Fossil Bones Reveal How Dinosaur Arms Evolved into Bird Wings

While we all know that birds evolved from a branch of dinosaurs, exactly how they accomplished this feat has long puzzled evolutionary biologists. Now, scientists have taken a closer look at how dinosaur arms evolved into wings over time.

Over the course of millions of years, wrists turned from straight to bend and hyperflexible, allowing birds to fold their wings neatly against their bodies while not flying. Underlying this transformation is a halving in the number of wrist bones. Exactly how this happened, though, has long remained a source of debate.

In order to learn a bit more about it, the researchers re-examined fossils stores in several collections while at the same time collecting new developmental data from seven different species of modern birds. Then, the scientists developed a new method that allowed them to study specific proteins in 3D embryonic skeletons. By combining the data, the researchers managed to take a major step forward in finding out how the avian wrist evolved.

Early dinosaur ancestors had as many as nine wrist bones. Birds, in contrast, have only kept four over the course of evolution. Now the researchers have found that the bird seminlunate, one of the write bones, was created by the fusion of two dinosaur bones. In addition, 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. This boned allowed for transmission of force of the downstroke while restricting flexibility on the upstroke.

The findings reveal a bit more about how evolution occurred in dinosaurs. More specifically, it reveals how birds managed to lose a number of wrist bones while at the same time re-gaining bones in evolutionary reversal.

The findings are published in the journal PLOS Biology.