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RE: Theropod limbs - how mobile?



Rutger Jansma wrote:

> But now explain to me what purpose the strong humerus+antebrachial
> elements is when you take the immobility into account of the fingers 
> that you mentioned in an earlier post? 

Immobile, rigid fingers are less useful for manipulation, to be sure.
However, the inflexible joints between phalanges *might* be useful for
resisting struggling prey held by the hands.  

> Reason for me to believe that the strong arm elements did not serve the 
> purpose, at least, the primary purpose to be usefull in hunting or 
> killing prey.

Let's say the hands were useful for seizing prey, with each hand positioned
on either side of the prey via execution of the 'predatory stroke' - as
suggested by Gauthier (and others).  The 'killing' function was carried out
by the jaws in combination with the lethal sickle-claw - as you describe
below.

> That's why I have revised my restoration of Deinonychus, partly due to
> your earlier post regarding Deinonychus' hand mobility. It's is now 
> pictured as a hunter which relies more or less on the action of 
> his "killer blade", instead of his hands. 

Sort of like what Ostrom proposed over 30 years ago.

> Large theropod evolution, wether you are talking about Carnosaurs or 
> Tyrannosaurs, seems to be focussing one increase in body size and 
> decrease the size of the forelimbs (Giganotosaurus in the Carnosaurs, 
> Tyrannosaurs in general).  These critters began to evolve into "walking 
> skulls" as Paul Serena said it one time. 

What about _Deinocheirus_ or _Therizinosaurus_ - some large theropods have
very long forelimbs.  Also, _Allosaurus_ shows no evidence of forelimb
reduction.

It is interesting to note that, based on _Eotyrannus_, the craniodental
specializations seen in tyrannosaurids preceded truncation of the forelimbs.

> This must be done for a reason, when something is useless in nature,
> in becomes lost, birds did the same thing with their tails

I'm glad you mentioned birds, because they shortened their tails *not*
because they became useless but because they were useful for a new purpose:
flight.  The tail (pygostyle) of birds supports the tail-fan, which is very
important in flight and acts in coordination with the wings.


>He clearly points out that the use of the forelimbs in, for example, 
> holding chunks of meat is not usefull. 

That's ONE purpose - and I never said the arms were used to bring chunks of
flesh to the mouth.  Of course, they were too short.  However, another
purpose is that the little arms were used sort of like grappling hooks to
help secure prey held by the head.


John Conway wrote:

>I  can't see T.rex's arms being used to hold a female during mating, 
> they would be ripped off.

Not too *hold* a female, but to help position the male during (*ahem*)
coitus.  Like the claspers of sharks.


Michael Lovejoy wrote:

> How does this grip work physically? and how is it "better" than a less 
> derived, say, ceolopysian arm at gripping?

Not 'better' - just a different way of doing things.  The forelimbs became
specialized for seizing and holding prey, and the hindlimbs (thanks to the
'sickle-claw') became specialized for subduing prey.  (Note: This is just
one hypothesis.)

> Is there any evidence either way for the presence of tertials in 
> Archie? All reconstructions I've seen show them, but are they just 
> assumed?

Tertials are just assumed to be present in _Archaeopteryx_.  However, I
think it did have tertials; some Tertiary birds from the Messel are also
preserved without tertials.  The absence of tertials from neornithines is
undoubtedly preservational, and the same is likely true for the
_Archaeopteryx_ specimens.
 



Tim