The wing of Archaeopteryx as a primary thrust generator

Since the late 1800s, the debate on the origin of flight in birds has centred around two antagonistic theories: the arboreal (take-off from trees) and cursorial (take-off from running) models(1-6) Despite broad acceptance of the idea that birds evolved from bipedal and predominantly terrestrial maniraptoriform dinosaurs(1.7) the cursorial model of flight origins has been less successful than the arboreal model. Three issues have contributed to this lack of success: the gap between the estimated maximum running speed of Archaeopteryx (2 metres per second) and its estimated minimum flying speed (6 metres per second)(8); the high energy demands of evolving flight against gravity(2,3); and the problem of explaining the origin of the 'flight' stroke in an earthbound organism(3,4). Here we analyse the take-off run of Archaeopteryx through lift-off from an aerodynamic perspective, and emphasize the importance of combining functional and aerodynamic considerations with those of phylogeny(1,9,10). Our calculations provide a solution to the 'velocity gap' problem and shed light on how a running Archaeopteryx (or its cursorial maniraptoriform ancestors) could have achieved the velocity necessary to become airborne by flapping feathered wings.