Did the Fossils Lie? The Dinosaurs Weren’t in Decline Before the Asteroid

A new study challenges decades of assumptions about the dinosaurs’ final days — and why their fossils are so hard to find

You never forget your first dinosaur bone in the field. It was a chunk of a sauropod femur, sun-bleached and crumbling, barely peeking out of a slope that had been eroding for who knows how long.

We weren’t even on a formal dig that day — just surveying a region where others had found fossils before. I was there to observe more than collect, but the moment still stuck with me. Not just for the thrill of discovery, but for what it made me realize: fossils don’t just happen. They depend on a perfect storm of events that most creatures will never experience. A whimsical lottery.

That same idea, that what we find in the fossil record is just a tiny, biased sliver of what once lived, is at the heart of a new study published in Current Biology this April. And it’s turning a long-standing dinosaur extinction narrative on its head.

But why?

For years, many paleontologists believed dinosaurs were already on the decline when the asteroid hit 66 million years ago. The data seemed to back this up: fossil diversity peaked about 76 million years ago and then steadily dropped off. By the time the asteroid slammed into Earth, the story went, dinosaurs were hanging by a thread.

But according to lead author Dr. Chris Dean and his colleagues at University College London, that drop in diversity wasn’t a biological signal — it was a geological mirage.

The team studied around 8,000 dinosaur fossils from North America, zeroing in on four families: tyrannosaurs, hadrosaurs, ceratopsians, and ankylosaurs. They focused on the Campanian and Maastrichtian stages of the Late Cretaceous, spanning from about 84 to 66 million years ago. This is a crucial time for understanding life before the impact.

At face value, the numbers tell a familiar story: fewer fossils in the final 6 million years before the asteroid, suggesting a shrinking dinosaur population. But Dr. Dean and his team dug deeper using a technique called Bayesian occupancy modeling. In simple terms, this method estimates not just how many dinosaurs lived, but how likely we are to detect them in the fossil record. It separates “how many there were” from “how many we can find.”

My PhD supervisor worked on this methodology, so I was happy to see the authors citing his work.

The results flipped the old narrative. Dinosaurs weren’t disappearing. Our ability to detect them was.

So what happened? It turns out, geology got in the way.

During the Maastrichtian — the final stage before the mass extinction — the Western Interior Seaway was retreating, mountain ranges were rising, and ecosystems were shifting. These changes disrupted the kind of sedimentary environments that preserve fossils well. On top of that, much of the rock from this time is still buried or covered by vegetation, especially in North America.

This means that even if dinosaurs were still abundant, we’re just not seeing them because the rocks we need either weren’t formed in the right way or aren’t accessible yet.

It’s a bit like trying to judge the popularity of books in a library where half the shelves are behind a wall. You might assume certain authors fell out of fashion, but it’s just that their books are hidden in the back.

That has big implications. For one, it suggests dinosaurs weren’t “already doomed” when the asteroid arrived. They were doing just fine. Occupying large areas, showing no ecological signs of collapse. It was the sudden impact that changed everything.

“Dinosaurs were probably not inevitably doomed to extinction at the end of the Mesozoic,” said co-author Dr. Alfio Alessandro Chiarenza in a statement. “If it weren’t for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds.”

The study also found that not all dinosaur families were equally affected by the fossilization bias. Ceratopsians, the horned dinosaurs like Triceratops, remained well represented, likely because they lived in drier, open environments more favorable to fossilization. On the other hand, Hadrosaurs preferred river systems, and declining river flow may have reduced the sediment needed to preserve their remains.

To me, this study is a reminder of just how fragile our window into deep time really is. The fossil record is an incredible gift, but it’s not a full archive, more like a series of scattered journal entries from different continents, written in disappearing ink. You need both a deep understanding of biology and a healthy skepticism of absence to interpret it well.

I didn’t work directly on this paper, but I’ve spent years thinking about how landscapes, ecosystems, and deep time intersect. My work involved data from the Paleobiology Database, a tool built to untangle these biases. And one of the last courses I taught before leaving academia was called “Dinosaurs and Their Environment.”

It was, and still is, one of my favorites. These creatures, massive and mighty, continue to teach us how science is shaped not just by what we find but by what we almost didn’t.

Published in Fossils et al. Follow to learn more about Paleontology and Evolution.

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Best,

Sílvia P-M, PhD Climate Ages