Some of the First Birds Chose the Ground Over the Sky

When we imagine the first birds, we often picture them on the cusp of flight—feathers stretching, wings testing the air, the sky opening before them. It is a compelling image, and for a long time it shaped how scientists understood the earliest chapters of avian evolution. But a new fossil from China is quietly rewriting that story, revealing that some of the earliest birds had already turned away from the sky and chosen the ground instead.

A team of researchers from the Chinese Academy of Sciences and the Fujian Institute of Geological Survey has described a new species of bird from the Late Jurassic period, approximately 148 to 150 million years ago, found in the Zhenghe Fauna of Fujian Province. Named Yuezhuornis yangyuanensis, the fossil represents the earliest known bird incapable of flight, and shows a remarkable mosaic of dinosaur and bird features . The research was published in the journal Nature .

The new species weighed about 821 grams and displays a highly mosaic pattern of evolution. Its skull has typical features of troodontid dinosaurs, including densely packed premaxillary and maxillary teeth, but its skull bones are slender overall, more similar to primitive birds . Most striking is its foot, which preserves the specialized second toe unique to deinonychosaurs—strongly curved backward for slashing or grasping prey—indicating powerful gripping or running ability .

In the evolutionary transition from dinosaurs to birds, lengthening of the forelimbs is a very obvious trend. However, Yuezhuornis has very short forelimbs. Macroevolutionary calculations show it is the known Mesozoic bird with the relatively shortest forelimbs, even shorter than some running dromaeosaurs . Its postcranial structure has many advanced bird features, including reduced tail vertebrae articulations, an elongated anterior ilium, and a shortened fibula, yet its well-developed pubic symphysis resembles that of troodontids .

The researchers note that the origin and early diversification of birds is inferred to have occurred in the Jurassic, yet fossil records from this critical geological period are extremely scarce . The long-standing view has been that flight was the dominant selective pressure driving early avian morphological evolution. However, the discovery of Yuezhuornis demonstrates that ecological specialization—specifically, a ground-dwelling, flightless lifestyle—appeared at the very beginning of bird evolution .

The species name is derived from Yangyuan Township in Zhenghe County, where the fossil was found, while the genus name comes from the ancient Chinese mythological bird Yuezhuo, mentioned in the Guoyu as a divine creature whose song heralded the rise of the Zhou dynasty . The fossil is the fourth important discovery from the Jurassic Zhenghe Fauna, following Fujianvenator, Baminornis, and Zhengheornis .

Writing in Nature, Xu et al. note that this discovery reveals "unexpected body-form innovations during the early evolution and diversification of bird ancestors" . The finding suggests that the story of early birds is not a single narrative of flight but a branching tree of experimentation, in which some lineages walked while others flew, and the ground was as much a frontier as the sky.

The illustrations in this report are AI-generated and serve only as visual aids.

Sources: Nature, Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology