The emergence of birds is often associated with the conquest of the skies. The earliest members of this group, close relatives of feathered dinosaurs, are thought to have gradually acquired the wings and muscles needed for flight. Yet a fossil discovered in China shows that their history did not unfold so simply.
This skeleton belongs to a previously unknown species, named Yuezhuornis yangyuanensis. It lived at the end of the Jurassic period, around 148 to 150 million years ago, in what is now Fujian Province. The animal probably weighed about 800 grams, roughly the weight of a small modern ground-dwelling bird.
What strikes researchers are its arms. In flying birds, the forelimbs bear wings developed enough to generate lift. In Yuezhuornis, they were far too short to allow efficient flapping flight. Its proportions instead resemble those of birds that spend their lives on the ground.
Its legs tell an equally surprising story. The second toe could be raised sharply, a feature also found in certain predatory dinosaurs, including close relatives of velociraptors. Its skull also had teeth, with characteristics that more closely recall dinosaurs than modern birds.
The animal therefore combined features that we tend to separate: an early member of the bird lineage, but with short forelimbs and traits inherited from its close dinosaur relatives. This combination led researchers to place it among the earliest birds, while concluding that it lived on land and probably did not fly.
One question remains: could its ancestors fly before losing that ability? The fossil does not provide an answer. Some early birds were developing adaptations for flight at the same time, but there is no reason to think that every branch followed the same path.
Only one incomplete skeleton is known so far. Yet it is enough to show that bird evolution took several paths very early on: some toward the sky, others toward a life on land.