The Feathered Dinosaur Archaeopteryx Could Fly — Just Not Like Today’s Birds

Illustration of Archaeopteryx preying on a dragonfly

Until the end of the 20th century, Archaeopteryx was thought of as the world's first bird. In more recent years, that distinction has shifted to an as-of-yet undiscovered ancestor of modern birds, which lived around 75 million years ago.

The confusion was understandable. The iconic creature of the Late Jurassic 150 million years ago had feathers, wings, and measured 1.7 feet long — a little larger than the length of a green heron.

It could also fly, reports a new study. The findings, reported in the journal Nature Communications, provide the first direct comparative evidence that Archaeopteryx was an active flyer. They also strengthen the belief that this animal was not just a bird, but a dino-bird.

"Archaeopteryx is now considered the oldest free-flying member of the clade Avialae that includes not only modern birds but also all extinct dinosaurs more closely related to the house sparrow than to Deinonychus, the terrestrial hunter that was adopted by the Jurassic Park franchise as a model for their ferocious 'Velociraptor,'" lead author Dennis Voeten told Seeker.

"All birds therefore belong to Avialae, but not all avialans are considered birds anymore," added Voeten, who is a researcher at the European Synchrotron Radiation Facility (ESRF).

Dennis Voeten indicates the bone wall thickness of the “Chicken Wing” specimen of Archaeopteryx on the top computer screen for comparison against the bone walls of a primitive pterosaur on the bottom screen. A three-dimensional model of the “Chicken Wing” is held up, the referred bone cross section is that of the humerus, the uppermost arm bone visible most right on the 3D-printed model.

Earlier speculation that Archaeopteryx was capable of active flight received little support because the dino-bird's skeleton lacks three things that were considered to be essential for a functional avian flight stroke.

"Firstly, everybody who has enjoyed eating a whole chicken or turkey will recognize the flat bone protruding from its rib cage," Voeten said. "This is the keeled breastbone, or sternum, of the bird that supports large flight muscles, which we know as chicken fillet or turkey breast. Nearly all modern flying birds have such a prominent sternum, but this appears to have been absent in Archaeopteryx."

Secondly, the flight stroke of modern birds involves raising the wings well over the backbone during the upstroke, yet the shoulder joint of Archaeopteryx seems not to have allowed for this motion.

Thirdly, most muscles involved in both the upstroke and downstroke of modern birds attach to the keel of the sternum.

"For the downstroke," Voeten said, "this is not extraordinary. When we move our arms together, we use a broadly similar muscle arrangement. For the upstroke, however, birds have a specialized tendon that loops over the shoulder before attaching to the upside of the upper arm bone. This pulley system allows for the quick upstroke of modern birds but was absent in Archaeopteryx."

It therefore seemed at first like Archaeopteryx was incapable of significant flight, except perhaps for gliding.

Unconvinced of this conclusion, Voeten and his team turned the flight question around and focused, not on the skeletal conditions that could have allowed for certain motions, but rather on bones that would have "recorded" volancy.

Even this proved to be challenging because most Archaeopteryx fossils are preserved in and on limestone slabs from the German state of Bavaria. They are among the most valuable in the world. This is due to their state of preservation, age, and relevance to understanding other species. Invasive probing to reveal obscured or internal structures has therefore been discouraged over the years.

"Fortunately, today it is no longer necessary to damage precious fossils," said Paul Tafforeau, who was co-senior author of the paper with Sophie Sanchez. Both are ESRF scientists.

The Munich specimen of the transitional bird Archaeopteryx. It preserves a partial skull (top left), shoulder girdle and both wings slightly raised (most left to center left), the ribcage (center), and the pelvic girdle, and both legs in a “cycling” posture (right); all connected by the vertebral column from the neck (top left, under the skull) to the tip of the tail (most right). Imprints of its wing feathers are visible radiating from below the shoulder and vague imprints of the tail plumage can be recognized extending from the tip of the tail.

The researchers were able to study the fossils non-invasively using a technique called phase-contrast synchrotron microtomography. This method provides high-resolution visualization of 3D structures in objects that are often opaque. The technology is particularly important for paleontology, since the contrast between rock and fossil bone — which has essentially been converted to rock as well during fossilization — is notoriously otherwise low.

The scientists focused on the cross-sectional architecture of Archaeopteryx's limb bones, co-author Jorge Cubo of the Sorbonne University said. He and Voeten explained that limb bones evolve to cope with the stress of an individual's locomotion and provide informative clues on how the animal moved.

The Munich specimen of Archaeopteryx at beamline ID19 at the ESRF. The limestone plate was mounted on a rotating sample stage and the beam is centered on the skull using lasers. The X-ray beam, coming from the right of the picture, travels through the sample and arrives at the detector (visible left) where a camera records the signal.

Through a statistical comparison with a broad selection of archosaurs — the group encompassing crocodiles, pterosaurs, and dinosaurs, including birds — the researchers found that the structure of the studied limb bones is shared exclusively with flying members of this group.

Additionally, they determined that the limb structure allies Archaeopteryx with modern birds that use incidental active flight to evade predators or to cross physical barriers, but that often exhibit a predominantly Earth-bound lifestyle.

The prehistoric dino-bird probably occupied an ecological niche somewhat similar to that of today's pheasants. It also possibly shared some similarities with the present-day secretary bird.

"The secretary bird is an African bird of prey with a predominantly terrestrial lifestyle, although it is much larger and has relatively much longer legs than Archaeopteryx," Voeten said.

Archaeopteryx's mode of flight was not identical to that of such modern birds.

"Archaeopteryx likely required a running start to take flight, whereas birds like pheasants and quails are capable of vertical takeoff,” Voeten explained. “In a scenario where early dinosaurian flight importantly contributes to fleeing behavior, vertical takeoff would bring an animal quicker outside the range of an earthbound predator than a running take-off would."

Based on the ancient dino-bird's shoulder girdle, the researchers believe that approximately the 2.2-pound Archaeopteryx had a flight stroke that was oriented forward and up, followed by a power stroke oriented rearward and down. The motion is midway between the grabbing abilities of small ancestral meat-eating dinosaurs, such as raptors, and the wing beat cycle of modern flying birds.

Once airborne, it is possible that Archaeopteryx could fly from anywhere between 66 feet and a mile. It could also probably fly in quick bursts.

At least one non-avian dinosaur appears to have exhibited a different form of flight. The enigmatic Yi qi, for example, sported bat-like wings that probably allowed it to glide. Yi qi, from China, also lived during the Late Jurassic.

"We feel our study supports that the evolution of dinosaurian flight was not simply a straight line towards the flight of modern birds, but involved an exotic diversity of alternative, experimental, and intermediate solutions that ultimately proved to be evolutionary dead-ends," Voeten said.

He added, "This illustrates that the diversity of locomotor strategies evolved in dinosaurs must have been larger than previously thought."

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