How did arms become wings during the evolutionary transition from dinosaurs to birds? Especially the short little useless arms of Tyrannosaurus rex?
A new study published today in PLoS Biology answers the first question and says it’s all about the wrists. Chilean researchers wanted to know how changes in wrist bones transformed from straight to bent and hyperflexible in the appendage, allowing birds to the ability to produce force on the downstroke and tension on the upstroke, as well as to fold their wings neatly against their bodies when not flying. They focused on determining how the nine wrist bones in dinosaur lineages became the four found in present-day birds.
In the past, researchers had studied the two (dinosaur fossils and bird embryos) separately but not together. The Chilean scientists examined fossils of both, each stored at several museum collections, and also gathered new developmental data from the embryos of seven different species of modern birds.
These researchers’ new techniques provide some exciting results. First, they discovered that the semilunate bone (a half-moon shaped wrist bone) in birds was formed by the fusion of the two dinosaur bones. Another bone called the pisiform was lost in bird-like dinosaurs, but then re-acquired in the early evolution of birds, probably as an adaptation for flight. And the other two bird wrist bones were commonly misidentified in both fields and the team has settled on a new identity for each. Voilà! C’est un oiseau.