Dinosaur Fossils May Reveal the Tendon Behind Modern Avian Flight

Dinosaur fossils with an unusual bend at the elbow and wrist may reveal evidence of a vanished tendon that supports the flight of every modern bird.

Scientists at the University of Tokyo suggest that this limb position may offer fresh insight into the evolutionary route that allowed vertebrates to become airborne.

The propatagium and the origins of flight

How wings became strong enough to raise a vertebrate from the ground remains one of palaeontology's biggest unanswered questions.

Pterosaurs are well known as the first vertebrates understood to have achieved genuine powered flight, almost 200 million years ago. However, these huge prehistoric reptiles were not dinosaurs, meaning birds' direct ancestors had to develop flight independently.

Avian dinosaurs emerged considerably later from bipedal, feathered theropods – at least 80 million years after pterosaurs had already taken to the air under their own power.

Although their evolutionary histories were profoundly different, birds rely on a remarkably similar feature to pterosaurs to remain airborne. Like feathers, it appears to have developed long before flight itself.

Known as the propatagium, this membrane occurs in every living vertebrate that flaps its wings, including birds and bats. Certain gliding mammals also possess a comparable feature spanning their parachute-like forelimbs.

To picture a propatagium, hold your arm out sideways while bending both your elbow and wrist. Then imagine a tendon running from your shoulder to your hand, forming a bridge – or the wing's 'leading edge'.

This 'bridge' enables flying birds to bend and straighten their wrists and elbows together while flapping. In effect, it supplies lift during flight by letting the bird control two joints simultaneously.

Its purpose in pterosaurs is less certain, although the propatagium appears to have influenced take-off and landing by changing airflow across the upper wing surface.

Without this tendon, some scientists believe birds, bats and dinosaurs may never have left the ground.

"It's not found in other vertebrates, and it's also found to have disappeared or lost its function in flightless birds, one of the reasons we know it's essential for flight," explains paleontologist Tatsuya Hirasawa from the University of Tokyo.

"So, in order to understand how flight evolved in birds, we must know how the propatagium evolved."

Dinosaur fossils reveal clues to the propatagium

The difficulty is that the propatagium consists of soft tissue, so it is seldom retained in fossils. In addition, the tendon is extremely thin and leaves little trace on the bones where it connects.

Fortunately, Hirasawa and his colleague Yurika Uno have devised a method for 'seeing' the tendon after it has disappeared. Their approach depends on recognising how the propatagium limits an animal's movement.

For example, after a modern bird dies, the membrane naturally holds its wrist and elbow in a flexed position.

By comparing the distinctive elbow angle in bird remains with the arm bends preserved in non-avian theropod fossils, the researchers uncovered signs that a propatagium-like feature probably extended between the shoulder and wrist in several land-dwelling dinosaurs.

The fossil angles of numerous 'maniraptorans' – a group that includes velociraptors – were slightly wider than those recorded in modern birds. Nevertheless, they still indicated the likely presence of an early propatagium-like structure.

To support these predictions, the team also found soft-tissue traces that could represent an early propatagium in two maniraptoran fossils: the turkey-sized Caudipteryx and the four-winged microraptor.

Maniraptorans, Archaeopteryx and powered flight

Caudipteryx was probably unable to fly, while scientists still dispute whether microraptor could do so. Even so, both dinosaurs evidently had structures that later became essential for powered flight.

Velociraptors were another likely flightless maniraptoran, but more research is required to determine whether their fossils preserve signs of a long-lost propatagium.

On the basis of these recent findings, the University of Tokyo researchers propose that the propatagium may have been ancestral to all maniraptorans, a lineage dating back around 150 million years.

The sole exception is the debated Archaeopteryx. It is difficult to position within the dinosaur family tree and may or may not be classified as a maniraptoran. There is also intense debate over whether the celebrated feathered dinosaur could fly at all, with some suggesting it merely used its limbs to glide.

The wrist and elbow angles in its fossils do not, however, indicate a propatagium-like structure.

"Therefore," the authors of the new research write, "Archaeopteryx was probably incapable of executing the kinematics of modern avian powered flight."

Perhaps, had Archaeopteryx possessed a propatagium, its descendants would still be flying today.

The study was published in Zoological Letters.