Dinosaur fossils containing arms with suspected flexion at the elbow and wrist may point to an unpreserved notochord underlying all modern bird flight.
If the University of Tokyo researchers are correct, this situation could provide clues to the path that flying vertebrates from Earth took to the skies.
The evolution of wings powerful enough to lift vertebrates off the ground is one of the great mysteries of paleontology.
Pterosaurs are famous for being the oldest known vertebrates, actually appearing around 200 million years ago. However, these massive ancient reptiles were not dinosaurs, so the direct ancestors of birds independently discovered all the mechanisms of flight.
Avian dinosaurs evolved much later than bipedal, feathered theropods, about 80 million years after pterosaurs had already achieved powerful flight.
Despite these vastly different origin stories, birds use a strikingly similar structure to pterosaurs to stay aloft and, like feathers, seem to have evolved long before flight itself.
Called the propatagium, this is a membrane present in all modern wing-flapping vertebrates, including birds and bats. Some gliding mammals even have a similar structure on their parachute-like upper limbs.
The best way to visualize the propatagium is to take the arm out to the side with the elbow and wrist bent. Now imagine a tendon running from your shoulder to your arm, forming the bridge or “front edge” of the wing.
This “bridge” allows flying birds to flex and extend the wrist and elbow simultaneously during the flapping motion. The skeleton, in fact, gives power to bird flight, allowing the animal to control two joints at once.
For pterosaurs, its role is less clear, but the propatagium appears to have controlled takeoff and landing by changing the airflow over the upper wing surface.
Some scientists believe that without the chord, birds, bats, and dinosaurs would not have been able to fly so high.
“It has not been found in other vertebrates,” explains paleontologist Tatsuya Hirasawa of the University of Tokyo. “It has also been found to have disappeared or lost its function in flightless birds, which is one of the reasons we know it is essential for flight.” So to understand how it evolved in birds, we have to know how propatagium evolved.”
The problem is that propatagium is soft tissue, which means it is rarely preserved in the fossil record. In addition, this tendon is very thin and does not leave marks on the bones to which it is attached.
Luckily, Hirasawa and his colleague Yurika Ono found a way to “see” the tendon even if it wasn’t there. The key is to learn how the propatagium restricts the animal’s movements.
For example, when a modern bird dies, this membrane naturally keeps the animal’s wrist and elbow flexed.
By comparing the apparent angle of the elbow to the curvature of the arms in non-avian theropod fossils, the researchers found evidence that a propatagium-like structure could extend over the shoulder and wrist of many land-dwelling dinosaurs.
For example, the angles seen in the fossils of many maniraptors (including velociraptors) were slightly larger than those of modern birds, but still hint at a similar structure to the early propatagium.
To support these predictions, the researchers also found soft tissue remains of what may have been an early propatagium in two maniraptoran fossils: a turkey-sized caudipteryx and a four-winged microraptor.
Caudipteryx probably couldn’t fly, and there is still controversy over whether a microraptor could. What is clear, however, is that both of these dinosaurs possessed the structures that later became necessary for flight.
The study is published in the journal Zoological Letters.
Source: Science Alert