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The ground-nowhere hypothesis on the origin of bird flight



I keep reading on this list that the ground-up/trees-down dichotomy is outdated. It is outdated as a dichotomy -- the number of hypotheses is greater than two. However, reality does not need to lie in the middle. Probably it lies somewhere else entirely.

Denver Fowler (2009): The grasping foot of *Deinonychus*: implications for predator ecology, evolution of the perching foot, and a new hypothesis for the origin of flight in birds, online-only supplement to JVP 29(3), 98A

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The notorious hypertrophied âkilling clawâ on pes digit (D) II of the maniraptoran theropod dinosaur *Deinonychus* was hypothesized by previous workers to have been a predatory adaptation for slashing or climbing. This led to the suggestion that *Deinonychus* and other velociraptorines were cursorial predators specialized for actively attacking and killing prey taxa several times larger than themselves. By making comparisons to modern birds of prey, this study offers a new alternative interpretation: that the enlarged claw of *Deinonychus* was functionally analogous to the enlarged talon also found on D-II of extant Accipitridae (hawks & eagles). Here it is used to maintain grip on prey of subequal body size to the predator, while the victim is pinned down by the body weight of the raptor and dismembered by the beak. Further analysis of predatory behavior and talon function in birds of prey reveals more profound implications. Here I propose a new hypothesis for the origin of avian powered flight: that it was exapted from âstability flappingâ executed for positioning during the initial stages of prey immobilization. This behavior is employed by accipitrids (keeping the raptor on top of its prey, so it is better able to use its body weight for pinning), and supported by the low aspect ratio wings seen in accipitrines (where this behavior is most commonly observed), *Archaeopteryx*, and many non-avian maniraptoran dinosaurs. In this new interpretation, the evolution of the flapping stroke is decoupled from the evolution of powered flight. Selection for more efficient stability flapping provides a viable selection pathway to true powered flight. Phalangeal proportions and elongation of digits (especially D-IV) in the foot of *Deinonychus* are adaptations towards a grasping function, further support for the accipitrid model of prey restraint. Selection for more efficient grasping ability provides a viable selection pathway for gradual reversal of the hallux. Placed in context of the evolution of flight, the grasping foot of Deinonychus and other terrestrial predatory maniraptorans was an exaptation for the grasping foot of arboreal perching birds.
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An important part of the talk was the fact that, while falconids kill their prey by severing the spinal cord (using the "falcon teeth" -- canine-shaped projections of the upper beak), accipitrids don't bother. They just put themselves on top and start eating, flapping vigorously in order to stay on top.

If nothing else, I think this explains why *Velociraptor*, a clearly flightless animal, had wings with big quill knobs which indicate that it was necessary to hold the wing feathers in place against strong forces.