[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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



> 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.

But it doesn't explain why they would need feathered hindlimbs, as is apparent 
in that member of the troodon family they just found in the Jurassic.

I suppose mammalian predators (such as a cat) can put most of their weight on 
their forelimbs, and use their hindlimbs to stabilize them, whereas theropods 
couldn't do this.

It could be a reason the larger dromeosaurs retained "wings", but I don't think 
it explains the apparent ancestral state.

--- On Thu, 10/8/09, David Marjanovic <david.marjanovic@gmx.at> wrote:

> From: David Marjanovic <david.marjanovic@gmx.at>
> Subject: The ground-nowhere hypothesis on the origin of bird flight
> To: "DML" <dinosaur@usc.edu>
> Date: Thursday, October 8, 2009, 3:58 AM
> 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 ground-up/trees-down dichotomy is outdated. It is outdated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 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 t
nd 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.
> <<
> 
> 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 cle
l, had wings with big quill knobs
> which indicate that it was necessary to hold the wing
> feathers in place against strong forces.
>