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.
“This new fossil changes our assumption about what a Jurassic bird looked like,” says Wang.
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 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 . Modern birds have lost these claws.
Unlike modern birds, Y. yangyuanensis had a long tail, similar to . 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 , 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. It saw early species almost immediately explore different niches and specialisations, including lifestyles that didn’t involve flying. This challenges the notion that the early evolution of the avian body plan was driven almost exclusively by the selective pressures of flight, Wang says.
“[Y. yangyuanensis] exhibits a striking ‘mosaic’ of primitive and derived features – blending classic avian traits with distinct characteristics inherited from non-[avian] dinosaurs,” he says.