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Re: Do we have dromaeosaurid evolution backward?

Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> WAIR is possible with anteroposterior flapping (modern birds do this, and 
> kinematically possible by non-ornithothoracine
> maniraptorans), whereas flapping flight requires dorsoventral flapping. Thus, 
> you can do WAIR without a full "above the shoulder"
> excursion of the humerus.
> (You--well, birds--can *also* WAIR using the full dorsoventral flap.)
> Also, WAIR has the advantage as a selective regime for the origin of flight 
> because it can work incremental: it isn't all "all or
> nothing" situation, but rather any slight change in feather size, power to 
> the forelimbs, etc. directly benefits to the angle of
> ascent possible. That is to say, a non-ornithothoracine need not be as good 
> at WAIR as any living bird but still get a selective
> advantage in its use. AND that advantage literally puts it in a situation 
> were selective pressures for other uses of pennate
> feathers and aerial locomotion are possible.

That's the case for the affirmative.  :-)  However... doubts have been
raised about WAIR as a plausible transitional stage for the evolution
of flapping flight.  For example: Nudds RL, Dyke GJ. (2009) Forelimb
posture in dinosaurs and the evolution of the avian flapping
flight-stroke. Evolution. 63: 994-1002.

Although the "proto-wings" of bird ancestors might well have been
quite similar to the incipient wings of modern bird chicks that use
wing-assisted-incline-running (WAIR), the wing movements required for
WAIR are actually quite sophisticated.  There are doubts over whether
the primordial flight apparatus of bird ancestors was up to the task
of performing WAIR.  Whereas the feathers of basal paravians are quite
advanced by avian standards, the skeletal (and by inference, muscular)
adaptations of these basal paravians were comparatively poorly

WAIR detractor's point out that the issue is not just the excursion of
the wing - i,.e. how far up, down, back, or forward the wing can be
moved - but the speed and complexity of the wing movements needed for
WAIR.  The bird that has been used as a model species for
demonstrating WAIR behavior is the chukar partridge.  Juveniles have
incipient wings that are capable of WAIR but not flapping flight.
These terrestrial birds employ flapping motions of their incipient
wings with symmetrical feathers to enhance hindlimb traction to
facilitate running up inclines.  However, partridges are flying birds,
and are therefore equipped with a thoroughly modern musculoskeletal
flight apparatus; even chicks can achieve a high wing-beat frequency
and wing rotation required for WAIR.

So although WAIR proposes an adaptive explanation for the development
of a "proto-wing", concerns have been raised over whether the
wingstroke kinematics (such as power and speed) were sufficiently
advanced in avian ancestors to perform WAIR.  The WAIR wingstroke can
be viewed more as a modification of the wingstroke for flapping flight
rather than as an incipient version.

Further, one of the criticisms of the alternative hypothesis that
flapping flight evolved from a gliding ancestor is that there are no
intermediates between gliders and flappers in the modern world,
despite gliding having evolved many times.  However, modern gliding
mammals tend to subsist on energy-poor diets - and flapping costs
energy.  In these forms, gliding is a time-saving measure, allowing
access to poor-quality food sources over a wide area.  But the first
bats appear to have been insectivorous, and there is no reason to
assume that the ancestors of birds had low calorific diets either.  So
the use of extant gliding mammals as analogs for pro-avians has its