How did dinosaurs' long tails shrink to a sparrow's stub? A fossil found in China holds the answer

Modern birds such as sparrows and pigeons have extremely short tail bones. At the tip sits a small bony knob from which tail feathers fan out like a folded hand.

Early birds — the ancient relatives of dinosaurs — were a different story, carrying long, lizard-like bony tails. How and when that tail shrank to its modern form has long puzzled scientists. Now a fossil unearthed in China may finally provide the answer.

A research team led by Dr. Wang Min and Dr. Zhou Zhonghe of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, along with Dr. Tang Jianrong of the Fujian Institute of Geological Sciences, published their findings in Volume 12, Issue 27 of the international journal Science Advances.

Birds descended from dinosaurs, most of which had long, bony tails extending behind their bodies. Today's birds have replaced that structure with a small fused bone at the tail's end — several short vertebrae merged into one.

Scientists call this fused structure the pygostyle. It allows tail feathers to spread and fold like a fan, playing a critical role in flight.

The precise steps of this transformation, however, remained unclear. The fossil record showed primitive birds with long tails and birds already possessing a short fused pygostyle appearing at roughly the same time and in the same region.

Yet no fossil had ever turned up showing an intermediate stage — one where the number of tail vertebrae had decreased but the bones had not yet fused. Some researchers had therefore speculated that the tail shortened abruptly, with no transitional phase.

The research team excavated a bird fossil from rock strata dating back approximately 148 million to 150 million years, found near a village in Zhenghe County, Fujian Province, China.

The specimen came out with most of the torso, hind limbs and wing bones still attached, and even preserved traces of feathers. The team named the new species Zhengheornis buyu.

Zhenghe refers to the county where the fossil was found, while buyu is a classical Chinese term meaning "unexpected" — a nod to the bird's surprisingly unusual tail and pelvis structure.

The creature is estimated to have weighed between 74 and 163 grams, slightly larger than a sparrow, making it one of the smallest birds yet discovered from this period.

The most striking feature of the fossil was its tail. It had only 15 tail vertebrae — fewer than half the count seen in other birds from around the same time. Archaeopteryx, long regarded as the earliest known bird, is known to have had 23 to 24 tail vertebrae.

Even more remarkable was the tail's tip. The last two vertebrae of Zhengheornis buyu had become blunt and box-like, in sharp contrast to the elongated, rod-like vertebrae seen in other long-tailed birds of the era. Crucially, the two bones remained separate — they had not yet fused into one.

The fossil thus shows that the reduction in vertebrae count and the shortening and blunting of individual bones came first, while the fusion of those bones into a pygostyle followed later. It is precisely the intermediate stage that had never been found before.

Based on this fossil, the research team said the evolution of the bird's tail was not a sudden event triggered by a handful of genetic changes, but a gradual process unfolding in multiple stages.

First, the number of tail vertebrae dropped from around 30 to 15. Then the individual bones grew shorter. Finally, the bones at the tail's tip fused together to form the pygostyle. Zhengheornis buyu, the team said, is a fossil frozen at exactly that middle point.

"Contrary to the long-held hypothesis that the transition to a short tail occurred abruptly with no intermediate stage, this fossil shows that such an intermediate stage did in fact exist," the team said.

The team added that multiple early bird fossils from the same region, including Zhengheornis buyu, "support the view that birds had already diversified considerably in body size and morphology by the close of the Jurassic period."

DOI: 10.1126/sciadv.aeb5202

Min Wang et al., Jurassic avialan reveals stepwise evolution of bony tail in birds. Sci. Adv. 12, eaeb5202 (2026).