Reorganization of the theropod wrist preceded the origin of avian flight.

The carpus (wrist) of birds has a complex evolutionary history, long known to involve carpal reduction and recently shown to include topological replacement of one carpal (the ulnare) by another (the pisiform). The pisiform plays a crucial role in stabilization of the distal wingtip during flight, and facilitates kinematic integration that 'automates' wing motion. The apparent absence of a pisiform in all but the earliest theropod dinosaurs led to the proposal that it was lost early in theropod evolution and regained only in birds as a key step in the origin of flight. Here, we describe the forelimb skeletons of two newly prepared theropod dinosaur specimens from the Gobi Desert of Mongolia, each of which preserves a pisiform, establishing its presence in Oviraptorosauria and Troodontidae in addition to birds. Reinterpretation of published material in light of these specimens shows a pisiform in a wide range of theropod species, including Microraptor, Ambopteryx and Anchiornis. Our results indicate that the pisiform replaced the ulnare by origin of the clade Pennaraptora, phylogenetically coincident with the hypothesized origin(s) of flight in birds and their closest relatives. Taken together, our results indicate that replacement of the ulnare by the pisiform was a terminal step in assembly of the dinosaurian flight apparatus that occurred close to the origins of flight in theropod dinosaurs, rather than a novelty restricted to birds.