Summary
A fossil from southeastern China represents a newly described Late Jurassic avialan with a mix of bird-like and troodontid-like features. Its short forelimbs and comparative anatomy support a terrestrial, flightless lifestyle.
A fossil from southeastern China has revealed a previously undescribed Late Jurassic avialan, Yuezhuornis yangyuanensis. The researchers’ anatomical and comparative analyses indicate that it lived on land and was flightless, adding evidence that early members of the bird lineage occupied more than one ecological niche.
The study, published in Nature on 14 October 2026, places Yuezhuornis among the earliest-diverging avialans. Avialans are members of the bird lineage, including extinct forms beyond the group containing living birds. The finding therefore concerns early bird relatives, rather than a bird species alive today.
A mix of features across the skeleton
The fossil combines characteristics associated with different groups. Its skull has a mixture of avialan-like and troodontid-like traits. The study’s anatomical description also notes serrations along the back edges of the upper-jaw teeth, with the teeth at the front of the upper jaw larger than those farther back.
Other parts of the skeleton show a different combination: derived postcranial traits alongside a second toe capable of marked extension, resembling the condition in deinonychosaurs, a group of theropod dinosaurs. This mosaic makes Yuezhuornis difficult to describe as simply an intermediate version of a familiar modern bird. Its anatomy records a distinctive combination of traits in an early part of avialan evolution.
Why the researchers infer flightlessness
The researchers’ phylogenetic analyses—a way of assessing evolutionary relationships using shared characteristics—found that Yuezhuornis had markedly short forelimbs. Short forelimbs are widespread among non-avialan theropods. The authors interpret the fossil’s limb anatomy, considered in its evolutionary context, as strong evidence that the animal was terrestrial and flightless.
They also compared its proportions with those of living birds. The study’s ecological-shape analysis included 381 independent specimens for comparison, and its supplementary figure describes a resemblance between Yuezhuornis and terrestrial birds in hind-limb proportions. Such comparisons help researchers infer likely modes of life from fossils; they do not provide a direct record of an animal in motion.
The result broadens the picture of early avialan diversification. Powered flight has often been treated as a major influence on changes in early bird-line skeletons. Yuezhuornis instead provides evidence that an early avialan could follow a terrestrial, flightless path while other members of the broader group evolved different combinations of traits. The fossil supports an early diversification with divergent ecological pathways, rather than a single flight-centred pattern.