Evolutionary biologists have long known that birds evolved from dinosaurs, but lacked a detailed understanding of the process. Data from a recent study sheds light on how dinosaur arms gradually evolved into wings over many millennia.
The new study, published in PLOS Biology on September 30th, elucidates the process of how dinosaur wrists evolved from straight to bendable over several million years, giving birds the ability to fold their wings in when not flying. This evolutionary process cut the number of wrist bones down from nine in dinosaurs to four in most birds.
More proof of the power of natural selection
This study demonstrates that the evolutionary process of dinosaur wrist into bird wing included the rarely seen disappearance and reappearance of a bone.
The pisiform, a small bone that helps keep a wing rigid on the upward movement of a wing flap, was present in flying dinosaurs, but disappeared as they evolved into the birdlike terrestrial dinosaurs that are modern birds’ closest ancestors. However, a few million years later, early birds eventually evolved the pisiform bone again as an adaption to flight.
“It is rare,” researcher Alexander Vargas, who leads the ontology and phylogeny lab at the University of Chile in Santiago, told Live Science in an interview this week. “This idea that a bone can disappear and reappear in evolution has been resisted a lot in evolutionary biology.”
Among the few known cases of a bone disappearing and reappearing, the bird pisiform is one of the clearest examples of the phenomenon, Vargas noted.
New dinosaur study took modern interdisciplinary approach
Vargas and his colleagues decided to take an interdisciplinary approach that involved both paleontology and developmental biology research on dinosaur and bird wrists/wings. The researchers studied museum collections, examining fossils of birdlike dinosaurs. Traditional examinations of modern bird anatomy were also undertaken.
This study also used a new technique to trace the prenatal development of the wrists of modern birds. The researchers traced proteins related to the formation of collagen, the protein that makes up connective tissue. This enabled them to develop a more clear understanding of the stages of development. The new protein-tracking method permitted then to determine when a bone originated as two separate components that fused together, versus bones that formed as a single unit.