For many years the identity of among the carpal bones in the wrist of birds was a clinical secret, up until scientists showed it was the pisiform.

This bone was initially a sesamoid bone like a kneecap and had actually moved from its initial position in the wrist to change the ulnare, another carpal bone.

Its position in contemporary birds appears to develop linkages that permit birds to fold their wing immediately when the elbow bends.

The bone’s shape, with a big V-shaped notch, likewise enables the pisiform of birds to grip their hand bones to stop them from dislocating throughout flight.

This bone is an essential part of a bird’s forelimb and vital to flight.

“The pisiform, in living birds, is an uncommon wrist bone because it at first forms within a muscle tendon, as do bones like your kneecap– however it concerns inhabit the position of a ‘typical’ wrist bone called the ulnare,” stated Dr. Bhart-Anjan Bhullar, a paleontologist at Yale University.

“Because it is so totally related to arm musculature, its incorporation into the wrist connects the muscular flight equipment to wrist movement.”

“This combination is especially essential for supporting the wing throughout flight.”

In their brand-new research study, Dr. Bhullar and associates evaluated 2 Late Cretaceous fossils: an unnamed types of troodontid (bird-like raptor associated to Velociraptorand Citipati cf. osmolskae (oviraptorid with a long neck and a toothless beak).

“We were lucky to have 2 immaculately maintained theropod wrists for this,” stated Yale University paleontologist Alex Ruebenstahl.

“Wrist bones are little and even when they are protected, they are not in the positions they would inhabit in life, having actually moved throughout decay and conservation.”

“Seeing this little bone in the best position split it large open and assisted us translate the wrists of fossils we had on hand and other fossils explained in the past.”

“We think this is the very first time a moved pisiform in a non-bird meat-eating dinosaur has actually been determined,” stated Dr. James Napoli, a vertebrate paleontologist and evolutionary biologist at Stony Brook University

“While we presently do not understand exactly the number of times dinosaurs discovered to fly, it is appealing that experimentation with flight in these animals appears just after the pisiform moved into the wrist joint.”

“Therefore, it is possible this developed the automatic systems discovered in present living birds, though we would require to evaluate this hypothesis with more research study and analysis of dinosaur wrist bones.”

Putting their findings in evolutionary context, the authors identified that the pisiform moved into its bird-like position not within birds however by the origin of Pennaraptora, a group of theropod dinosaurs that consists of dromaeosaurids like Velociraptortroodontids and oviraptorosaurs.

In general, this is the group of dinosaurs in which bird-like characteristics such as feathered wings started to appear, and in which flight developed a minimum of two times, and perhaps approximately 5 times.

“The topological and practical replacement of the ulnare by the pisiform happened much deeper in theropod history than has actually been formerly comprehended and was a step-by-step procedure,” the scientists stated.

“Over the previous couple of years, our understanding of theropod dinosaur anatomy and advancement has actually increased greatly, much of it exposing that classically ‘bird’ characteristics such as thin-walled bones, a bigger brain, and plumes, all define more inclusive groups of theropod dinosaurs.”

“Our outcomes recommend that the building of the bird wrist is no exception and follows topological patterns put down by the origin of Pennaraptora.”

The group’s paper was released July 9, 2025 in the journal Nature

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J.G. Napoli et alReorganization of the theropod wrist preceded the origin of bird flight. Naturereleased online July 9, 2025; doi: 10.1038/ s41586-025-09232-3