A bird as old as Archaeopteryx had already traded flight for running

A bird that lived in the age of Archaeopteryx, the most famous early bird of all, seems to have spent its life on the ground. Named Yuezhuornis yangyuanensis, it had wings too short for real flight, limbs proportioned like a pheasant’s and a curved claw on each foot like a Velociraptor’s. Its fossil comes from rocks in Zhenghe County, in China’s southeastern Fujian province.

That matters because the textbook story of bird origins is a story about flight: feathered dinosaurs got lighter, their arms grew into wings, and the bird body took shape as an engine for flying. Yuezhuornis suggests that some of the very first birds were already heading the other way. “Yuezhuornis challenges the traditional paradigm that the avian body plan evolved almost exclusively under the selective pressures of flight,” paleontologist Min Wang of the Chinese Academy of Sciences’ Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), the study’s corresponding author, told Reuters.

The animal lived in the Late Jurassic, roughly 148 to 150 million years ago, about the same time as Archaeopteryx in what is now Germany. It weighed an estimated 821 grams. The team, from the IVPP and the Fujian Institute of Geological Sciences, describes it as the earliest known example of a bird losing the ability to fly.

How you tell that a fossil bird could not fly

There is no single bone that says “flightless.” The clue in Yuezhuornis is proportion. Its forelimbs are about a third shorter than its hind limbs, a pattern common in ground-running dinosaurs but unusual in birds built to fly. According to the Chinese Academy of Sciences, they are relatively the shortest forelimbs known in any bird of the dinosaur era. Archaeopteryx, by contrast, had long, well-proportioned wings.

To test what those proportions mean, the researchers measured the limbs of Yuezhuornis and compared them with 377 living bird species and with other Jurassic relatives of birds. Its limb proportions fell closest to those of ground-dwelling birds such as pheasants, and to some non-bird dinosaurs close to the bird line. The team read that as strong evidence for a terrestrial, flightless lifestyle.

That method is statistical, not a direct observation. It works because limb length tracks how an animal moves: flapping fliers need long arms relative to their legs, runners do not. It cannot see muscles or feathers, so it tells you what the body was best suited for, not what the animal did on any given day. Wang told Reuters that Yuezhuornis may have lived like a roadrunner, mostly running and flying only in short bursts when needed. Asked whether that was the picture, he said: “Yes, that is likely.”

A raptor’s toolkit on a bird’s body

The skeleton mixes two kinds of animal. The hips, hind legs and slender skull bones are bird-like. But the jaws carry closely packed, serrated teeth of a kind found in deinonychosaurs, the group of small predatory dinosaurs that includes Velociraptor, Deinonychus and Troodon, and not in birds. The second toe could flick up and back, carrying a sickle-shaped claw, another deinonychosaur trademark. Like all the earliest birds, it also kept a long bony tail.

“When we examined Yuezhuornis, we were struck by its extraordinary mix of features,” Wang said in an IVPP statement. “It possesses bird-like pelvic and hindlimb structures yet retains the specialized predatory foot and teeth of a small meat-eating dinosaur. It shows us that early bird evolution was far more complicated than we previously realized.”

Wang told Reuters the animal may have competed for prey with small non-bird dinosaurs. That reading rests on its teeth and feet; the fossil preserves no direct record of what it ate.

Where it sits next to Archaeopteryx

To place the species on the family tree, the team ran two separate kinds of analysis, one that finds the tree needing the fewest evolutionary changes and one that also uses the ages of the fossils. Both gave the same answer. “The results consistently placed Yuezhuornis firmly within Avialae (the group containing modern birds and their closest extinct relatives), just crownward of Archaeopteryx and as a sister taxon to Zhengheornis,” said co-author Zhonghe Zhou. In plain terms, Yuezhuornis is a true member of the bird lineage, a step closer to modern birds than Archaeopteryx, and it groups with other early birds from the same Fujian site. The team also tried forcing it into the raptor-like deinonychosaurs instead; according to the Chinese Academy of Sciences, that version of the tree needed 14 extra evolutionary steps, which counts against it.

