Jurassic fossil fills gap in evolution of bird tails

A 150-million-year-old fossil unearthed in Fujian province has revealed what researchers say is the smallest known long-tailed bird from the Late Jurassic period, filling a critical gap in scientists' understanding of how dinosaurs' long tails evolved into the short, compact tails of modern birds.

The findings show that the number of caudal vertebrae decreased before the remaining bones fused into a single structure — the fused pygostyle typical of modern birds — helping resolve a long-standing debate in evolutionary biology. The study was published in the journal Science Advances on Thursday.

The fossil was discovered on March 24, 2024, by a joint research team from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and the Fujian geological science research institute. The discovery followed over 400 days of field work at the Zhenghe Fauna site, a fossil-rich area in Zhenghe county, Nanping, Fujian province, known for yielding well-preserved ancient animal fossils.

Researchers named the new species Zhengheornis buyu. The first part of the name refers to the discovery site, while buyu is derived from Guoyu, an ancient Chinese book, and means "unexpected" — a tribute to the specimen's surprising tail anatomy.

That unexpected anatomy addresses one of the most enduring paleontological questions: how ground-dwelling, long-tailed dinosaurs evolved into today's lightweight, short-tailed birds. While the origin of feathered wings is well documented, the evolution of short bird tails has long remained a mystery because relevant fossils are incredibly rare.

Zhou Zhonghe, an author of the study and a professor at the Institute of Vertebrate Paleontology and Paleoanthropology, said genetic studies have long suggested that tail shortening could result from just a few simple genetic mutations.

"Because long-tailed and short-tailed birds appear almost simultaneously in the fossil record, without intermediate forms, many evolutionary biologists argued that a transitional species having a shortened, but no fused pygostyle was biologically unlikely and probably never existed," Zhou said.

Previous fossil evidence appeared to support that view. The earliest birds with modern, fused pygostyles emerged abruptly near the end of the Jurassic period, at the dawn of bird evolution, while other contemporary birds and their dinosaur relatives kept their long tails.

However, the discovery of Zhengheornis buyu challenges the long-held assumption by providing the long-sought transitional form.

Dating between 148 and 150 million years old, the well-preserved, fully connected skeleton has a tail composed of only 15 caudal vertebrae and lacks a fused modern pygostyle. That is significantly fewer caudal vertebrae than in other early birds, such as Archaeopteryx, which had 23 to 24, Jeholornis, which had 22 to 27, and other early bird-like dinosaurs, which typically had more than 30.

"This mix of ancient and advanced features proves a step-by-step evolutionary path: the loss of caudal vertebrae and overall tail shortening happened before the remaining bones fused together in early bird evolution," said Wang Min, the study's lead author and a professor at the institute.

The researchers said the shortened but flexible tail likely offered important functional advantages. By reducing body weight, shifting the center of gravity forward, and making the tail less rigid, Zhengheornis buyu could move its tail feathers more effectively. This would have improved its flight stability and control compared with its longer-tailed contemporaries like Archaeopteryx.

The species also sets a new record as the smallest known member of the broader dinosaur-bird family tree that includes birds and closely related bird-like dinosaurs. Using size-scaling equations, the researchers estimated that the animal weighed only 74 to 163 grams in life. Its thigh bone measured just 63 percent of the length of that of Microraptor zhaoianus, previously considered the smallest known birdlike dinosaur, and was about 10 percent smaller than the smallest known Archaeopteryx specimen.

The team said such extreme miniaturization suggests that early bird-like dinosaurs shrank in size much more rapidly than previously believed.

Zhengheornis buyu is the fourth ancient bird species reported from the Zhenghe site. Family tree analyses indicate that the bird was not specially adapted for a life spent purely on the ground or purely in trees, suggesting it was an ecological generalist capable of thriving in various environments. The coexistence of multiple, vastly different body shapes also indicates that birds had already split into many diverse species by the end of the Jurassic period.

"This landmark discovery not only settles long-standing academic debates regarding when early birds began to diversify but also adds a crucial piece to the puzzle of how dinosaurs transformed into birds," Wang said.

limenghan@chinadaily.com.cn