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Re: The ground-nowhere hypothesis on the origin of bird flight


2009/10/8 Michael Erickson <tehdinomahn@live.com>:
> 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.
> ~ Michael
> ----------------------------------------
>> Date: Thu, 8 Oct 2009 12:58:07 +0200
>> From: david.marjanovic@gmx.at
>> To: dinosaur@usc.edu
>> 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 outdated. It is outdated as a dichotomy -- the number of hypotheses is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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