That site, the Zhenghe Fauna, has become one of the most important windows on Jurassic birds. It has already produced Baminornis, described as the earliest known bird with a short tail, and Fujianvenator, a long-legged early member of the bird lineage. It is the southernmost place on Earth where Late Jurassic bird fossils are known. In the Jurassic it was a landscape of lakes and basins between volcanic eruptions, where turtles, fish and plants are preserved alongside the birds.

For comparison, flightlessness is common among living birds, from ostriches and emus to kiwis and penguins, and it has evolved many times. Among birds of the dinosaur era, the best-known flightless forms, such as the diving Hesperornis, lived tens of millions of years after Yuezhuornis. Finding flight loss right at the base of the bird family tree means early birds were already experimenting with very different ways of life.

What the fossil does not settle

Yuezhuornis is known from a single skeleton, and the Nature press office notes that it is incomplete, with more fossils needed to pin down its lifestyle and its place in bird evolution. One specimen cannot show how much individuals varied or whether this one was fully grown.

The biggest open question is how it became a ground bird. It may descend from flying ancestors and have given flight up, or it may belong to a side branch that never fully committed to flying. Wang said Yuezhuornis may share a common ancestor with Archaeopteryx earlier in the Jurassic, and that it could also represent an evolutionary experiment that arose apart from the main bird lineage. Paleontologist Steve Brusatte of the University of Edinburgh, who was not involved in the study, put the uncertainty bluntly. “Yuezhuornis is one of those discoveries that throws up more questions than it gives answers,” he told Reuters. “Yet this is a weird bird. Its skeleton looks like a Frankenstein monster mashup.”

Brusatte still sees it as significant. In his view, it hints that some of the first birds “were not particularly good fliers, maybe even that some of them had evolved from flying ancestors but reverted back to living on the ground, rarely if ever taking to the skies.” The family tree of the earliest birds is also notoriously unstable, and each new Jurassic fossil can shuffle which species count as birds and which as their dinosaur cousins.

Common questions about Yuezhuornis and early birds

What is Yuezhuornis yangyuanensis?

A crow-sized bird from the Late Jurassic of Fujian, China, about 148 to 150 million years old. It had bird-like hips and legs, short wings, serrated teeth and a sickle claw on its second toe, and its limb proportions suggest it lived on the ground.

Was Yuezhuornis older than Archaeopteryx?

They lived at roughly the same time, around 150 million years ago. On the family tree, Yuezhuornis sits one step closer to modern birds than Archaeopteryx does.

Could Yuezhuornis fly at all?

The study concludes it was flightless, based on wings about a third shorter than its legs and limb proportions like those of ground birds. Its corresponding author says it may still have managed short bursts, the way a roadrunner does.

Why did it have a claw like a Velociraptor?

The hyperextensible second toe with a sickle claw is a trait it shares with deinonychosaurs, the predatory dinosaurs closest to birds. Reuters described it as a dangerous weapon for a hunter that lived on the ground.

What comes next

The fossil, catalogued as IVPP V34427, was found in August 2024 near Yangyuan in Zhenghe County, and the study was published on 14 October 2026 in Nature, alongside a commentary by paleontologist Fernando Novas. Wang says the steady stream of early birds from Zhenghe shows the site’s potential to fill a poorly documented chapter of Jurassic bird evolution. The next skeleton from those rocks will decide whether Yuezhuornis was an oddity or the first sign that the earliest birds came in many shapes, and not all of them took to the air.

Reference: Xu et al., “Jurassic flightless bird reveals early ecological diversity of avialans,” Nature, 2026. DOI: 10.1038/s41586-026-10972-z

Tags: dinosaurs, paleontology, Archaeopteryx, bird evolution, flightless birds, Yuezhuornis