The ground-nowhere hypothesis on the origin of bird flight

[David Marjanovic <david.marjanovic@gmx.at>]

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outdated. It is outdated as a dichotomy -- the number of hypotheses is 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 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.
<<

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.