All modern birds capable of flight have in common a specialized wing structure called the propagation without which they could not do it. The evolutionary origin of this structure remains a mystery, however new research now suggests that it evolved in non-avian dinosaurs. The finding was made possible by exhaustive statistical analysis of arm joints preserved in numerous fossils of flying and terrestrial birds, as well as dinosaurs, and helps fill some gaps in knowledge about the origin of bird flight.
From dinosaurs to birds
We have known for a long time that modern birds evolved from certain lineages of dinosaurs that lived millions of years ago. This has led researchers to turn their gaze towards dinosaurs to explain some of the unique features of birds and birds, for example, their feathers or their bone structure. But there’s something special about bird wings that piqued the interest of researchers at the University of Tokyo’s Department of Earth and Planetary Sciences.
“At the leading edge of a bird’s wing we find a structure called a propagium which houses a muscle that connects the shoulder and wrist that helps the wing flutter and makes flight possible for the bird,” explains associate professor Tatsuya Hirasawamain author of an article under the title Origin of the propatagium in non-avian dinosaurs It was recently published in the specialized magazine Zoological Letters. “This structure is not found in other vertebrates, and was also found to have disappeared or lost its function in flightless birds, one of the reasons we know it is essential for flight. So, to understand how flight evolved in birds, we must know how flight evolved. propagation. and eThis is what prompted us to explore some distant ancestors of modern birds, the theropod dinosaurs.”
He propagium is found only in birds, and has also lost its function in flightless birds.
The problem is that theropod dinosaurs like Tyrannosaurus rex or the Velociraptorthey had arms, not wings, therefore, finding evidence of propagation in these dinosaurs would help explain how modern birds made the transition from arms to wings. Hirasawa’s team’s initial approach was simple, however, propagation is made up of soft tissues that do not fossilize well, if at all, so as research has shown, it is highly unlikely to find direct evidence of this. structure in the fossil record.
Yurika Uno / Tatsuya Hirasawa
This schematic shows how the researchers believe propagation enabled different types of movement as the former evolved from theropods to birds, leading to the evolution of wings.
To save this stumbling block, the researchers had to find an indirect way to identify the presence or lack of propagation in a specimen. “The solution we came up with to assess the presence of a propagation was to collect data on the joint angles along the arm or wing of a dinosaur or bird,” explains Yurika Uno, graduate student in Hirasawa’s lab. “In modern birds, the wings cannot fully extend due to propagation, which restricts the range of possible angles between connecting sections.”
Pterosaurs: the path to winged perfection
“If we could find a similar specific set of angles between joints in dinosaur specimens, we could be pretty sure they also possessed propagation. And through quantitative analyzes of fossilized bird and dinosaur postures, we found the joint angles that we expected”.
maniraptors and the origin of wings
Following up on this clue the team discovered that propagation likely evolved in a group of dinosaurs known as tmaniraptor eropods, among which is the famous Velociraptor, something that seems supported by the presence of propagation in specimens of other maniraptor species such as the feathered oviraptorosaur Caudipteryx or the winged dromaeosaur Microraptor. In fact, youAll the specimens in which scientists found evidence of propagation existed before the evolution of flight in that lineage.
Yurika Uno / Tatsuya Hirasawa
Thus, scientists think they know approximately when propagation arose, which inevitably leads them to new questions: How did it arise? whatWhy did these particular theropod species need such a structure to better adapt to their environment? Hirasawa’s team has already begun to explore possible connections between the fossil evidence and the embryonic development of modern vertebrates to find connections and new clues in this regard.