When the first fossil of the blue-jay-sized Longipteryx chaoyangensis was found in 2020, paleontologists thought its elongated skull with an extended, toothed beak suggested it ate fish.

But a more recent look inside a specimen’s stomach showed the bird — which lived 120 million years ago in what’s now northeastern China — fed on fruit-like plants, according to a report in Current Biology.

Comparing Longipteryx to Other Ancient Birds

Paleontologists initially compared the ancient bird to the contemporary kingfisher because of its similarly-shaped skull and beak, and diet of small fish. That resemblance turned out to be a red herring.

“It was very superficial,” says Jingmai O’Connor, a curator at Chicago’s Field Museum of Natural History and the lead author of the study.

As paleontologists uncovered and studied other species of fossilized birds — some of which had bits of fish preserved in their stomachs — they started noticing patterns. Few of those patterns matched Longipteryx.

O’Connor visited the Shandong Tianyu Museum of Nature in China. She noticed two Longipteryx specimens that appeared to have something in their stomachs. She consulted Field Museum colleagues. They told her that the round contents in the fossilized bird stomach were flesh-covered seeds (true fruits found on flowering plants were just emerging during Longipteryx’s time), probably from gymnosperms, relatives of today’s conifers and gingkos.

The group was fortunate to find such solid evidence. “In general, finding stomach contents is incredibly rare,” O’Connor says.

Read More: What Are Fossils and Where Are They Found the Most?

Why Did Some Ancient Birds Have Teeth?

Although scientists now have a better understanding of Longipteryx’s diet, mysteries remain. For instance, the disproportionately large teeth toward the front of the beak and the thickness of those teeth’s enamel resembles that of a hyper-carnivore, akin to a meat-eating dinosaur like Allosaurus.

If those features weren’t meant for eating, what were they used for? Alex Clark, a graduate student and bird enthusiast working with O’Connor had a theory: Longipteryx was using its head as a weapon, just like modern hummingbirds wield their long, narrow beaks as air-born swords to fight off competition for food.

Read More: Beak Evolution Gives Insight Into the Beginning of Birds

The Evolution of Toothless Birds

The presence of teeth in Longipteryx poses an interesting evolutionary question. How and why did birds eventually lose them? Archeopteryx had a complete set, Longipteryx’s teeth were limited to the very front of its beak, and contemporary birds have remained toothless.

That question remains up for speculation. But O’Connor says the initial mischaracterization of Longipteryx ‘s diet holds a cautionary tale for paleontologists. Just as one shouldn’t judge a book from its cover, scientists can’t necessarily know what a bird eats solely based on the shape of its skull.

Read More: The Oldest Bird Relative Ever Found

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Before joining Discover Magazine, Paul spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.