A 150-million-year-old fossil discovered in southeastern China has revealed new evidence about tail evolution, shedding light on how birds evolved from their dinosaur ancestors.
Researchers say the find supports the idea that bird tails changed gradually over time instead of through a sudden evolutionary shift. The study was published in the peer-reviewed journal Science Advances on July 1.
The fossil belongs to a newly identified bird-like dinosaur named Zhengheornis buyu. Scientists uncovered it in Zhenghe County, Fujian Province, during a field survey led by researchers from the Chinese Academy of Sciences and the Fujian Province Geological Science Research Institute. The discovery fills an important gap in the fossil record of early bird evolution.
Researchers explained that one of the biggest changes during bird evolution involved the transformation of the tail. Modern birds have short tails that end in a fused bone known as the pygostyle. This structure supports tail feathers and muscles, helping birds maintain balance and improve flight performance.
Tail Evolution Process
Most non-avian dinosaurs had long, bony tails made up of many separate vertebrae. Modern birds, in contrast, have only a few tail vertebrae that fuse together to form the pygostyle. This shorter tail reduces weight while providing a stable base for flight feathers.
Until now, scientists believed that long-tailed and short-tailed species appeared almost at the same time. That evidence led many researchers to suggest that the transition occurred quickly, without clear intermediate forms. The newly discovered fossil presents a different picture.
The fossil shows a shortened tail with fewer vertebrae than similar bird-like dinosaurs from the same period. However, it does not yet have a pygostyle, making it an important transitional example. This indicates that tail shortening happened before the vertebrae fused into the modern bird structure.
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Wang Min, the study’s first author and a researcher at the Chinese Academy of Sciences Institute of Vertebrate Paleontology and Paleoanthropology, said the fossil represents a unique tail structure not seen before.
He explained that the reduction in the number of tail vertebrae came before the formation of the pygostyle. According to the researchers, this supports a step-by-step pattern of bird tail evolution.
Unique Fossil Features
Scientists determined that the fossil belonged to a subadult or an adult animal, with no signs of skeletal deformities. The specimen has 15 short tail vertebrae, placing it between long-tailed dinosaurs and later birds with fused tails. This intermediate anatomy makes the fossil especially valuable for evolutionary research.
For comparison, Archaeopteryx, one of the earliest known bird-like animals, had about 23 tail vertebrae. Some other dinosaurs possessed around 30 vertebrae in their tails. The gradual reduction seen in Zhengheornis buyu highlights a clear evolutionary transition.
Researchers believe the shorter tail may have improved flight by reducing body weight and increasing tail stiffness. A more rigid tail helps control movement during gliding and flying. These advantages may have played an important role in the early birds’ adaptation to life in the air.
The fossil itself measures only about 20 centimeters in length. Scientists estimate that the animal weighed between 74 and 163 grams, making it the smallest known adult non-pygostylian theropod discovered so far. Its small size suggests that some early bird relatives became much smaller earlier than previously thought.
Wider Evolution Impact
Despite its lighter body and shorter tail, the fossil lacks specialized features associated with advanced flight or efficient ground locomotion. Researchers believe the species occupied an intermediate stage between terrestrial dinosaurs and highly adapted flying birds. This makes it a valuable example of evolutionary experimentation.
The researchers noted that Zhengheornis buyu is the fourth avialan discovered within the Late Jurassic Zhenghe Fauna. Avialans include modern birds and their closest extinct relatives. Together, these fossils show that bird relatives had already become diverse around 150 million years ago.
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The discovery also highlights the growing importance of Fujian Province in the study of prehistoric life. Several significant dinosaur and early bird fossils have already been recovered from the region. Each new discovery helps scientists build a clearer picture of how birds gradually evolved from dinosaurs.
The findings also demonstrate how rare transitional fossils can reshape long-standing scientific ideas. Instead of a rapid change, the evidence points to a gradual sequence of evolutionary steps that produced the efficient tail structure seen in birds today.
As more fossils emerge from China and other regions, researchers expect to uncover additional details about the origins of powered flight and the remarkable evolutionary journey from dinosaurs to modern birds.
