An analysis of two theropod dinosaur fossils has revealed the presence of a carpal bone in their wrists, which is considered important for flight in birds. This discovery was made by a team led by James Napoli from the Department of Anatomical Sciences at Stony Brook University’s Renaissance School of Medicine. The research, published in Nature, challenges previous findings that suggested theropods did not possess a bird-like carpal bone known as the pisiform.
The identity of one carpal bone in bird wrists had been a mystery until researchers identified it as the pisiform. Originally a sesamoid bone similar to a kneecap, it moved to replace another carpal bone called the ulnare. In modern birds, this positioning helps them fold their wings automatically when the elbow flexes and prevents dislocation during flight.
The study analyzed fossils from two types of dinosaurs: a troodontid, related to Velociraptor, and an oviraptorid, an omnivore with birdlike features. The identification was possible due to exceptional fossil preservation and high-resolution CT scanning techniques used to digitally isolate wrist bones. These specimens were provided through collaboration with the American Museum of Natural History and the Mongolian Academy of Sciences.
The 3D visualizations showed that tiny carpals are migrated pisiforms — marking the first identification in non-bird dinosaurs captured during their evolutionary transition.
“We believe this is the first time a migrated pisiform in a non-bird meat-eating dinosaur has been identified,” said Napoli. He further explained that while it’s unclear how many times dinosaurs learned to fly, experimentation with flight appears only after pisiform migration into the wrist joint.
The findings suggest that these changes occurred not within birds but at the origin of Pennaraptora — theropod dinosaurs including dromaeosaurids like Velociraptor, troodontids, and oviraptorosaurs. This group saw bird-like traits such as feathered wings appear and flight evolve multiple times.
Napoli and his co-authors concluded that “the topological and functional replacement of the ulnare by the pisiform occurred much deeper in theropod history than has been previously understood.” They noted that our understanding of theropod anatomy has increased significantly over recent decades, revealing many ‘avian’ traits across more inclusive groups within these dinosaurs.
