An analysis of two theropod dinosaur fossils has revealed the presence of a carpal bone in their wrists, previously thought to be unique to birds. This finding challenges earlier research that concluded theropods lacked a bird-like carpal bone, known as the pisiform. The study was led by James Napoli from the Department of Anatomical Sciences at Stony Brook University's Renaissance School of Medicine and published in Nature.
The pisiform bone, initially a sesamoid bone similar to a kneecap, shifted from its original position in the wrist to replace another carpal bone called the ulnare. In modern birds, this positioning helps facilitate wing folding when the elbow flexes and prevents dislocation during flight.
Researchers analyzed fossils from two types of dinosaurs: a troodontid and an oviraptorid. Their identification of the pisiform in these theropods was aided by exceptional fossil preservation and high-resolution CT scanning technology. The specimens were provided through collaboration with the American Museum of Natural History and the Mongolian Academy of Sciences.
The study's 3D visualizations indicated that these carpals are migrated pisiforms — marking their first identification in non-bird dinosaurs — captured during an evolutionary transition. "We believe this is the first time a migrated pisiform in a non-bird meat-eating dinosaur has been identified," said Napoli.
Napoli added that while it remains unclear how many times dinosaurs developed flight capabilities, it's intriguing that such experimentation appears only after the pisiform's migration into the wrist joint. Further research is needed to explore this hypothesis.
In an evolutionary context, researchers determined that the pisiform moved into its bird-like position within Pennaraptora, a group including dromaeosaurids like Velociraptor, troodontids, and oviraptorosaurs. Bird-like traits such as feathered wings emerged within this group where flight evolved multiple times.
Napoli and his co-authors noted that their findings clarify that "the topological and functional replacement of the ulnare by the pisiform occurred much deeper in theropod history than has been previously understood."