Illustration by Davide Bonadonna
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Reconstruction of a floodplain in North America from about 66 million years ago, where dinosaurs such as Tyrannosaurus rex, Edmontosaurus, and Triceratops roamed.

Illustration by Davide Bonadonna

Would dinosaurs have died without an asteroid strike? Here's the science.

A new look at how dinosaurs fared before the catastrophe may also help scientists better understand responses to modern climate change.

One day 66 million years ago, life came to a sudden, apocalyptic halt when an asteroid impact violently closed the book on the age of dinosaurs. Birds are the only members of the dino family tree that survived the ordeal, and the open niches left behind gave them and our early mammal ancestors their time in the ecological spotlight.

But what if calamity hadn't befallen the dinosaurs? Would they still have gone out not with a bang, but a whimper?

Maybe not, according to a new study that says dinosaurs still had plenty of vim and vigor leading up to the mass extinction at the end of the Cretaceous period. Revealed using huge simulations that are new to paleontology, the finding marks the latest turn in a debate over whether dinosaurs were already in “terminal decline” by the time doomsday struck.

Dinosaurs 101 Over a thousand dinosaur species once roamed the Earth. Learn which ones were the largest and the smallest, what dinosaurs ate and how they behaved, as well as surprising facts about their extinction.

In addition, the study's cutting-edge approach could help us better look back at past environmental turmoil and learn in finer detail what we might expect from modern climate change.

“The results are very important—the whole story of decline and contradicting that—but it's also good that we've been devising and applying new methods. It's this multi-angled thing,” says paleontologist Alfio Alessandro Chiarenza, a Ph.D. student at Imperial College London who led the new study, published today in Nature Communications.

Fall of the dinosaurs

Watch the 1940 Disney movie Fantasia, and you'll get a look at how paleontologists once thought the dinosaurs died out. In the film, familiar species flourish in lush swampland but eventually succumb to a more hellish climate. That vision changed in the 1980s, when Walter and Luis Alvarez argued that dinosaurs didn't simply march across a desert to their deaths; instead, a combination of geologic and fossil evidence suggested that their reign ended with a cataclysmic asteroid strike.

Years later, scientists found the smoking gun: the remnants of the resulting crater off the coast of Mexico. Ever since, most paleontologists have agreed that the asteroid bears the bulk of the blame for the dinosaurs' demise. (But even that's up for debate: Two recent studies build on the case that volcanoes also helped kill the dinosaurs.)

That said, paleontologists debate what would have happened if the asteroid had never crashed into Earth. Getting hard numbers on this question is challenging because of the fossil record's fragmentary nature. Environmental conditions must be just right for an organism's body to end up buried and isolated long enough to form a fossil. As a result, telling life's history with fossils is like reconstructing the plot of an epic when all you have are tatters of the only surviving transcription. What if the pages fall apart, or the ink fades?

When tallying up the number of ancient species, paleontologists must therefore account for biases in the fossil record. And when you look at raw tallies, it looks like the number of dinosaur species declined in western North America in the last 17 million years of the Cretaceous. That suggests the dinosaurs were already in their twilight years when the asteroid struck.

But the time period closer to the mass extinction, what's called the Maastrichtian epoch, has not yielded enough fossils to show the finer details. Many studies have tried to account for this bias, and when they do, they find that dinosaur diversity in western North America holds steady or was even increasing until the last gasp. In this scenario, dinosaurs were doing fine—until they suddenly weren't.

This emerging consensus suffered a blow in 2016, when University of Reading biologist Manabu Sakamoto published a paper claiming that, tens of millions of years before the extinction, dinosaur species were dying out faster than new ones were emerging. According to this picture, based on a global family tree of dinosaurs, the heyday for some dinosaur groups had passed well before asteroid-induced armageddon.

Sakamoto's study isn't directly comparable to others, as it looks at longer timescales. Nevertheless, his work once again stoked the flames of debate.

Big bones, bigger data

To tackle big questions, it helps to have big databases, and for decades, paleontologists have been assembling huge public databases of fossil occurrences. Now, a new generation of computer-savvy paleontologists can slice and dice the ancient world like never before, gaining new insights on a global scale.

“We’re at an age of big data and data science now, right?” Sakamoto says. “If you want to make these grandiose kinds of studies and claims, you do need really big data to back it up, so the databases are vital.”

If you're picturing database-driven paleontology as a mix of Jurassic Park and The Matrix, you'd be sorely mistaken. It's tedious work carefully vetting and re-vetting databases that can house hundreds of thousands of entries.

“We spend years on this kind of stuff—it's day in and day out of failed models, failed runs, data cleaning, and if I see a misspelling of 'Maastrichtian' another time, I'll go crazy,” says paleontologist Emma Dunne, a Ph.D. student at the University of Birmingham who uses climate models to study dinosaurs' evolutionary origins. “But it's so worth it. It's super-exciting.”

Chiarenza's journey followed a similar path. He just wanted to study dinosaurs, but to answer his questions, he had to learn subjects ranging from Earth systems models to cutting-edge ecology.

For the new study, he first combined high-resolution models of ancient Earth's terrain with state-of-the-art climate models, the same kind that scientists use to understand human influence on today's climate. He and his colleagues then plotted where dinosaur fossils had been found across this ancient terrain, focusing on three groups: tyrannosaurs, ceratopsians such as Triceratops, and “duck-billed” hadrosaurs.

The researchers trained algorithms on the massive dataset to associate a given group of dinosaurs with a type of topography and climate. With these habitat models in hand, Chiarenza's team could zoom out to all of North America and project which regions were theoretically suitable for dinosaurs. Their model shows that toward the end of the Cretaceous, much of North America would have still been dino-friendly.

At the same time, the researchers modeled where in this region dinosaur fossils were most likely able to form. They simulated the flow of sediments off the baby Rocky Mountains into a seaway that once covered parts of western North America. As the Cretaceous ended, this seaway shrank—and so did the volume of sediments needed to preserve fossils.

Based on their results, Chiarenza and his colleagues argue that the apparent dino decline in western North America isn't a result of evolution cutting dinosaurs from the story; instead, it's geology being a sloppy scribe.

What could have been

Though debate is sure to continue, Chiarenza's work falls in line with other studies that also fail to show a long-term dino decline. In 2018, a study led by Ph.D. student Klara Nordén found that based on their teeth, plant-eating dinosaurs of the late Cretaceous were as ecologically diverse as ever.

“It fits really nicely with what we already know from other lines of evidence,” she says.

And since Chiarenza's models simulate dinosaurs' response to climate, his work could let researchers tease out what exactly killed them; scientists could throw a simulated asteroid or mega-volcano at the model and see the effects on habitat. Chiarenza is currently working through this very question. Similarly, researchers could use the model to dive into other past instances of climate variation to see how habitats respond and what may be happening today with ongoing climate change.

“This kind of technique could be very valuable for [giving] a baseline for how we understand changes that we might be facing with anthropogenic global warming,” says Paul Barrett, a paleontologist at the Natural History Museum in London who was not involved in the study.

Steve Brusatte, a paleontologist at the University of Edinburgh, adds that the study very clearly implicates the asteroid as the reason the dinosaurs died out. But he also notes that the study hints at what could have been if not for the sudden catastrophe.

“The most touching part of the paper [is that] it’s really clear that there was still plenty of potential niches for the dinosaurs—but the dinosaurs just weren't there, because the asteroid killed them off,” he says. “You kind of get this sad sense of the dinosaurs that could have been.”