Theropod dinosaurs had the pisiform, a small carpal bone in birds that plays a critical role in flight, researchers find

A group of researchers led by a comparative biologist from Stony Brook University has made an important discovery that may change the timeline of evolutionary traits in dinosaurs.

James Napoli, a vertebrate paleontologist and evolutionary biologist, found theropod dinosaurs had the pisiform, a small carpal bone that — while found in all limbed vertebrates — has a unique positioning in birds and plays a critical role in flight.

Theropods are one of the three major dinosaur groupings; the T. rex, velociraptor and allosaurus all belong to the group.

Birds also are theropods, the only major group of dinosaur still alive.

Napoli said  the discovery, published in the journal Nature, fits into an emerging pattern found in theropod dinosaurs.

Bird traits that early anatomists and evolutionary biologists thought evolved specifically as adaptations for flight, such as hollow air-filed bones, feathers and wishbones, "actually appeared much earlier in theropod evolution and started appearing in nonflying theropod dinosaurs." Napoli said in an interview on July 16.  

"And now what we've seen is that this pisiform, which is a very characteristic feature of the bird wing, is also something that adopted its present day bird position in theropod dinosaurs as well," he said.

In birds, the pisiform moves out of its original position and "occupies a unique place within the wrist that gives it some special properties," Napoli said.

His team determined  this also was true for theropod dinosaurs, despite the most recent previous study on the pisiform concluding it was not present in dinosaurs.

"The prior study had looked at a bunch of fossils and didn't find [a pisiform], and so they concluded that dinosaurs must have lost it and that their bird descendants re-evolved, having a pisiform form and moved it into a birdlike position in the wrist at about the same time," Napoli said. Their research showed it was "probably never really lost" and" moved into a birdlike position much earlier," he said.

Small bone

Since the pisiform is a small bone, it is easy to lose in preservation, which is probably why it is not found in many fossils, leading scientists to believe the dinosaurs did not have the bone, Napoli said. 

The discovery was made in late 2021 after the team examined fossils collected in the early 1990s and contained in a field jacket, which is used to transport them safely.

The fossil’s wrist was articulated, with all the carpals in place. Napoli immediately recognized a pisiform in a birdlike position, which was "the first time it was identified in a non-bird dinosaur," he said.

At first he was worried the bone was actually just a fragment of another bone that had been pushed out of position, or he was missing part of the three-dimensional story, so after extracting the bones from the rock they were in, they used a micro CAT scanner, which scans at a very high resolution and allowed them to visualize the whole wrist in 3D.

"That allowed us to determine for sure that the pisiform that I identified was not a fragment of another bone and had not been pushed out of position," he said.

Yale doctoral candidate Alex Ruebenstahl helped to discover the pisiform in the other specimen used for the study. He said he came across it while segmenting out the forearm of a troodontid, a theropod dinosaur. 

"And so I was like, oh, there's this bone I don't really recognize here in the wrist that doesn't look like it's supposed to be there," he said.

After contacting Napoli, they realized they now had two cases of the pisiform in the birdlike position.

"It wasn't really planned, it's just a fun, happy coincidence of science, which is exciting," Ruebenstahl said.

Similar to feathers, which "had been assumed to have evolved for flight but then were found later to kind of show up in a more simple form earlier on," Ruebenstahl said the pisiform found was not as complex as the one seen in living birds.

"It's simpler, but it's also present earlier than we expected, so birds were kind of coming preloaded to flight with all the features they needed and weren't just throwing themselves off the trees," he said.

The next steps will be to do more research into what the pisiform does in modern birds, Napoli said, "and to really understand its functions using new technologies we have in the field of biomechanics."