After decades of debate, most scientists now agree: there was a single day 66 million years ago that ranks among the all-time worst days in the history of our planet. On that day, an asteroid collided with the Yucatan Peninsula in Mexico, triggering the extinction of the dinosaurs and more than two-thirds of all animal species.

But that consensus raises other questions. Why did the dinosaurs die out while other groups survived? Were the dinosaurs already in bad shape before the impact? Most intriguingly of all, why did the ancestors of modern birds squeeze through, when other feathered dinosaurs that would have looked just like birds didn’t make it?

On that last question, scientists now believe they have an answer:

Seeds.

Granivory — a diet of seeds — may have allowed the ancestors of modern birds to dodge the sickle of mass extinction, according to a new study co-authored by three Canadian paleontologists that examined over 3,100 fossilized dinosaur teeth. The research is published in the journal Current Biology.

“We think birds that survived the extinction were able to access this resource,” says Derek Larson, the study’s first author and an assistant curator at the Philip J. Currie Dinosaur Museum. “Whereas all of these mostly animal-eating (birdlike) dinosaurs with teeth would have gone extinct, because they couldn’t.”

“It’s hard to know for sure, it’s hard to test these things with fossils, but I think it’s very plausible,” said Stephen Brusatte, a University of Edinburgh paleontologist who authored an accompanying article.

“It adds to the weight of evidence that dinosaurs were thriving right up to that moment in time, that Tuesday morning, when that six mile wide rock fell out of the sky and exploded with the force of a billion Hiroshima bombs, and changed the planet in a day.”

For their analysis, Larson and his colleagues collected a database of 3,104 dinosaur teeth from collections housed at the Royal Tyrrell Museum, the University of Alberta, the Royal Ontario Museum and beyond. All the teeth belonged to four different families from a group of dinosaurs called maniraptorans, which are small-bodied, feathered, bipedal, and include raptors and other birdlike dinosaurs.

Teeth are gold to a paleontologist for two reasons. Unlike the delicate skeletons of maniraptorans, which do not fossilize well and are incredibly rare, their hardier teeth are salted throughout the fossil record. And teeth link an animal to its environment, holding clues as to the tooth-owner’s diet and ecological niche.

The researchers painstakingly catalogued the shape and size of thousands of these teeth, looking for patterns of variations across the 18 million years before the asteroid. Constant variations would indicate a stable, diverse ecosystem. But if variations between the teeth decreased, that could suggest the animals and ecosystems were stressed and losing diversity — the same way climate change and other human pressures are triggering biodiversity losses today. In recent years, paleontologists have used to similar techniques to suggest the large-bodied herbivorous dinosaur groups were already declining leading up to the asteroid impact.

But that’s not what the paleontologists found with maniraptorans. The teeth indicated stability throughout those 18 million years — until suddenly all the animals disappear.

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That suggests something about these toothed, birdlike dinosaurs doomed them all. So next, Larson and his colleagues pieced together family trees and dietary information for modern birds, and worked backwards. They predicted that the ancestors of modern birds must have had beaks, not teeth. And that lead them to a novel conclusion: that the ability to crack open nuts and seeds allowed birds to access a critical dietary resource.

“In the conditions in the wake of that major catastrophe, the asteroid, the ecosystem would quickly lose foliage and animal life. But those seeds would be these high-energy packets of food that would persist on the landscape, and any animal that could access them would have an ecological advantage,” says David Evans, curator of vertebrate paleontology at the ROM and another of the paper’s co-authors.

Evans added that the team discovered similar trends from studies of modern forest fires. “It turns out that seed-eating birds are very typically the first vertebrates back into these disturbed habitats. This makes sense, because a forest fire will burn all the foliage and decimate the animal population, but seeds are in their protective shell. They survive these major disruptions, and they can survive for decades.”

Other paleontologists have concluded that birds’ small body size, large brain size, and even the shape of their eggs allowed them to survive changing conditions after the asteroid. But the new study is the first to examine the contribution of granivory.

“We’re pretty excited about that, and we’re interested to hear what the scientific community has to say,” says Larson.