Do you regularly wonder how to carry the weight of your body with just your arms, while having them flapping frantically to stay in the air? No ? We probably underestimate the muscular strength and anatomical prowess that is necessary for the flapping flight accomplished by birds. The birth of their wing was until recently very mysterious, but this origin has just been clarified thanks to exceptional fossils!
The great anatomical innovation that most characterizes birds is undoubtedly their wing, a modified and highly specialized chiridian forelimb. Yes, if we are interested in the bones of the wing of birds, or that of bats, we realize that they are in fact extremely modified arms! They are said to be homologous structures because they have a common origin. But how might such a modification have appeared, and when? This is what the authors of a study published in the journal Zoological Letters.
Understand the structure of the wing
Modern birds are mostly able to move through the air thanks to a so-called flapping flight, unlike gliding which does not require the same morphology, even if any animal capable of the first can do the second. , but not vice versa. Because to flap your wings, you need a solid specialized musculature. In birds, one of the structures without which it would be impossible for them to perform this active flight is the propatagium. The word patagium simply designates the skin membrane that gives shape to the wings of bats, or pterosaurs, it serves to provide a lift surface to the wing. Birds have more feathers to fulfill this role, but they also have a little patagium, especially between their shoulder and their wrist. The propatagium contains a muscle, the propatagialis, unique among vertebrates, which helps birds to flap their wings.
It is not very impressive, so thin and sinewy that we often consider it to be a tendon, so why be interested in it? We observe in current birds that no longer fly, such as ostriches or penguins, the loss or reduction of the propatagialis. This is used to extend the wing. When the elbow opens, this muscle automatically causes the simultaneous opening of its wrist, and facilitates the control of the flapping of the wing. To better understand the origin of bird flight, the authors of the study therefore tried to identify the moment in the evolutionary history of this group when the propatagialis appeared by looking at the fossils.
Interpret bird fossils
This was no small feat, as soft tissue is very rarely preserved during fossilization. To circumvent this problem, the researchers had a clever idea, they were interested in fossils of skeletons in connection, where the bones are still articulated as during the life of the animal, and observed their elbow. This is because the propatagialis, which attaches to the wrist, prevents birds from fully extending their elbow joint. The scientists were able to demonstrate that the opening angle of the elbow was therefore a good indicator of the presence or absence of the propatagium. While studying articulated fossils of very ancient birds and their close cousins, they made a startling discovery.
Small reminder: birds are classified among the dinosaurs, they are even theropod dinosaurs, such as tyrannosaurs, therizinosaurs or the famous Velociraptor ! This one, like modern birds, is classified among the smaller group of Maniraptorians, and the study shows that the latter probably all had a propatagium. It is therefore non-avian cousins older than birds that have seen this structure appear, and therefore the wings themselves. It is now a question of what was the use of this structure by the basal Maniraptorians who presumably did not fly. The authors do not think that the initial reason was adaptation to flight, since their front paw was much better able to grasp than to fly!
