150-million-year-old fossil sheds light on bird tail evolution

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

"Because long-tailed and short-tailed birds appear almost simultaneously in the early fossil record, without clear intermediates, many evolutionary biologists argued that a transitional species having an abbreviated but entirely unfused bony tail was biologically improbable and unlikely to have ever existed," Zhou said.

Previous fossil evidence appeared to support that view. The earliest birds with a pygostyle emerged abruptly in the Late Jurassic, at the dawn of avian evolution, while other contemporary birds and their dinosaur relatives still retained long tails.

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

Dating to the Tithonian stage of the Late Jurassic, about 148 million to 150 million years ago, the well-preserved articulated skeleton has a tail composed of only 15 shortened vertebrae and lacks a pygostyle. That is significantly fewer than in other long-tailed birds, such as Archaeopteryx, which possessed 23 to 24 caudal vertebrae, Jeholornis, which had 22 to 27, and other early bird-like dinosaurs, which typically exceeded 30.

"This anatomical mosaic proves a stepwise evolutionary path: vertebral reduction and tail shortening preceded pygostyle fusion in early bird evolution," said Wang Min, the study's lead and corresponding author and a professor at the Institute of Vertebrate Paleontology and Paleoanthropology.

The researchers said the shortened but unfused tail likely offered important functional advantages. By reducing body weight, shifting the center of mass forward and increasing tail rigidity through fewer vertebral joints, Zhengheornis buyu could manipulate its tail feathers more effectively, improving flight stability and maneuverability compared with longer-tailed contemporaries such as Archaeopteryx.

The species also sets a new record as the smallest known long-tailed paravian, a group that includes birds, troodontids and dromaeosaurids. Using empirical scaling equations, the researchers estimated that the animal weighed only 74 to 163 grams in life. Its femur measured just 63 percent the length of that of Microraptor zhaoianus, previously considered the smallest known non-avian dinosaur, and was about 10 percent smaller than that of the Chicago specimen of Archaeopteryx, the smallest known individual of the species.

The team said such extreme miniaturization suggests that body-size reduction among early bird-like dinosaurs occurred much more rapidly than previously believed.

Zhengheornis buyu is the fourth avialan reported from the Zhenghe Fauna. Phylogenetic analyses indicate that the bird lacked specialized adaptations for either a strictly ground-dwelling or tree-dwelling lifestyle, suggesting it was an ecological generalist. The coexistence of multiple highly divergent body plans also indicates that birds had already undergone substantial adaptive radiation by the end of the Jurassic.

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