Jurassic avialan reveals stepwise evolution of bony tail in birds

The evolutionary assembly of the flight-adapted bird body plan encompasses some of the most profound morphological changes in terrestrial vertebrate history. Beyond feathered wings, the short pygostyle-bearing tail has been pivotal to the clade's ecological success. However, transition from the long bony tail to the short pygostyle-bearing tail remains a mystery, hindered by the scarcity of early branching avialans with transitional morphologies. Here, we report on a new avialan, Zhengheornis buyu, gen. et sp. nov., from the Upper Jurassic of southeastern China, suggesting that the vertebral reduction and shortening preceded pygostyle fusion in early avialan evolution, providing critical evidence for the stepwise evolution of the bird tail. Z. buyu is smaller than all known non-pygostylian paravians, expanding the species and body size diversity of stemward taxa.