Tiny 149-Million-Year-Old Bird Rewrites the Story of Bird Evolution

Chinese researchers have identified the smallest known long-tailed bird, shedding new light on how birds evolved from their dinosaur ancestors nearly 150 million years ago.

The newly described species, Zhengheornis buyu, lived about 149 million years ago during the Late Jurassic period. Researchers say the fossil provides the strongest evidence yet that early birds began shortening their tails before evolving the fused tail bone, known as the pygostyle, found in modern birds.

The study was led by Wang Min of the Institute of Vertebrate Paleontology and Paleoanthropology under the Chinese Academy of Sciences, in collaboration with the Fujian Institute of Geological Sciences. The findings were published in the journal Science Advances.

A fossil that reshapes bird evolution

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.

Chinese researchers have discovered the smallest known long-tailed bird, Zhengheornis buyu, from 149 million years ago. The fossil shows birds began shortening their tails before evolving the fused tail bone seen in modern birds. pic.twitter.com/fK8suIZxE2

— Tom Marvolo Riddle (@tom_riddle2025) July 2, 2026

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.

Shorter tails may have improved early flight

A shorter but unfused tail would have made the body lighter and shifted its center of mass forward, improving flight. Fewer tail joints also made the tail stiffer, allowing better control in the air.

At the same time, the shorter tail enabled birds to move their tail feathers more efficiently, giving them greater stability and agility than earlier birds such as Archaeopteryx, one of the oldest known birds.

Tiny bird points to rapid diversification

Researchers estimate Zhengheornis buyu weighed between 74 and 163 grams and measured about 20 centimeters (8 inches) in length, making it the smallest long-tailed bird known to science. Its thigh bone was only 63% as long as that of M. zhaoianus, previously considered one of the smallest known dinosaurs. It was also about 10% smaller than the recently described Chicago specimen of Archaeopteryx.

The team says its small size suggests some bird-line dinosaurs shrank much faster than previously believed. That rapid reduction may have helped them adapt to new habitats, including life in trees and powered flight.

Earliest-known species in bird lineage

Researchers also identified Zhengheornis buyu as the fourth known species from this early bird lineage. Unlike some of its contemporaries, it showed no clear adaptations for either ground-dwelling or tree-dwelling lifestyles. Instead, it occupied its own ecological niche despite living alongside other early birds in the same region.

The researchers say the wide variety of body forms seen among these Jurassic birds indicates that birds had already undergone a major adaptive radiation by the Late Jurassic, rapidly diversifying into a range of ecological roles long before modern birds emerged.

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