Questions and answers about the Archaeopteryx study published in Nature Communications What are the conclusions of the study? Why are they important? Do they inform on modern birds? Our study concludes that Archaeopteryx was indeed volant and that its wing bone geometry is most consistent with short bouts of active flight. Earlier studies have proposed, suggested, and even assumed volancy in Archaeopteryx through circumstantial evidence, but direct evidence was thus far lacking. We bring clear independent evidence into the discussion and demonstrate that dinosaurian flight had already evolved by the latest Jurassic, which not only offers new insight into the locomotion of Archaeopteryx but also establishes the oldest active dinosaurian volancy. This implies that the search for the first free flying dinosaurs, which will add to our understanding of the very origin of dinosaurian flight, should focus on fossils older than Archaeopteryx. Our conclusions do not directly inform on modern avian flight, but do illuminate on the early precursors of bird flight. Is this the first time that researchers found evidence that Archaeopteryx was an active flyer? To our knowledge, we are the first to provide strong and direct comparative evidence that Archaeopteryx was an active flyer. It is common knowledge that modern birds descended from extinct dinosaurs. The dino-bird Archaeopteryx is one such extinct animal that has fascinated researchers since its discovery 150 years ago, as it was pivotal in the realisation that birds are, in fact, dinosaurs. However, research methods have, to this point, been unable to conclusively establish whether it actually flew or not. Our study reveals that Archaeopteryx’ wing bone architecture is most consistent with what we see in modern birds that incidentally undertake active flight, such as the pheasant. 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. What does “active flyer” mean? Active flight implies that an animal is capable of powered ascent, which can be followed by gliding phases, horizontal flight, or more aerobatic manoeuvres. This contrasts passive gliding, in which an animal moves up onto a perch by walking or climbing and then either parachutes (near-vertical falling while air-braking, resulting in a safe landing) or glides (controlled descent with limited directional control towards a landing site) down to the ground or a lower perch. Soaring birds involve an advanced form of passive flight in their aerial locomotion, but are importantly also capable of flapping flight, rendering them active flyers as well. What can we learn from this study about its flight? Can we compare it to some modern birds’ flight? Do we know how far it could fly? Since Archaeopteryx is extinct, we cannot directly observe its flight in person. Modern birds use active flight through a wing-beat cycle in which the roughly downwards-oriented powerstroke is alternated with a more or less upwards-oriented recovery stroke. Archaeopteryx was likely incapable of this fashion of flapping, so we suggest that it may have moved its wings more foreward and up, followed by a downwards backstroke. Such a motion is intermediate between the grabbing movement of earthbound dinosaurs related to Archaeopteryx and the flight stroke executed by modern birds. However, more research is needed to determine the exact mode of flight that Archaeopteryx had adopted. Reconstructing flight distance not only involves the question whether or not an animal was capable of independent lift-off, but also relies on its metabolic and muscle performance. It has been proposed that Archaeopteryx’ may have flown distances between 20 and 1500 m. Nevertheless, that study assumed a particular reptilian metabolic regime, which has not yet been conclusively confirmed for Archaeopteryx. Future studies focusing on such aspects, but also on aerodynamics, may provide more information to that effect. How did you proceed to analyse the fossils of Archaeopteryx? In order to shed more light on the employment of its feathered wings, we sought independent indicators that would have recorded rather than (potentially) enabled flight. Limb bones evolve to cope with the stress regime they experience as a function of strength and weight and are even capable, to a certain extent, to adapt to the forces they are subjected to during life. These effects are strongest in the middle of the bone where its diameter is smallest. Because Archaeopteryx fossils are very valuable, physical interference, such as cutting into a fossil bone to reveal its interior, is highly discouraged. Luckily, the European Synchrotron Radiation Facility in Grenoble is the foremost facility for visualising obscured properties of fossils without causing damage. We were therefore able to use microtomography with powerful synchrotron X-ray radiation to reveal the cross-sectional geometry of the humerus and ulna in three specimens of Archaeopteryx without harming the fossils. We conservatively restored the bones and studied, quantified, and subsequently compared these valuable data against equivalent comparative data of other archosaurs. The reason we looked at humeri and ulnae is that these bones reflect the clearest flight-related signal in birds. How did the study of Archaeopteryx’ wing bones allow for scientists to conclude that Archaeopteryx was an active flyer? Two parameters defining the cross sections of the humerus and ulna were quantified, namely the amount of bone present relative to the cross-sectional area of the complete bone and the mass- corrected resistance to torsional forces. These data were subjected to statistical comparison against numerous other archosaurs (the group containing crocodiles, pterosaurs, and dinosaurs including birds), which revealed that the degree of “hollowness” of Archaeopteryx’ bones is exclusively shared with archosaurs that flew or fly. We also found that relatively low torsional resistance characterises birds that use incidental flapping flight whereas higher torsional resistance is associated with avian prolonged active flight and endured soaring. The low torsional resistance of Archaeopteryx’ wing bones closely allies it with burst flyers, such as pheasants and roadrunners. This led us to the conclusion that Archaeopteryx must also have actively, albeit occasionally, used its wings to take flight. What were the environment and lifestyle of Archaeopteryx, and which dinosaurs lived around it? All fossils of Archaeopteryx have been found in marine sediments that were deposited in a sea in- between reefs and islands. However, it is highly unlikely that Archaeopteryx lived in the water itself, and its fossils are found alongside not only marine fossils but also those of land-living plants, insects, lizards, and small carnivorous dinosaurs such as Compsognathus, Sciurumimus, and Juravenator. Based on these fossils and the reconstructed Late Jurassic geography of Bavaria, the environment of Archaeopteryx is believed to have been formed by semi-open shrubland not too far from the seashore and possibly extending onto the islands in the “Solnhofen lagoon”. Importantly, Archaeopteryx also lived alongside a variety of flying pterosaurs. Finally, a very recent study has concluded that one of the fossils previously assigned to Archaeopteryx actually represents a different family of bird-like dinosaurs, and was named Ostromia. Does this study provide new insights into the environment and lifestyle of the Archaeopteryx ? The capability of active flight allows for a refinement of the ecology of Archaeopteryx. Its habitat is believed to have been formed by semi-open shrubland nearby or on islands in the Jurassic Solnhofen lagoon (situated in modern-day Germany) based on geological information and associated fossils. Earlier studies have proposed that flight could, in theory, have enabled migration between islands, for example when foraging. The capacity of active flight indeed brings interinsular movements within the realm of possibility. Can we say that the Archaeopteryx was the “ancestor” of modern birds? The “world’s oldest bird”? “The first bird”? Until the end of the 20th century, Archaeopteryx was thought of as the world’s oldest bird. In more recent years, the title of “first bird” has shifted to the last common (yet thus far undiscovered) ancestor of modern birds. The class of birds still includes many extinct forms but excludes, for example, all toothed representatives of the larger group earlier assigned to birds. 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”. All birds therefore belong to Avialae, but not all avialans are considered birds anymore. When did Archaeopteryx live? All unequivocal Archaeopteryx fossils data back to the Late Jurassic and have been encountered in circa 150 million-year-old limestones that crop out within the German state of Bavaria. What did we already know about Archaeopteryx? Archaeopteryx has been studied for over 150 years now and is therefore actually a relatively well- studied fossil animal. The external morphology of its skeleton