Research Highlight - Ancient Dinosaur Wrist Bones Reveal Evolutionary Leap Toward Avian Flight

A newly published study in Nature reveals that a crucial transformation in wrist anatomy took place among theropod dinosaurs before the emergence of birds, reshaping our understanding of how flight evolved.

Fig 1. From YaleNews; Credit: Henry S. Sharpe/University of Alberta

Led by James Napoli and colleagues, the research reports the discovery of a small, once-overlooked wrist bone, the pisiform, in two exceptionally preserved theropod fossils from Mongolia: one an oviraptorosaur (Citipati osmolskae), the other a troodontid. Using high-resolution 3D imaging, the team identified these bones in the same anatomical position as in modern birds, suggesting the pisiform replaced another wrist bone, the ulnare, earlier than previously believed.

The pisiform, a sesamoid bone embedded in tendon, plays a vital role in stabilizing the bird wing during flight. It was long thought to be absent in non-avian dinosaurs and to have reappeared uniquely in birds. However, this study shows that the pisiform had already evolved into the ulna's functional and positional replacement in Pennaraptora, a clade that includes birds and their closest feathered relatives.

Fig.2: Evolutionary history of the pisiform in Theropoda.

These findings indicate that one of the last pieces of the avian flight apparatus was in place well before powered flight arose. Although the pisiforms in these fossils are smaller and lack the complex muscle structures seen in modern birds, they mark a significant evolutionary transition.

The study also highlights that birds’ specialized wrists evolved gradually through a mosaic of changes, rather than as a sudden leap. The evolutionary relocation of the pisiform, once considered a hallmark of birds, now appears as a broader innovation among flight-capable theropods, hinting that avian flight may indeed have been “all in the wrist.”

———

Reference

Napoli, J.G., Fabbri, M., Ruebenstahl, A.A. et al. Reorganization of the theropod wrist preceded the origin of avian flight. Nature (2025). https://doi.org/10.1038/s41586-025-09232-3