What is the evolutionary origin of birds’ wings?

All currently existing species of birds that are capable of flight have a specialized wing structure called the propatagium, without which they could not fly. For a long time, the evolutionary origin of this structure has remained a mystery. Now, by conducting statistical analyses of arm joints preserved in fossils of nonavian dinosaurs, a team of researchers from the University of Tokyo has found that this structure may have first emerged in these prehistoric creatures.

Scientists have long known that modern birds evolved from various lineages of dinosaurs, which had some of the features now unique to birds, such as feathers or particular types of bone structures. The recent discovery of the possible origins of the propatagium sheds new light on the origins of bird flight.

“At the leading edge of a bird’s wing is a structure called the propatagium, which contains a muscle connecting the shoulder and wrist that helps the wing flapping and makes bird flight possible,” said study senior author Tatsuya Hirasawa, an associate professor of Evolution and Vertebrate Paleontology at Tokyo. 

“It’s not found in other vertebrates, and it’s also found to have disappeared or lost its function in flightless birds, one of the reasons we know it’s essential for flight. So, in order to understand how flight evolved in birds, we must know how the propatagium evolved. This is what prompted us to explore some distant ancestors of modern birds, theropod dinosaurs.”

Theropod dinosaurs – such as the well-known Tyrannosaurus rex and Velociraptor – had arms instead of wings and were incapable of flight. Thus, if scientists could find evidence of the existence of the propatagium in these dinosaurs, it would help explain the evolutionary transition from arms to wings. However, since the propatagium consists of soft tissues that don’t fossilize well, scientists needed an indirect method of identifying the presence or lack of this structure in theropods. 

“The solution we came up with to assess the presence of a propatagium was to collect data about the angles of joints along the arm, or wing, of a dinosaur or bird,” said lead author Yurika Uno, a graduate student from Hirasawa’s lab. “In modern birds, the wings cannot fully extend due to the propatagium, constraining the range of angles possible between connecting sections. If we could find a similarly specific set of angles between joints in dinosaur specimens, we can be fairly sure they too possessed a propatagium. And through quantitative analyses of the fossilized postures of birds and nondinosaurs, we found the telltale ranges of joint angles we hoped to.”

This method revealed that the propatagium most likely evolved in a group of dinosaurs called maniraptoran theropods, which included the Velociraptor. This hypothesis was confirmed when the experts identified this structure in preserved soft tissue fossils of the feathered oviraptorosaurian Caudipteryx and winged dromaeosaurian Microraptor. 

Although it is now known when the propatagium emerged, a more difficult question which will need further research has to do with the reasons for its emergence and further development in non-flying prehistoric creatures.

The study is published in the journal Zoological Letters.

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By Andrei Ionescu, Earth.com Staff Writer

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