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

Re: Evidence For a Feathered Velociraptor...



On Tue, Sep 25, 2007 at 05:31:45PM -0500, Tim Williams scripsit:
> Graydon wrote:
> > Quill knobs imply substantial aerodynamic load and structured
> > feathers,
> 
> I can understand why quill knobs imply structured feathers, but not
> aerodynamic load.

Because that's what quill knobs _do_, and they're expensive; growing new
feathers is expensive, and so is growing connective tissue to anchor
those new feathers to bone.  So if the quill knobs are excessive, at
least in context of the peak force experienced, one would expect
(especially in a flightless species) that there would be selection
pressure to remove them as an unnecessary metabolic expense.

> At least not for _Velociraptor_.  Sure, the forelimb feathers might
> have been exposed to certain aerodynamic forces, such as when rapidly
> extended or retracted during a predatory strike; or (even more
> dramatically) if _Velociraptor_ was leaping through the air toward
> large prey.  But it's a tough sell to say that these forces qualify as
> "aerodynamic load". (I'm not suggesting that you are saying this, BTW;
> I'm justing setting the scene.) 

If the feathers are being flapped through the air, at speed, for
whatever reason, they're going to be under aerodynamic load.  It doesn't
matter if it's leaping to claw something's face off or fanning the nest.

> The presence of quill knobs means that the feathers were anchored to
> the ulna with ligaments.  The selective pressure that originally drove
> the evolution of quill knobs might indeed have been aerodynamic load,
> such as WAIR, or phugoid gliding, or primary thrust generation, or
> whatever.  But for _Velociraptor_ (and maybe _Rahonavis_ too) quill
> knobs may have evolved (or been retained) for a non-aerodynamic
> reason.  For example, having the feathers more tightly anchored to the
> ulna would help them stay attached during predatory tussles with large
> prey.

That only works if there _were_ such tussles, and I don't think there
were.  It's a bad idea to wrestle if your advantages are being fast and
spiky and you're smaller than your prey animal.

>  Because _Velociraptor_ has broken the correlation between quill knobs
>  and powered flight,

But I don't think it _is_ a correlation between quill knobs and powered
flight.  I think it's a correlation between quill knobs and aerodynamic
load on retrices.

We know there are other ways to deal with feather control and anchoring,
because there are modern volant species as don't have quill knobs, but
the presence is pretty tightly correlated with keeping anchored against
airflow.

>  we have to come up with other (non-aerodynamic?) explanations for why
>  flightless maniraptorans might have had quill knobs.  Or else, it
>  could be argued that the quill knobs of _Velociraptor_ are just
>  remnants of an ancestry from maniraptorans which were aerodynamic
>  (and maybe powered fliers).

But the quill knobs are expensive, and we know that other linages of
velociraptorin theropods lost them, so some sort of developmental lock
isn't the least hypothesis; the least hypothesis is that they were used
to support retrices under aerodynamic load.

[snip]
> You know, I'm coming around to your way of thinking.  In support of
> your argument, Gishlick (in the Ostrom Symposium volume) noted that
> feathers on the hands would not have intefered too much with
> predation, because the feathers were attached roughly perpendicular to
> the claws, and oriented tangentially to the prey.

I can tell that you, personally, have not been assaulted by geese. :)

> He also suggested that large forelimb feathers might have advantages
> in predation - they could cause the hands and arms to seem larger (and
> prevent small prey from dodging to either side); or for pack-hunters
> this same ploy could be used to "herd" prey.  Of course, both ideas
> are highly speculative.  They would certainly make for interesting
> illustrations.

My personal guesses are intraspecies competition for leap height, and
predation advantage by assisted leaping out of the plane of the ground.

No idea how to test any of that, and as you say, highly speculative.

-- Graydon