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.
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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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The authors included two pterosaurs — one primitive and one more advanced form — in the comparative study. The differences between the wing bone geometry of the primitive and the more advanced pterosaur qualitatively agree with those observed between Archaeopteryx and modern-day birds that spend most of their time in the air.
Although the wings of pterosaurs are quite different to those of flying dinosaurs, which implies a different force regime as well, this does suggest that broadly comparable developments occurred during the evolution of pterosaur flight as in the evolution of dinosaur flight towards the emergence of birds.
With its feathers, wings, and ability to fly — however awkwardly — Archaeopteryx was still unlike any bird alive today. Its teeth, claws, and long dinosaur-like tail gave it a distinct appearance reminiscent of its non-avian dinosaur ancestors.
In the future, the ESRF scientists plan to study the fossils of other animals. Their facility, located in Grenoble Cedex, France, is scheduled for a major upgrade over the coming years. The upgrade will improve the non-destructive visualization of important fossils.
"My main interest lies with the exploration of innovative adaptations of Mesozoic dinosaurs and marine reptiles,” Voeten said, “and it is my ambition to continue my study of vital fossils with novel research techniques towards uncovering more secrets of extinct vertebrate life."