in the fossil record?
To find out, they modeled how different geological conditions across time and space affected the chances of fossilization. They used occupancy models, which estimate the chances of a species being present but undetected.
Their findings revealed something striking. The period just before the extinction — the Maastrichtian age, from 72.1 to 66 million years ago — was geologically hostile to fossil preservation.
Much of the landscape had changed. The vast Western Interior Seaway, which once split the continent in two, was retreating, and the Rocky Mountains were rising. These events reshaped river systems and sedimentation patterns, conditions that strongly influence whether a dead dinosaur becomes a fossil or simply disappears.
A World Still Full of Dinosaurs
In other words, dinosaurs may have still been present across much of the continent, but the geological and environmental conditions weren’t always ideal for preserving their bones — or for us finding them.
“If it weren’t for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds,” said Alfio Alessandro Chiarenza, another co-author from University College London.
The researchers found no signs of environmental stress, habitat loss, or shrinking ranges that would typically precede a mass extinction. In fact, all four families appeared widespread and ecologically secure.
So, why do we see so few fossils from this time?
The answer, they argue, lies partly in what geologists call “outcrop bias.” The more exposed sedimentary rock there is, the more likely we are to find fossils. However, rocks from the Maastrichtian are often buried under vegetation or eroded away. That makes it harder for paleontologists to find and excavate fossils, no matter how many there are.
Hadrosaurs and ankylosaurs, for instance, seemed to vanish from many parts of the map. But the model revealed their actual occupancy — their likelihood of being alive in a region — stayed the same or even rose. It was the rocks, not the reptiles, that had changed.
Modern land cover also matters. Forests, fields, and even roads influence which parts of the landscape are explored by paleontologists. The researchers also layered in other variables, like rainfall and distance from roads, to show how current conditions — and not just ancient ones — can shape our view of the deep past.
Even the best-sampled dinosaur fossil zones, like Alberta’s Dinosaur Park Formation, can give a skewed impression. These areas act as “hotspots,” masking the absence of fossils in poorly preserved or inaccessible regions.
Rethinking the Final Days of the Dinosaurs
All of this reframes one of paleontology’s most dramatic questions: were dinosaurs already falling before the asteroid pushed them over the edge?
For years, scientists debated whether the non-avian dinosaurs were already weakened, outcompeted by mammals, or suffering from climate change. This new study suggests none of that may be true. The asteroid wasn’t the final blow — it was the first and only one.
Fossils don’t always tell the full story. They’re a patchwork stitched together by luck, geology, and time.
Dinosaurs were likely thriving when the sky fell. And had the asteroid missed? This world might still be theirs.
The findings appeared in the journal Current Biology.
