Modern birds have long been known to have evolved from certain lineages of dinosaurs that lived millions of years ago. Researchers have looked to dinosaurs to explain many of the unique features of birds, including feathers, bone structure, and more. However, the wings of birds have proven to be particularly interesting to researchers at the University of Tokyo’s Department of Earth and Planetary Science.
The propatagium is key
The propatagium is a specialized wing structure found in all modern birds that is essential for their ability to fly. It contains a muscle connecting the shoulder and wrist, which helps the wing flap and generates lift. The propatagium is not found in any other vertebrates and has disappeared or lost its function in flightless birds, demonstrating its importance for flight. As such, understanding the evolutionary origin of the propatagium is crucial for understanding how flight evolved in birds.
Theropod dinosaurs, such as Tyrannosaurus rex and Velociraptor, are distant ancestors of modern birds that had arms instead of wings. Therefore, if scientists could find evidence of an early example of the propatagium in these dinosaurs, it would help explain how the modern avian branch of the tree of life transitioned from arms to wings.
Searching for a needle in a haystack
However, it’s not so simple, as the propatagium is made up of soft tissues that do not fossilize well, if at all, making direct evidence hard to find. As a result, the researchers had to find an indirect way to identify the presence or lack of a propatagium in a specimen.
The solution they developed 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. In modern birds, the wings cannot fully extend due to the propatagium, constraining the range of angles possible between connecting sections. If researchers could find a similarly specific set of angles between joints in dinosaur specimens, they could be sure that they too possessed a propatagium. Through quantitative analyses of fossilized postures of birds and non-dinosaurs, the researchers found the telltale ranges of joint angles they hoped to.
Based on this clue, the team found that the propatagium likely evolved in a group of dinosaurs known as the maniraptoran theropods, which includes the famous Velociraptor. This was backed up when the researchers identified the propatagium in preserved soft tissue fossils, including those of the feathered oviraptorosaurian Caudipteryx and winged dromaeosaurian Microraptor. All the specimens in which the propatagium was found existed before the evolution of flight in that lineage.
Scientists now know when, but why?
This research means that scientists now know when the propatagium came into being. Still, it raises further questions about how it evolved and why these particular theropod species needed such a structure to better adapt to their environment.
The team believes that some theropods might have evolved the propatagium not because of any pressure to learn to fly, as their forelimbs were made for grasping objects, not for flying. The researchers are already exploring possible connections between the fossil evidence and embryonic development of modern vertebrates to see if that sheds any light on subject.
Associate Professor Tatsuya Hirasawa, said:
“Dinosaurs portrayed in popular media are becoming more and more accurate. At least now we get to see features like feathers, but I hope we can see an even more up-to-date representation soon where theropods have their propatagium too.”
Image Credits
In-Article Image Credits
The way arm joints are articulated in fossils gives away the presence or absence of the propatagium via Yurika Uno and Tatsuya Hirasawa with usage type - Creative Commons License. Credit must be given to the creator.Rare examples of soft tissue such as that found in the propatagium via Yurika Uno and Tatsuya Hirasawa with usage type - Creative Commons License. Credit must be given to the creator.
The evolution of birds from dinosaurs illustrated via Yurika Uno and Tatsuya Hirasawa with usage type - Creative Commons License. Credit must be given to the creator.
Propatagium old and new via Yurika Uno and Tatsuya Hirasawa with usage type - Creative Commons License. Credit must be given to the creator.
Featured Image Credit
The evolution of birds from dinosaurs illustrated via Yurika Uno and Tatsuya Hirasawa with usage type - Creative Commons License. Credit must be given to the creator.
