Scientists propose wild new theory for what originally killed the dinosaurs

The sky burned with streaks of crimson red and dark orange. The dinosaurs looked up — just in time to see a burning, bright flash streak across the sky.

Then the asteroid hit, wiping out three-quarters of life on Earth.

These events describe the Cretaceous–Paleogene (K–Pg) mass extinction event, a famously monumental moment that occurred approximately 66 million years ago. It’s a riveting tale we’ve heard time and time again since grade school.

But research published Tuesday in the journal Nature Communications finds it’s not exactly accurate — at least when it comes to the extinction of dinosaurs. New research suggests their death was more gradual than sudden.

“I actually did not expect such a long decline,” lead author Fabien Condamine, a research scientist at the French National Center for Scientific Research, tells Inverse.

What’s new — Reports published earlier this year questioned the idea that dinosaurs declined due to a two-punch asteroid impact, finding that one of the smaller asteroids actually emerged later than the bigger asteroid. This bigger asteroid is known by the name of its impact crater, Chicxulub.

This new Nature Communications study takes it a step further, debunking the commonly held belief that asteroids primarily caused the dinosaur extinction.

The asteroid may have been a death knell, but dinosaurs were on their way out long before Chicxulub made an appearance on the Yucatán Peninsula in Mexico, the study suggests.

In fact, the decline in dinosaur populations likely began 76 million years ago during the Campanian period — 10 million years before the asteroid hit. The researchers write that two factors may have had the most impact on the dinosaur decline:

• A changing climate (including cooling global temperatures)
• The declining diversity of herbivorous dinosaurs

The dual blows that killed the dinosaurs were not necessarily the double asteroids, but environmental and population factors that emerged much earlier, the researchers say.

“Further analyses indicate that the global dinosaur decline could have been precipitated by the decline of herbivores,” Condamine explains, adding that herbivores are essential “keystone species” in ecosystems.

His research suggests dinosaurs weren’t able to recover from these dual blows. The rates of new species of dinosaurs emerging could not keep up with the extinction rates, leading to a decline in dinosaur diversity.

In fact, the dinosaur decline was so severe, Condamine and colleagues posit that the T-rex may have only had one species left on planet Earth by the time the asteroid made impact.

How they did it — Condamine has been fascinated by dinosaurs since childhood. He originally studied butterflies as a postdoctoral student at the University of Alberta — before pivoting to dinosaurs.

“I thought the time was ripe to test again the hypothesis of an extinction ...”

He spent years working alone and putting together a fossil record of the six major dinosaur families from the Late Cretaceous Period.

“I am passionate about the debate on the extinction of dinosaurs, and I thought the time was ripe to test again the hypothesis of an extinction due solely to the asteroid impact versus the hypothesis of a long-term diversity decline,” Condamine says.

He’s not a paleontologist by profession, so he solicited the help of co-author and Canadian paleontologist Phil Currie — whom he likens to “Professor Graham in Jurassic Park” — to help with his preliminary analyses of the dinosaur fossil record. This analysis suggested dinosaur populations were in decline before the asteroid event.

Led by Condamine, the subsequently four-person research team compiled a dataset of 1,600 fossils representing 247 dinosaur species from six major families, including the Tyrannosaurs and the herbivores known as Hadrosaurs.

“Although the dinosaur fossil record provides invaluable data [...] it is biased and incomplete,” the researchers write in the paper.

They turned to a tool called PyRate to estimate how the diversity of dinosaur species changed over time. The tool helps researchers analyze the rates of speciation — the emergence of new species — and extinction from incomplete fossil records.

Next, the researchers looked at differences in diversity among different dinosaur groups, specifically comparing carnivores and herbivores.

Finally, they used statistical models to determine the effects of certain factors, such as changes in dinosaur diversity and warming temperatures. This allowed them to determine what may have caused dinosaur decline.

Why it matters — The asteroid tale of dinosaur extinction has taken on an almost legendary status in our pop culture.

But this study, like others before it, suggests that the most popular rendition of their demise isn’t the most accurate one. This research also challenges the timeline of dinosaur extinction, proposing it starting far earlier than previously assumed.

“From the perspective that our results add a new stone in our understanding of dinosaur diversification until the end, it's likely the study will be controversial among paleontologists,” Condamine says.

“... we also support the idea that the meteorite was the fatal coup de grâce.”

Condamine explains that paleontologists have typically viewed dinosaur extinction in one of two ways: sudden extinction or a more gradual decline.

“Perhaps our study will be seen as a proponent of one side,” Condamine says. He also adds that “even if we show the dinosaurs were in decline well before the asteroid hit the ground, we also support the idea that the meteorite was the fatal coup de grâce” that wiped out the remaining dinosaurs. (A asteroid fragment that lands on Earth is known as a meteorite.)

As we stare down our own climate crisis, it’s startling to think that dinosaurs were contending with their own changing climate — albeit, a cooling world rather than a warming one.

“Such a debate is not only about dinosaurs, but the diversification of organisms in general and how they respond to global events,” Condamine says.

Dinosaurs’ large forms could not maintain constant body temperatures in a chillier world. Their inability to adapt to their changing climate seems like a grim precursor when we consider how many animal species are failing to adapt to fluctuating global temperatures today.

Among the species most affected by the climate crisis are some dinosaurs’ descendants: birds.

What’s next — New dinosaur fossil discoveries are still happening, and they might further challenge our previous understanding of dinosaurs.

These future fossil discoveries might provide insight into other environmental or genetic factors that could have led to the dinosaurs’ demise, the researchers write. Although the study covers six important dinosaur families, it doesn’t provide a full picture of what happened to all dinosaur species.

“I think future studies can build upon our results by adding more dinosaur families, for instance, or testing other environmental variables as putative causes of the extinction,” Condamine says.

But it’s inarguably an important first step to figuring out what led to their extinction.

“Studying dinosaurs is a complex but passionate topic, and I anticipate that our results will motivate people to look at their data and results through a new lens,” Condamine says.

Abstract: The question why non-avian dinosaurs went extinct 66 million years ago (Ma) remains unresolved because of the coarseness of the fossil record. A sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dinosaurs were in decline or not before the impact. We analyse the speciation-extinction dynamics for six key dinosaur families, and find a decline across dinosaurs, where diversification shifted to a declining-diversity pattern ~76 Ma. We investigate the influence of ecological and physical factors, and find that the decline of dinosaurs was likely driven by global climate cooling and herbivorous diversity drop. The latter is likely due to hadrosaurs outcompeting other herbivores. We also estimate that extinction risk is related to species age during the decline, suggesting a lack of evolutionary novelty or adaptation to changing environments. These results support an environmentally driven decline of non-avian dinosaurs well before the asteroid impact.