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Re: Evidence For a Feathered Velociraptor...



On Mon, Sep 24, 2007 at 09:32:07PM -0500, Tim Williams scripsit: [reply
to Mike Keesey snipped]
> Graydon wrote:
> > Seen that video of an eagle driving off a wolf? The eagle is (among
> > other things) beating the wolf about the head with its wings.
> 
> Yes, but the eagle isn't trying to *catch* the wolf with its wings.

That doesn't really affect the question of fragility, though; the eagle
is trying to biff the wolf with the same part of the limb that would
have had claws if the eagle was a velociraptorine dinosaur, and the
wolf substantially out-masses the eagle and wants to object.  It's not
able to do so effectively, including in a way that damages the flight
feathers, which is a much larger problem for the eagle than it would be
for a ground-running dinosaur.

> This is where the analogy breaks down.  For dromaeosaurids (or at
> least dromaeosaurines and velociraptorines) the forelimbs appear to
> have played a major role in catching and subduing prey.  And yet,
> _Velociraptor_ presumably had two featherdusters strapped to its arms.

Quill knobs imply substantial aerodynamic load and structured feathers,
not feather dusters.

So at least one of:
        1 these are a big help in catching up to the prey before subduing it

        2 these are absolutely required to have a shot at successful
          breeding (display leaping, tucked-wrist feather-battering fights for
          breeding rights, fanning the nest, who knows...)

        3 if you don't have these, you can't get away from your larger
          theropod relatives

has to be true, along with "these don't make it more difficult to subdue
prey".

> > Geese and swans do that as a defence, or when having disputes with
> > one another; all sorts of ducks do that, too. Feathers aren't that
> > fragile.
> 
> True.  But in a battle between the beak of a desperate _Protoceratops_
> and an arm feather, I think ther feather would come out second best.

Sure, but that's much better than the *arm* coming out second best, and
target confusion, while it must have been a secondary purpose, isn't a
bad thing.  And they're not actively *flight* feathers, they can't be,
so feather loss is not an immediate death sentence the way it is for a
volant bird that loses enough feathers to prevent flight.  (Leaving
aside waterfowl that are seasonally flightless and some other special
cases.)

> > It's also not clear that the very limited range of motion in the
> > maniraptoran manus was used for holding the struggling prey at all;
> > the proto-flight-stroke looks more like a mechanism for slashing
> > something apart.
> 
> That's a cool idea.  Has it been looked at in detail (biomechanics,
> etc)?

Not that I know of, but, if recollection serves:

1 no pronation of the wrist _at all_; pretty much a rigid hand on a
rigid forearm from a rotation-round-the-axis-of-the-radius standpoint,
which is really bad if you're hooked into a struggling prey animal and
it, quite naturally, twists; the hand can fold, but that doesn't handle
an entire expected range of prey motions.  Rather than postulate
regularly dislocated elbows and shoulders among velociraptor
populations, I'm going to suggest that they didn't suffer that kind of
load very often.

2 sweep distance is about the same as the modern bird wing, stopping on
the mid-line.  For anything other than very small prey, this is going to
make grabbing _really hard_, because you have to get it perfect to get
both limbs in on it, *or* you have to have enough arm strength to
forcibly align yourself if one set of hand claws hits (while running at
speed.  Not an ideal scenario...).  Not much torso flexion, hips are
straight back-to-front, the pubic boot is generally down to knee level,
and the arms still point significantly _down_ when fully extended
forward.  This makes grabbing large prey without standing on it pretty
much impractical, and standing on it tricky; a full belly-down squat
might required all four limbs for stability, and then the poor beast is
presumably reduced to trying to gnaw through the protoceratops' back.

3 tetrapod large-prey predation generally involves either not letting
the air in or letting the blood out; theropods in general seem never to
have developed a 'not the let the air in' approach, and velociraptorine
dentition isn't much adapted to the purpose.  Hook and cling while
kicking with the sickle claws doesn't work if the sickle claws (as
recently reported) are unsuited to slashing.

4 the forelimb prey-capture (or prey slashing) motion was suitable to be
exapted into a flight stroke; whatever they did with those feathered
forearms, it was subjected to selection pressure for speed, power, and
endurance.  Display can explain that; so can a 'will this die before it
bites me?' repeated rapid slashing use.  (This would give the display
activity some connection to a measure of fitness, too, which is
typically the case.)  Grabbing once swiftly doesn't go with flight
stroke exaption so well.

In the absence of careful biomechanic study, that all seems to me to be
suggestive.

-- Graydon