The evolutionary path from dinosaurs to birds included the development of a tiny wrist bone that ultimately proved crucial for stabilizing wings in flight. A new study suggests that the bone appeared in bird ancestors millions of years earlier than first thought.
Paleontologists at Yale and Stony Brook University led a research team that made the discovery after examining fossils from two species of bird-like dinosaurs — an unnamed troodontid and a Citipati from the Late Cretaceous period 66 to 100 million years ago — found in the Gobi Desert in Mongolia. The findings were published in the journal Nature.
“We were fortunate to have two immaculately preserved theropod wrists for this,” said Alex Ruebenstahl, a student in Yale’s Graduate School of Arts and Sciences and member of the lab of Yale paleontologist and YIBS Faculty Affliate Bhart-Anjan Bhullar. Both Ruebenstahl and Bhullar, as well as Norell, are co-authors of the new study.
“Wrist bones are small and even when they are preserved, they are not in the positions they would occupy in life, having shifted during decay and preservation,” Ruebenstahl said. “Seeing this little bone in the right position cracked it wide open and helped us interpret the wrists of fossils we had on hand and other fossils described in the past.”
The evolution of theropod dinosaurs into birds included significant anatomical modifications, such as the enlargement of the brain, changes in the pelvis and its surrounding musculature — and a transformation of the dinosaur forelimbs.
One of the key changes in the forelimb transformation was the replacement of a particular dinosaur wrist bone — the ulnare — with a bone called the pisiform in birds. In the fossil record, pisiform bones appeared in very early theropods, then disappeared, only to return in birds.
“The pisiform, in living birds, is an unusual wrist bone in that it initially forms within a muscle tendon, as do bones like your kneecap — but it comes to occupy the position of a ‘normal’ wrist bone called the ulnare,” said Bhullar, associate professor of Earth and Planetary Sciences in Yale’s Faculty of Arts and Sciences. “Because it is so intimately associated with arm musculature, its incorporation into the wrist ties the muscular flight machinery to wrist motion. This integration is particularly important for stabilizing the wing during flight.”
“This discovery pulls back the origin of the integrated pisiform on the bird evolutionary lineage by tens of millions of years,” he added.
For more information, click here for an article published by Yale News or here for the full study in Nature.
