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Re: semilunate carpal



On Tue, Aug 07, 2001 at 05:39:58PM -0500, Tim Williams scripsit:
> Graydon wrote:
> >Some maniraptorans are obviously evolved to attack prey as large or
> >larger than themselves; in these cases, seizing isn't going to be a good
> >description of the interaction with the prey animal.
> 
> "Grappling" is probably a better word.

Ok.

The point I'm trying to get to here is that the predator was likely
moving *itself* relative to the prey animal.

> >The other thing is that the greatest relative strength would have had to
> >evolve in small forms, since those are the ones that became volant, and
> >minimal levels of useful for flight arm strength would already have to
> >be enough to move *them*; locomotor strength levels, but in a limb that
> >couldn't be used for walking.
> 
> Yes, but just look at the size of the sternal plates in these small 
> maniraptorans.  They anchored some pretty heavy-duty muscles.

Certainly; they *are* strong.  The question is why did they get that
way?

The usual answers come down to 'they were waving their arms a lot',
which I think covers the question of strength but not the question of
the particular restricted range of movement.

> Imagine how much strength would be needed by a _Deinonychus_ to cling
> on to a bucking, heaving _Tenontosaurus_ fighting for its life
> (literally).  

Well, really, imagine how much strength it takes to keep the kick with
the foot claw from bouncing the _Deinonychus_ *away* from the
_Tenontosaurus_, bucking or not; those legs are the primary locomotor
organ of a grapple-and-slash predator, presumably one with a
considerable short sprint capability.

Modern clawed predators mostly pounce, and pin prey to the ground, or
support their own bodies to position a bite (big cats attacking big
things); the closest analog to that sort of sickle-claw attack I can
think of is a bear swiping with a clawed forepaw, and that's *across* the
axis of the body, rather than *along*, so it's a rotten analogy.
(sometimes also *down*, but still bad, since bears are not bipeds.)

The ancestral form that started developing the sickle-claw would have
had this same problem; how do you keep a clawed kick from making the
*second* kick much harder to deliver by bouncing you back from the prey
animal?

I think that problem is a possible explanation for the hand
specialization, especially in conjunction with the hip morphology of
theropods; they had to kick straight ahead, so anything that gave better
straight on alignment would presumably be an advantage to a kicking
predator.

> Or a _Velociraptor_ trying to hold a _Protoceratops_ so it can kick it
> with its slashing foot-claw - while trying to avoid being mauled by
> the _Protoceratops_'s beak.

I think that one is actually a bad example of typical behaviour; whoever
started that fight, it went really wrong for the _Velociraptor_.

(Is there any consensus on how they got into that position?  I don't
have a good sense of inital relative heights of things; it seems to make
the most sense if the _Velociraptor_ got to attack the _Protoceratops_
while the later was rearing up into a bipedal stance.)

> >Is there a known biomechanical reason why the use of the 'predatory
> >stroke' might not have been to align the maniraptoran, relative to the
> >prey, rather than to move a small prey animal toward the jaws?
> 
> The predatory stroke might have co-opted in arboreal theropods to negotiate 
> predatory descents, from trees to the ground.  But that's another story.

Well, yes, but that's not quite what I was trying to get at.  If the
primary killing weapon is the foot, and the hips will only swing the
foot directly fore-and-aft, and if the prey animal is large, relative to
the maniraptoran, there's a fairly obvious application of the stroke in
aligning the feet.  (and incidentally also the jaws.)

This would appear to fit better with the semi-lunate carpal -- adding
elasticity, and a motion that stops the hands on the center line of the
animal and increases speed.

In such a scenario, the maniraptoran jumps, the arms both -- any
evidence that it would ever not be both? -- swing forward, the body gets
aligned to the prey animal, and the clawed feet start slashing.

Something like _Utahraptor_, a bit large for jumping, would make the
hand slash and bite, perhaps, or just slash;

> >That's reasonably consistent with tree climbing, and a steady
> >increase in stength to the point where the arms were capable of
> >becoming locomotor organs.
> 
> I think predatory adaptations in the forelimb and manus pre-adapted
> small theropods to tree-climbing.  Darren Naish wrote a nice paper on
> this recently.

That makes sense.

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.