Not all Jurassic birds evolved the ability to fly, including this ferocious predator

Yuezhuornis, a 150-million-year-old Jurassic bird from China, had serrated teeth unlike any other bird and probably hunted on the ground.

A bird with a mouthful of sharp teeth would look strange beside the birds outside a window today. But during the Jurassic Period, about 150 million years ago, an animal like that lived in what is now southeastern China.

It had short arms, a long, bony tail and possibly a raised toe like those of Velociraptor's relatives. From its bones, researchers think it was a small hunter that ran on the ground and probably couldn't fly.

They named it Yuezhuornis yangyuanensis. It is one of the oldest birds yet found, and it gives scientists a rare chance to see how differently early birds could live.

Paleontologists Liming Xu and Min Wang led the study in Beijing. "It shows us that early bird evolution was far more complicated than we previously realized," Wang said.

Jurassic bird was a small ground hunter

Researchers found the skeleton in Fujian Province during the summer of 2024. Earlier work had dated the fossil beds there to between 148 million and 150 million years, near the end of the Jurassic period.

Yuezhuornis had arms about a third shorter than its legs. That is an unusual shape for an early bird, particularly one living at a time when other Jurassic birds already had features suited to flight.

The team estimated that it weighed about 1.8 pounds (821 grams), using the width of its thigh bone. They also examined a thin slice of its shin bone and found signs that it had almost or completely finished growing.

So the researchers were looking at the body of a nearly or fully grown bird. Its short arms were part of that adult body.

Why flight seems unlikely

The case against flight in this Jurassic bird involved more than the length of its arms. The shape of its wishbone and upper arm bone, along with its long tail, also pointed toward a bird that stayed on the ground.

Short wings would have made flying difficult. They would have had to support the bird's weight while producing relatively little lift.

The researchers also compared its arm and leg bones with those of 377 living birds, grouped by where they find food. Yuezhuornis came closest to two pheasant species, suggesting that it searched for meals on the ground.

That comparison helped the team work out where it probably fed. The rest of the evidence suggested a small meat-eater suited to running.

Teeth like tiny blades

There is a lot to learn from its mouth, too. Very few Jurassic bird skulls have turned up, so finding one gives researchers a rare look at the teeth and jaws of a bird this old.

The front teeth were small and crowded together. Behind them sat flatter, blade-like teeth with sharp tips that curved backward.

Along the back edge of those teeth were tiny serrations, rather like the uneven cutting edge of a kitchen knife. No other bird known to science has serrated teeth, according to the researchers.

For comparison, Archaeopteryx had teeth that were more like cones. Even among these very early birds, their mouths could look quite different.

A familiar dinosaur toe

The foot brought another surprise. Among the surviving bones was a short, thick toe bone with a large bump at one end.

It looked familiar because relatives of Velociraptor have a similarly shaped bone in their second toe. They could bend that toe sharply upward, lifting the large, curved claw it carried.

To check the match, the team compared the fossil with 70 toe bones from early birds and their dinosaur relatives. The results suggested that Yuezhuornis also had this kind of raised toe.

The trouble is that the bone had come loose from the foot, leaving its exact position uncertain. Another fossil with the bones still connected could show whether the researchers have placed it correctly.

Flight's complicated beginnings

The bigger question concerns how birds first took to the air. On the family tree the researchers built, Yuezhuornis sat slightly closer to modern birds than Archaeopteryx.

Yet Archaeopteryx lived a little earlier and had several features associated with flight. Finding an apparently flightless bird on the next branch makes that history harder to untangle.

Perhaps the ancestors of Yuezhuornis could fly and its family later lost the ability. Or flight may have developed more than once among early birds.

The researchers cannot choose between those explanations using this fossil alone. Calling it the earliest bird to lose flight would therefore go further than the evidence allows.

More Jurassic bird bones to find

There is still plenty this animal cannot tell us, since the study rests on one incomplete skeleton. Its movements and feeding habits are the researchers' best reading of its bones, not behaviors they could observe.

But it is not the only early bird from these rocks. Three others have turned up there, including one with unusually long legs and another with a short tail.

Each find adds another animal to the picture of what birds were like near the beginning of their history. Already, that picture includes more than just creatures developing better wings.

The study is published in the journal Nature.

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