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Archaeopteryx was an active flyer

20 March 2018

Archaeopteryx fossil with parts of the skull, both wings, rib cage and legs preserved.

The question of whether the Late Jurassic dino bird Archaeopteryx was a feathered ground dweller, a glider, or an active flyer has fascinated palaeontologists for decades. An international team of scientists has now used synchrotron microtomography to answer this question without damaging the fossils. The wing bones of Archaeopteryx were shaped for incidental active flight, but not for the advanced style of flying mastered by today’s birds.

Was Archaeopteryx capable of flying, and if so, how? Although it is common knowledge that modern day birds descended from extinct dinosaurs, many questions on their early evolution and the development of avian flight remain unanswered. Traditional research methods have thus far been unable to answer the question whether Archaeopteryx flew or not. Using synchrotron microtomography at the European Synchrotron Radiation Facility (ESRF) to probe inside Archaeopteryx fossils, an international team of scientists from the ESRF, Palacký University, Czech Republic, CNRS and Sorbonne University, France, Uppsala University, Sweden, and Bürgermeister-Müller-Museum Solnhofen, Germany, shed new light on this earliest of birds.

Fossils from southeastern Germany

Reconstructing extinct behaviour poses substantial challenges for palaeontologists, especially when it comes to enigmatic animals such as the famous Archaeopteryx from the Late Jurassic sediments of southeastern Germany that is considered the oldest potentially free-flying dinosaur. This well-preserved fossil taxon shows a mosaic anatomy that illustrates the close family relations between extinct raptorial dinosaurs and living dinosaurs: the birds. Most modern bird skeletons are highly specialised for powered flight, yet many of their characteristic adaptations in particularly the shoulder are absent in the Bavarian fossils of Archaeopteryx. Although its feathered wings resemble those of modern birds, the primitive shoulder structure is incompatible with the modern avian wing beat cycle.

“By statistically comparing the bones of living animals that engage in observable habits with those of cryptic fossils, it is possible to bring new information into an old discussion”, says senior author Dr. Sophie Sanchez, researcher at Uppsala University, Sweden.

Archaeopteryx skeletons are preserved in and on limestone slabs that reveal only part of their morphology. Since these fossils are among the most valuable in the world, invasive probing to reveal obscured or internal structures is therefore highly discouraged. Fortunately, today it is no longer necessary to damage precious fossils. The exceptional sensitivity of X-ray imaging techniques for investigating large specimens that is available at the ESRF offers harmless microscopic insight into fossil bones and allows virtual 3D reconstructions of extraordinary quality.

Illustration of Archaeopteryx
Artists impression of what a flying Archaeopteryx may have looked like. Illustration: Jana Růžičková

Wing bones similar to those in pheasants

Scanning data unexpectedly revealed that the wing bones of Archaeopteryx, contrary to its shoulder girdle, shared important adaptations with those of modern flying birds.

“We immediately noticed that the bone walls of Archaeopteryx were much thinner than those of earthbound dinosaurs but looked a lot like conventional bird bones”, says lead author Dennis Voeten of the ESRF. “Data analysis furthermore demonstrated that the bones of Archaeopteryx plot closest to those of birds like pheasants that occasionally use active flight to cross barriers or dodge predators, but not to those of gliding and soaring forms such as many birds of prey and some seabirds that are optimised for enduring flight.”

The region around Solnhofen in southeastern Germany is known to have been a tropical archipelago, and such an environment appears highly suitable for island hopping or escape flight. Archaeopteryx shared the Jurassic skies with primitive pterosaurs that would ultimately evolve into the gigantic pterosaurs of the Cretaceous. The researchers found similar differences in wing bone geometry between primitive and advanced pterosaurs as those between actively flying and soaring birds.

Insight into early evolution of flight

Since Archaeopteryx represents the oldest known flying member of the avialan lineage that also  includes modern birds, these findings not only illustrate aspects of the lifestyle of Archaeopteryx but also provide insight into the early evolution of dinosaurian flight. “Indeed, we now know that Archaeopteryx was already actively flying around 150 million years ago, which implies that active dinosaurian flight had evolved even earlier!” says Prof. Stanislav Bureš of Palacký University in Olomouc. “However, because Archaeopteryx lacked the pectoral adaptations to fly like modern birds, the way it achieved powered flight must also have been different. We will need to return to the fossils to answer the question on exactly how this Bavarian icon of evolution used its wings”, concludes Voeten.

Full article: Dennis F.A.E. Voeten et al. (2018) Wing bone geometry reveals active flight in Archaeopteryx, Nature Communications, http://dx.doi.org/10.1038/s41467-018-03296-8