All modern, flying birds rely on a distinctive wing feature called the propagium, a structure essential for taking off. The evolutionary origin of this organ remains a topic of debate, yet recent analyses suggest it evolved from non-avian dinosaurs. By statistically examining the joints in fossil limbs, researchers have gained new insight into how bird flight began and how these ancient limbs transitioned toward powered flight.
The consensus has long been that modern birds descended from specific dinosaur lineages that lived millions of years ago. This lineage connection helps explain shared traits such as feathers and bone configuration. Scientists have studied extinct species to illuminate unique features, and the propagium in particular has drawn close attention from researchers at the Department of Earth and Planetary Sciences at the University of Tokyo.
A bird’s wing includes a front-end structure named the propagium. One muscle links the shoulder to the wrist, promoting wing flapping and enabling flight, as explained by Tatsuya Hirasawa, an associate professor in the lab.
In a captioned note, findings point to the propagium as a feature not present in other vertebrates and showing loss of function in some flightless birds. This underscores its necessity for flight and frames the question of how dispersal — the evolution of wing-like capabilities — evolved within the dinosaur lineage. The investigation pointed toward theropod dinosaurs as distant ancestors, helping to map the transition from arms to wings in the bird lineage.
Theropod dinosaurs, such as Tyrannosaurus rex and Velociraptor, possessed arms but not fully developed wings. If researchers uncover evidence of early reproduction or development of wing-like structures in these species, it would illuminate how the modern bird branch diverged from the rest of the theropod family.
Difficulty detecting propagium in fossils
The challenge is real because the propagium comprises soft tissues that do not fossilize readily, making direct evidence scarce. To overcome this, scientists pursued indirect methods to infer the presence of the propagium in each specimen.
Yurika Uno, a graduate student in Hirasawa’s lab, explained that data on joint angles along the arm or wing can serve as a proxy for dispersal capability in both dinosaurs and birds. This approach hinges on observing whether a dinosaur’s joints maintained specific angular ranges that would reflect a wing-like setup.
In modern birds, wing movement is constrained by the way joints connect, limiting the possible range of angles. If a similar, tightly defined set of joint angles is found in dinosaur specimens, it provides a strong indication that those dinosaurs possessed a dispersal-ready limb configuration. Through quantitative analyses of fossil postures in birds and non-bird dinosaurs, researchers identified distinct angle ranges consistent with a propagium-like arrangement.
Building on this clue, the study suggests that the propagium likely evolved within a subgroup of dinosaurs known as maniraptorans, which includes Velociraptor. The discovery gains support from preserved soft-tissue fossils showing feathered limbs, including species such as Caudipteryx and a winged relative known as a dromaeosaurid, all existing before the emergence of true flight.
These findings push the timeline of propagium emergence earlier and prompt new questions about why these theropods needed such a structure to adapt to their environments. Could it be that a wing-like apparatus offered advantages beyond flight, or did flightlessness in certain lineages only later erase those traits from the record?
Researchers are pursuing connections between fossil evidence and the embryonic development of modern vertebrates to shed light on these questions. They also consider whether some theropods evolved dispersal features without a direct pressure to fly, perhaps to handle grasping or other environmental challenges with forelimbs built for manipulation rather than flight.
According to Hirasawa, the portrayal of dinosaurs in the public eye is becoming more accurate, and these findings hint at future representations that show theropods bearing propagia-like structures as part of their anatomy.
Reference work: https://zoologicalletters.biomedcentral.com/articles/10.1186/s40851-023-00204-x#Fig1
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