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An analysis of two theropod dinosaur fossils has shown that they had a type of carpal bone (pisiform) in their wrists鈥攁 bone considered important to flight in birds.

This discovery by a team of researchers led by James Napoli, Ph.D., of Stony Brook University, counters previous research that concluded theropods did not have a bird-like pisiform. Their finding, in Nature, opens the possibility that the evolution of flight in dinosaurs was "all in the wrist."

For years, the identity of one of the carpal bones in the wrist of birds was a scientific mystery, until researchers proved it was the pisiform. This bone was originally a sesamoid bone like a kneecap, and had moved from its original position in the wrist to replace the ulnare, another carpal bone. Its position in modern birds appears to establish linkages that allow birds to fold their wings automatically when the elbow flexes.

The bone's shape鈥攚ith a large V-shaped notch鈥攁lso allows the pisiform of birds to clasp their hand bones to stop them from dislocating during flight. Therefore, this bone is an important part of a bird's forelimb and critical to flight.

The two types of dinosaur fossils analyzed were a troodontid, a birdlike raptor related to the Velociraptor; and an oviraptorid, an odd birdlike omnivore with a long neck and a toothless beak.

The team's identification of the pisiform bone in theropods was possible because of exceptional preservation of the dinosaurs, in combination with the use of high-resolution CT scanning that enabled them to digitally isolate the bones of the wrist. The dinosaur specimens in the analysis were available under a collaborative agreement with the American Museum of Natural History and the Mongolian Academy of Sciences.

The 3D visualizations make it clear that tiny, bead-like carpals are migrated pisiforms鈥攖he first ever identified in non-bird dinosaurs鈥攃aptured in a transitional stage of their evolution.

"We believe this is the first time a migrated pisiform in a non-bird meat-eating dinosaur has been identified," says Napoli, lead author, a vertebrate paleontologist and evolutionary biologist, and Research Instructor in the Department of Anatomical Sciences in the Renaissance School of Medicine at Stony Brook University.

"While we currently do not know precisely how many times dinosaurs learned to fly, it is intriguing that experimentation with flight in these creatures appears only after the pisiform migrated into the wrist joint. Therefore, it is possible this established the automated mechanisms found in current living birds, though we would need to test this hypothesis with more research and analysis of dinosaur wrist bones," he explains.

Putting their findings in evolutionary context, they determined that the pisiform moved into its bird-like position not within birds but by the origin of a group called Pennaraptora鈥攁 group of theropod dinosaurs that includes dromaeosaurids like the Velociraptor, troodontids, and oviraptorosaurs. Overall, this is the group of dinosaurs in which bird-like traits such as feathered wings began to appear, and in which flight evolved at least twice, and possibly up to five times.

Napoli and co-authors write that their results "make clear that the topological and functional replacement of the ulnare by the pisiform occurred much deeper in theropod history than has been previously understood and was a stepwise process鈥�. Over the past few decades, our knowledge of theropod dinosaur anatomy and evolution has increased exponentially, much of it revealing that classically 'avian' traits such as thin-walled bones, an enlarged brain, and feathers, all characterize more inclusive groups of theropod dinosaurs. Our results suggest that the construction of the avian wrist is no exception and follows topological patterns laid down by the origin of Pennaraptora."