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 radiation^1,2, a profound scarcity of fossils from this period has hampered how the distinctive bird bauplan was assembled^3-5. Selection for powered flight has been viewed as the primary driver of early avialan skeletal modifications^3,6,7, 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.