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A 150-million-year-old bird fossil discovered in southeastern China is challenging conventional theories about bird evolution, suggesting that some early birds may have lost the ability to fly as they adapted to life on the ground.
The newly identified species, Yuezhuornis yangyuanensis, lived during the Late Jurassic period, around 150 million to 148 million years ago. Described in the journal Nature, the crow-sized creature provides evidence that early bird evolution followed more diverse paths than previously understood.
Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and the Fujian Institute of Geological Sciences discovered the fossil in the Zhenghe Fauna of Fujian Province.
The findings suggest that Yuezhuornis represents the earliest known example of a bird lineage that lost its ability to fly.
The fossil has an unusual combination of bird-like skeletal structures and features associated with small predatory dinosaurs. Its jaws contained closely spaced, serrated teeth resembling those of deinonychosaurs, a group of meat-eating dinosaurs closely related to birds.
Its second toe could also bend unusually far backwards and carried a curved, sickle-shaped claw, a feature commonly associated with predatory dinosaurs such as dromaeosaurids and troodontids.
“When we examined Yuezhuornis, we were struck by its extraordinary mix of features,” said Min Wang, the study’s corresponding author.
He said the fossil possessed bird-like pelvic and hindlimb structures while retaining the specialised teeth and feet of a small predatory dinosaur, highlighting the complexity of early bird evolution.
To establish the species’ evolutionary position, researchers used two analytical methods — maximum parsimony and Bayesian tip-dating — to compare anatomical features and reconstruct relationships among ancient birds and dinosaurs.
Both methods placed Yuezhuornis within Avialae, the evolutionary group that includes modern birds and their closest extinct relatives. The species was positioned just beyond Archaeopteryx on the evolutionary tree and as a close relative of Zhengheornis, according to co-author Zhonghe Zhou.
Researchers also analysed the fossil’s skeletal proportions to determine how the animal might have moved. Comparisons with modern ground-dwelling birds and other prehistoric animals suggested that its limbs were better suited to terrestrial movement than to flight.
The findings indicate that Yuezhuornis probably lived primarily on the ground and was likely flightless.
The discovery challenges the idea that early bird evolution was driven mainly by the development of increasingly efficient flight. Instead, some lineages may have adapted to different ecological niches, including environments where flying was unnecessary.
Its combination of bird-like anatomy, dinosaur-style predatory features and apparent flightlessness offers new insights into the evolutionary diversity of early birds.
Researchers said further discoveries from the Zhenghe Fauna could help clarify how the earliest birds evolved and adapted to different ways of life during the Jurassic period.