RE: Neoflightless Theropods and Roadrunners

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Kris (MariusRomanus@aol.com) wrote:
 
> I was thinking about the recent roadrunner posts..... Very cool stuff by
> the way..... But, I have a little bit of doubt about all of what this
> could be implying. An animal that is a flyer and already uses wings to
> change directions in the air would easily utilizes this same behavior
> when evolving a cursorial way of life. {snip} What I'm
> saying is that I don't really see a connection between the raodrunner
> and a non-avian theropod when it comes to the use of the arms....

BTW, the idea that feathered forelimbs were initially employed for stability
during running turns, before being co-opted for ground-to-air aerial forays
(and then on to powered flight), goes back to Peterson (1985), I believe.

The issue you raise is whether or not the way a roadrunner uses its wings
while running is a reversion to (or a recapitulation of) the incipient
pre-flight behavior of avian ancestors.  In the same vein is Greg Paul's
suggestion that the weak gliding-flight of kakapo parrots (which is derived
for Neornithes, which began fully-flighted) might be similar to the
primitive flight capabilities of early birds.  I think both scenarios are
entirely credible - and I have seen no data which directly refutes them.

> If the non-avian theropod had ancestors that once flew.... those
> that already used their arms when it came to directional changes... then
> we could have a winner for what it being implied as with the roadrunner.
> It makes little sense to me when looking at it any other way.

There are two issues here.  (1) In order to exclude the possibility that
primitive flightless theropods DID NOT use feathered forelimbs to assist in
cursorial turning motions, you have to demonstrate that this behavior is
neither biomechanically or biophysically feasible.  I think the roadrunner's
behavior undermines this assertion.  However, actually demonstrating that
feathered non-avian theropods exhibited this behavior is probably
impossible.  But demonstrating that they DID NOT is just as impossible - and
for the same reasons...  We can't observe these extinct critters in motion,
and such behavior doesn't necessarily require specific and unique
adaptations (in the skeleton or feathers) that would indicate that such
behavior was expressed.  This is the caveat attached to most of the
hypothetical functions proposed for feathered forelimbs in flightless
dinosaurs (leaping, insect-trapping, gliding, grooming, swimming[!]).  

(2) Can this behavior be proposed as an intermediate behavior for the origin
of powered flight in birds?  It can, but providing evidence in order to
advance it as a credible scenario is a little trickier.  Like most
hypotheses in the origin of avian flight, and the attendant premise for the
initial enlargement of feathers along the forelimb (leaping,
insect-trapping, gliding, grooming, swimming), they are principally
"just-so" stories, with the available evidence marshalled in support of the
given hypothesis.  This doesn't invalidate such hypotheses necessarily -
it's just a recognition of what we're up against when building hypotheses on
the basis of fossil specimens.  In other words, we are never going to KNOW
how flight evolved.
 


Tim


----------------------------------------------------------------- 




Timothy J. Williams, Ph.D. 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 9359