A 148-million-year-old Jurassic fossil discovered in China may have solved one of evolution's biggest mysteries about how birds evolved from dinosaurs

How did the long bony tail of a dinosaur become the short, compact tail of a modern bird? It is one of the more stubborn open questions in palaeontology, partly because fossils showing the in-between stages have been almost impossible to find. A new discovery from China may have just changed that.

Researchers have described a previously unknown bird species from the Late Jurassic period, named Zhengheornis buyu, whose fossil was recovered from Fujian Province in southeastern China. The findings were published in the journal Science Advances on 1 July 2026.

The specimen is estimated to be around 148 million years old.


What makes this fossil different

What sets Zhengheornis buyu apart from other ancient birds is its tail. The fossil preserves 15 short caudal vertebrae but no pygostyle, the fused bone structure found at the base of a modern bird's tail that anchors its tail feathers and supports flight.

Most early bird relatives known to science fall into one of two categories: those with long tails carrying more than 20 vertebrae, like Archaeopteryx, and those with a fully fused pygostyle. Until now, no fossil had been found that clearly sat between those two conditions.

Zhengheornis buyu does exactly that. Its tail is shorter than those of other long-tailed bird relatives but the vertebrae at the end have not fused into a pygostyle.

"This fills a morphological gap between the ancestral long bony-tailed avialans and the derived short-tailed pygostylians," the research team wrote.

What the tail tells us

The finding supports the idea that bird tail evolution happened in stages rather than all at once.

Previously, some researchers had argued that the pygostyle formed quickly through a small number of genetic mutations, which would explain why intermediate forms had never been found. The discovery of Zhengheornis buyu challenges that view directly.

According to the study, the evidence now suggests that the number and length of tail vertebrae reduced first, and the fusion of those bones into a pygostyle came later, as a separate step.

The researchers also noted that the tail of Zhengheornis buyu is proportionally shorter than its hindlimb, a distinction from most other long-tailed bird relatives where the tail is longer than the leg. The terminal two vertebrae are described as box-like rather than splint-shaped, another feature not seen in similar animals at the same evolutionary stage.

A notably small animal

Beyond its tail, the fossil is notable for its size. The researchers describe Zhengheornis buyu as the smallest known adult individual among non-pygostylian theropods, the broad group that includes the earliest birds and their closest dinosaur relatives.

Its femur is shorter than that of the Chicago specimen of Archaeopteryx, which had previously been described as the smallest long-tailed avialan on record. The team estimates the animal's body mass at somewhere between 74 and 163 grams.

The small body size is considered significant because miniaturisation in the lineage leading to birds is thought to have played a role in the development of powered flight and in opening up new ecological niches.

Where the fossil was found

The specimen, catalogued as IVPP V34168, was recovered during fieldwork in 2024 near Yangyuan Village in Zhenghe County, Fujian Province. It is now held at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences in Beijing.

It is the fourth avialan species identified from the Late Jurassic Zhenghe Fauna, a site that has already produced Fujianvenator, Baminornis, and a third specimen represented only by a furcula. The researchers say the four species together show that ancient birds were already considerably varied in body shape, size, and likely behaviour by the end of the Jurassic period, roughly 145 million years ago.

The study was led by Min Wang and Zhonghe Zhou and involved researchers from multiple institutions in China. It was supported by the National Natural Science Foundation of China and several other funding bodies.