40 Million Years Before the Asteroid Hit, Dinosaurs Were Already On the Decline

“What doesn’t kill you makes you stronger.” A nice adage, but not true for the dinosaurs.

The asteroid that presumably obliterated the dinosaurs might not have played such a critical role in their demise as we once thought. Researchers in the U.K. have announced that, based on a statistical analysis of what they call “speciation events” (the splitting of a species into two daughter species), dinosaur populations were already suffering losses at least 40 million years before the cataclysmic Chicxulub impact 66 million years ago.

The scientists, led by Manabu Sakamoto of the University of Reading, looked at speciation events because they deemed it a more reliable indicator of overall dinosaur health than other measures; counting the mere number of species at any given point, for example, has its flaws—namely, some rocks are better at preserving fossils than others.

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An artist’s rendition of the Chicxulub impact

Sakamoto and his team focused on 614 dinosaur species. They discovered that many of these dinosaurs—but especially theropods and sauropods—may have reached a deadly “tipping point” up to 90 million years ago (at least 40 million years before the asteroid), at which their extinction rates were surpassing speciation rates. New species weren’t evolving quickly enough to replace the species that were in decline. Ceratopsians and hadrosaurs might have been exceptions to that rule, though a different study by Stephen Brusatte of the University of Edinburgh showed that they, too, were being phased out.

Their findings, now published in the Proceedings of the National Academy of Sciences, suggest that the famed asteroid may not have been the exact cause of dinosaurs’ extinction, but rather the final event that precipitated their collapse.

The research also illustrates the complexity of extinction processes. Here’s Ed Yong, reporting for The Atlantic:

[Brusatte] suspects that the results say more about dinosaur booms than dinosaurs busts. They emerged during the Triassic period and after an extinction event killed off most of their competitors (like the obscure but highly successful crurotarsans), they evolved like mad during the Jurassic. After that burst, a slowdown was inevitable. “That doesn’t mean the economy is necessarily doomed; it just means things aren’t growing as insanely fast anymore,” says Brusatte. He suspects that you’d see the same pattern in other groups that suddenly rose to power, including mammals like us.

“I think we sometimes have a tendency to overthink the dinosaur extinction, myself included,” Brusatte adds. “The way I see it, it came down to the asteroid. Simple as that. Diversity declines may have made dinosaurs somewhat more susceptible to the asteroid impact, but probably nothing was going to save them.”

The climate changes that occurred after the asteroid impact certainly would have contributed to the unbearable conditions on Earth—but geologic forces were also already at play. Earth’s two supercontinents, Laurasia and Gondwana, were pulling apart, sea levels were fluctuating, and megavolcanoes were probably erupting in India. A dinosaur wipe-out was bound to happen—the Chicxulub asteroid probably just sealed the deal.