From Dinosaur Arms to Bird Wings, an Evolutionary Twist

Although we can now appreciate that dinosaurs led to the rapid rise of birds over the course of millions of years, one of the last nagging doubts about their link may be settled by a new study that shows how dinosaur arms evolved into bird wings.

The key is all in the wrist. In order for dinosaurs to take flight, their skeletal structure had to be altered. This process involved the rare disappearance and reappearance of a bone, called the pisiform - a crumb of bone that was lost in bird-like dinosaurs, but then re-acquired in the early evolution of birds.

"It is rare," study researcher Alexander Vargas, who leads the ontology and phylogeny lab at the University of Chile in Santiago, told Live Science. "This idea that a bone can disappear and reappear in evolution has been resisted a lot in evolutionary biology."

Skeletal similarities between dinosaurs and birds have long been considered evidence for paleontologists that both species were of the same evolutionary tree. But how exactly straight dinosaur wrists morphed into hyperflexible wrists, to allow birds to fold their wings when not flying, has been a subject of much debate.

The new study, published in the journal PLOS Biology, shows how nine dino wrist bones were reduced over millions of years of evolution to just four wrist bones in modern day birds.

To solve the evolutionary puzzle, Vargas and his colleagues examined fossils stored at several museum collections, and also conducted traditional examinations of modern bird anatomy. Using a technique called immunostaining, the research team could trace certain proteins found inside embryonic cartilage to get a clearer picture of the stages of development.

First of all, they found that the semilunate, a half-moon-shaped bone in birds, is the fusion of two wrist bones from dinosaurs. But more striking than that, was the pisiform bone, a tiny bone that develops inside a wrist tendon. The pisiform was present in early dinosaurs but had disappeared by the time birdlike dinosaurs, known as theropods (the famous Tyrannosaurus rex is one example) came around. However, it was resurrected in today's birds.

"We think the pisiform was lost when dinosaurs became bipedal," Vargas told ABC Science. "Quadrupedal animals used this bone because they walk with their forelimbs, but bipedal dinosaurs no longer walked with their forelimbs and lost the bone. However they regained it when they began using their forelimbs for locomotion in flight."

Vargas and his team now plan on moving on from the wrist to the ankle with their technique.