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RE: The ground-nowhere hypothesis on the origin of bird flight
This is extraordinarily interesting. And now that Greg Paul's secondarily
flightless maniraptor hypothesis has proven correct (Now? There was ALWAYS good
supporting evidence for it, it's just that most everybody refused to except
it), it is really starting to become clear that the origin of bird flight is
far more complicated than we ever thought.
Can't wait for the paper, awesome stuff.
> Date: Thu, 8 Oct 2009 12:58:07 +0200
> From: firstname.lastname@example.org
> To: email@example.com
> Subject: 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 outdated. It is outdated as a dichotomy -- the number of hypotheses is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
> The notorious hypertrophied “killing claw” 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 “stability flapping” 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.
> 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.
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