One of the biggest changes during the evolution from dinosaurs to birds was the shortening of the tail. Modern birds have only four to nine tail vertebrae and a pygostyle, a fused structure that supports the tail feathers and helps improve flight.

For decades, many scientists believed there was no intermediate stage between long-tailed early birds and modern birds with a pygostyle. The fossil of Zhengheornis buyu challenges that view.

The Jurassic bird had only 15 tail vertebrae, far fewer than the more than 23 found in other long-tailed early birds. However, its tail vertebrae had not yet fused into a pygostyle. Comparisons with other fossils also showed it had the shortest tail, relative to its body size, of any known long-tailed avialan.

Researchers say the discovery shows that birds first reduced both the number and length of their tail vertebrae before the fused tail bone evolved. The findings suggest tail evolution happened gradually rather than through a single anatomical change.