This fossil bird choked to death on rocks, and no one knows why

A fossil only tells part of the story. When an animal’s body is preserved as a fossil, there are often pieces missing, and even a perfectly-preserved body doesn’t tell the whole story of how that animal behaved, how it lived, and how it ultimately died. But the cause of death for one unlucky bird that lived about 120 million years ago is clearer: the cluster of rocks in its throat tells scientists that it probably choked to death. The reason why this bird was swallowing rocks in the first place is more of a mystery, and one that gets into the bigger picture of dinosaur and bird evolution.

Jingmai O’Connor, the associate curator of fossil reptiles at Chicago’s Field Museum and lead author of a new paper describing the new species in the journal Palaeontologica Electronica, came across the fossil at the Shandong Tianyu Museum of Nature in China.

“There are thousands of bird fossils at the Shandong Tianyu Museum, but on my last trip to visit their collections, this one really jumped out at me,” says O’Connor. “I immediately knew it was a new species.”

The fossil was tiny— about the size of a modern sparrow— but it had features in common with a larger fossil bird called Longipteryx. “It had really big teeth at the end of its beak, just like Longipteryx, but it’s a tiny little guy. So based on that, I knew it was something new.”

When O’Connor examined the fossil under a microscope, she saw something puzzling. “I noticed that it had this really weird mass of stones in its esophagus, right up against the neck bones,” says O’Connor. “This is really weird, because in all of the fossils that I know of, no one has ever found a mass of stones inside the throat of an animal.” The placement and chemical composition of the rocks suggested that they really were swallowed by the animal during its life, instead of just washing up near its body in the lakebed where its fossil formed.

It’s common enough for animals to swallow stones that there’s even a term for rocks swallowed by animals, either intentionally or by accident: gastroliths. Some birds, including chickens, swallow small stones that are stored in a muscular stomach called a gizzard, which they use to help grind up their food. But in the thousands of fossil birds in the same group as this little fossil, none have ever been found with gizzard stones.

To help determine if the new little bird was simply the first of its kind to be found with gizzard stones, O’Connor drew upon previous research in which she and her colleagues had CT scanned fossils of birds that definitely did have gizzards. “We had quantified the average volume of the stones, the number of stones that these other fossil birds had in their gizzards, the size of the gizzard stone mass compared to the total size of the bird,” says O’Connor. “We CT scanned this new fossil so we could compare it to these other birds with gizzards.”

The CT scan data indicated that whatever the mass of rocks in the little bird’s throat were, they were not gizzard stones. “We found over 800 tiny stones in this bird’s throat— way more than we would have expected in other birds with gizzards. And based on their density, some of these stones weren’t even really stones, they seemed to be more like tiny clay balls,” says O’Connor. “With these data, we can very clearly say that these stones weren’t swallowed to help the bird crush its food.”

Since the bird’s gastroliths were not gizzard stones, it must have swallowed them for another reason. The researchers can’t be sure, but they have an idea of what that might be.

“When birds are sick, they start doing weird things,” says O’Connor. “So we put forth a tentative hypothesis that this was a sick bird that was eating stones because it was sick. It swallowed too many, and it tried to regurgitate them in one big mass. But the mass of stones was too big, and it got lodged in the esophagus.”

The finding is unusual, not just because it’s the first time, to O’Connor’s knowledge, that a fossil animal has been discovered with a throat full of stones. “It’s pretty rare to be able to know what caused the death of a specific individual in the fossil record,” says O’Connor. “But even though we don’t know why this bird ate all those stones, I’m fairly certain that regurgitation of that mass caused it to choke, and that’s what killed that little bird.”

In addition to its unusual gastroliths, the fossil represents a new species of bird, and thus, a new species of dinosaur. O’Connor named the fossil Chromeornis funkyi (kroh-me-or-nis fun-key) after the techno-funk duo Chromeo, one of her favorite bands, since birds are also known for making beautiful music. (Scientists can’t tell exactly what sounds Chromeornis made— it probably didn’t sing like a modern bird, but O’Connor says that it probably made some sort of vocalizations.)

 Chromeo, comprised of David “Dave 1” Macklovitch and Patrick “P-Thugg” Gemayel, were happy to learn that a fossil had been named after them, saying, “We’ve been doing this for 20 years but this is the first time someone’s called us a dinosaur! Jokes aside, this is an incredible honor to add to a career full of surprises. We’re glad to bring a little fossil funk to the great science of paleontology.”

Beyond the fate of this one unlucky bird that choked on rocks, Chromeornis is a piece of a bigger picture of extinction and survival. It’s part of a group of birds called the enantiornithines. During the Cretaceous, this was the most prevalent group of birds, but when the asteroid hit 66 million years ago, enantiornithine birds went extinct along with nearly every other kind of dinosaur— the only group that remained is the one containing the modern birds still alive today.

“During that environmental disaster, the enantiornithines went from being the most successful group of birds to being wiped out,” says O’Connor “Understanding why they were successful but also why they were vulnerable can help us predict the course of the mass extinction we’re in now. Learning about Chromeornis and other birds that went extinct could ultimately help guide conservation efforts today.”