An article published in the journal “Science Advances” reports the identification of a bird that lived approximately 150 million years ago, during the Jurassic period, and shows a step in the evolution from the tail type typical of dinosaurs to that of birds, equipped with a so-called pygostyle. A team of researchers led by Wang Min of the Chinese Academy of Sciences, who named it Zhengheornis buyu, studied a fossil discovered in 2024 that has a tail with 15 vertebrae, while early bird relatives had tails with over 30 vertebrae.
The image (Courtesy Min Wang, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences. All rights reserved) shows a photo and a drawing of the holotype of Zhengheornis buyu.
Previously, bird fossils had shown species with both long and short tails that appeared around the same time. This had led to the hypothesis of a quick evolution up to the fusion of the last caudal vertebrae into the bone called the pygostyle. The newly discovered species indicates that there was at least one intermediate stage, without the pygostyle. These changes are important in the transition from dinosaurs to birds because the pygostyle represents an advantage in flight.
The authors of this study examined the fossil discovered in the Nanyuan Formation. This is an important site for paleontological studies focused on the evolution of birds because the species Zhengheornis buyu is not the only one identified thanks to fossils discovered there.
The species Zhengheornis buyu is important because its tail has 15 vertebrae. For comparison, a species that has become famous in studies of the evolution of dinosaurs to birds, such as Archaeopteryx, had a tail with 23 or 24 vertebrae and also lived about 150 million years ago. Other species related to primitive birds had tails with more than 30 vertebrae.
In flight, the weight of a large tail is a disadvantage, so a long tail becomes a burden. The pygostyle is lightweight, providing a solid bony base for the tail feathers that act as a rudder. Muscles develop around it, allowing for more maneuvers, and also better transferring the power of the wing muscles.
A tail with 15 vertebrae, like the one found in the species Zhengheornis buyu, offers less aerodynamic control, less thrust, and less lift than a pygostyle. However, it represents an advantage over the tail of Archaeopteryx and even more so over that of various winged dinosaurs and primitive birds.
Discoveries of winged dinosaurs and primitive birds in recent decades, especially in China, are increasingly revealing a series of adaptations for flight. They’re also showing that a taxonomic separation between dinosaurs and birds makes little sense. In the case of Zhengheornis buyu, the study authors included it in the Avialae group, which is precisely a group of dinosaurs that includes their only surviving descendants, birds. This is now a consolidated practice in the world of paleontology, and each new discovery adds a piece to the evolutionary history of birds.
