A reconstruction of the Late Jurassic bird _Yuezhuornis yangyuanensis_ A remarkable fossil found in China raises the possibility that some of the earliest bird species never took wing or abandoned flight almost immediately after they had evolved the ability. Min Wang at the Chinese Academy of Sciences says the discovery of the new flightless species, _Yuezhuornis yangyuanensis_, could mean that powered flight didn’t drive the evolution of birds’ unique anatomy at all. Recent discoveries are disrupting the view that the evolution of birds and flight proceeded in a straight line, including another recently announced bird-like dinosaur, also found in China, that had evolved to fly but emerged from a completely different lineage.
The almost-complete new fossil, which is about 148 million to 150 million years old, was discovered in August 2024 near Yangyuang village, Fujian Province, and has been named by Wang and his colleagues after Yuezhuo, a mythological Chinese bird. When alive, _Y. yangyuanensis_ was around half a metre tall, weighing just over 800 grams – probably as big as a large crow, Wang says. He suspects the creature most likely lived in a swampy environment and was probably a good runner.
After comparing the ancient bird’s anatomy with nearly 400 living birds, the researchers concluded _Y. yangyuanensis_ is most like modern ground-dwelling pheasants. These modern birds have forelimbs shorter than their hindlimbs, and _Y. yangyuanensis_ did, too. While pheasants can fly in short bursts, they are effectively ground-dwellers.
The aerial abilities of some other early birds, including _Archaeopteryx,_ are sometimes questioned, but they have forelimbs longer than their hindlimbs, and probably had stronger flying abilities than _Y. yangyuanensis_. _Y. yangyuanensis_ also had a distinctive sickle-shaped claw on its second toe that was “highly functional”, says Wang, and probably used in prey capture. It was very similar in appearance to the claws of its distant relatives, including _Velociraptor_. Modern birds have lost these claws.
Unlike modern birds, _Y. yangyuanensis_ had a long tail, similar to _Archaeopteryx_. It also had closely packed, serrated teeth like those typically present in deinonychosaur dinosaurs. But in other anatomical features, _Y. yangyuanensis_ resembles other birds.
Its hip bones and fibula – one of the lower leg bones – closely mirror those of later Cretaceous birds, says Wang. The mix of dinosaur and bird features present in _\_Y. yangyuanensis\__ demonstrates that the bird body plan wasn’t a steady progression towards flight optimisation, say the researchers – particularly given that their analysis suggests _Archaeopteryx_, a stronger flier, was actually more primitive than _\_Y. yangyuanensis\__. Rather, they say, bird evolution was a more complex and rapid process than previously thought.
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