Jurassic flightless bird reveals early ecological diversity of avialans.

The origin and early diversification of birds remain highly debated in vertebrate evolution. Although estimates of the evolutionary timescales suggest a Jurassic period radiation1,2, a profound scarcity of fossils from this period has hampered how the distinctive bird bauplan was assembled3-5. Selection for powered flight has been viewed as the primary driver of early avialan skeletal modifications3,6,7, attesting to the independent occurrences of aerodynamic features among paravian lineages8. Here we report on the discovery of a Late Jurassic avialan, Yuezhuornis yangyuanensis, gen. et sp. nov., from the Zhenghe Fauna of southeastern China9. As one of the earliest diverging avialans, Y. yangyuanensis exhibits a striking mosaic of features, combining a mixture of avialan- and troodontid-like cranial characters, paired with derived postcranial traits and a deinonychosaur-like hyperextensible second pedal digit. Phylogenetically informed analyses show that Y. yangyuanensis possesses markedly short forelimbs-a condition present widely among non-avialan theropods. These findings provide strong evidence for a terrestrial, flightless lifestyle of Y. yangyuanensis. Our study demonstrates that evolution works intensively as exemplified by the occurrence of several, divergent ecological pathways being exploited in the initial diversification of birds.