A remarkably preserved 150-million-year-old bird fossil discovered in China is challenging long-standing ideas about how birds evolved. Rather than developing increasingly sophisticated flight abilities, some of the earliest birds appear to have taken a very different evolutionary path, including losing the ability to fly altogether.
The newly identified species, Yuezhuornis yangyuanensis, lived during the Late Jurassic period (150-148 million years ago). Described in a study published in Nature, the crow-sized creature provides evidence that early bird evolution was much more complex than scientists once believed. Instead of evolving primarily to become better flyers, ancient birds were adapting to a variety of environments and ways of life, including those that favored living on the ground.
Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS) and the Fujian Institute of Geological Sciences uncovered the fossil in the recently recognized Zhenghe Fauna of Fujian Province in southeastern China. According to their findings, Yuezhuornis represents the earliest known example of a bird lineage losing its ability to fly.
An Ancient Bird With Dinosaur Teeth and Claws
One of the most remarkable features of Yuezhuornis is its unusual combination of bird and dinosaur characteristics. Although it possessed skeletal features associated with birds, it also retained anatomical traits that would have been familiar among small predatory dinosaurs.
Its jaws contained closely spaced, serrated teeth resembling those of deinonychosaurs, a group of meat-eating dinosaurs that included agile predators related to birds. Such teeth are not normally associated with early birds.
Even more striking was its second toe, which could bend backward unusually far and carried a curved, sickle-shaped claw. This specialized feature is strongly associated with deinonychosaurs, including dromaeosaurids and troodontids, two groups of predatory dinosaurs.
“When we examined Yuezhuornis, we were struck by its extraordinary mix of features,” said Dr. Min Wang, corresponding author of the study. “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.”
Where Does This Strange Jurassic Bird Fit in Evolution?
To understand how Yuezhuornis was related to other prehistoric birds and dinosaurs, scientists carried out detailed analyses of its evolutionary relationships. They used two approaches, maximum parsimony and Bayesian tip-dating, to compare anatomical characteristics and reconstruct the species’ position on the evolutionary tree.
Maximum parsimony identifies evolutionary relationships that require the fewest changes in characteristics, while Bayesian tip-dating also incorporates statistical estimates of evolutionary history and information about when fossils lived.
Both methods produced consistent results, placing Yuezhuornis within Avialae, the evolutionary group that includes modern birds and their closest extinct relatives.
“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 Dr. Zhonghe Zhou, a co-author of the study.
This placement is particularly important because Archaeopteryx, which also lived roughly 150 million years ago, is among the most famous early bird relatives. The findings indicate that Yuezhuornis belonged to an early branch of bird evolution rather than simply being a small predatory dinosaur with bird-like characteristics.
Evidence That Some Early Birds Lost the Ability to Fly
The researchers also examined how Yuezhuornis might have lived. Using measurements of its skeletal proportions and statistical comparisons with other animals, they investigated whether its anatomy was better suited for flying or moving along the ground.
The results showed that its limb proportions closely resembled those of modern ground-dwelling birds and certain other prehistoric dinosaurs closely related to birds. This similarity suggests that Yuezhuornis spent its life on the ground and was likely unable to fly.
The findings point to an unexpected development in the history of birds. Rather than simply progressing toward stronger wings and more efficient flight, some early bird lineages appear to have moved in the opposite direction, adapting to lifestyles in which flying was no longer necessary.
This challenges the traditional assumption that the earliest stages of bird evolution were driven primarily by the demands of flight. Instead, birds were already exploring different ecological roles and developing a surprisingly wide range of body forms.
A 150-Million-Year-Old Discovery Rewrites Bird Evolution
The discovery of Yuezhuornis suggests that the evolution of birds was never a simple, one-directional process. Even during the Late Jurassic, when the earliest bird lineages were emerging, some species were developing adaptations that favored life on land rather than in the air.
Its unusual combination of bird-like anatomy, dinosaur-style predatory features, and apparent flightlessness provides a revealing glimpse into that evolutionary experimentation. The fossil shows that early birds were more anatomically and ecologically diverse than previously recognized.
It also underscores the scientific importance of the Zhenghe Fauna, a fossil assemblage that is helping researchers investigate a poorly understood period in bird evolution. As additional discoveries emerge from this region of China, scientists may gain an even clearer picture of the surprising evolutionary paths taken by the earliest birds.
