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Re: Theropod limbs - how mobile?



You wrote:

>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.
>
Was there in the paper an image that I could possibly have or you can give
additional information about it, for example, what the range of finger
movement was?
>
>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.
>
Let's say he didn't, the following makes perfect in a neoflightless sort of
way...: take Deinonychus, big bad Dromie. The immobility in it's hands make
it difficult for capturing prey, our poor Dromie sees all those delicious
Hypsies just walking right by him and he is getting hungry. In a while, he
solves it: he evolves an alternate tool to slice his prey up. This in the
shape of of an enlarged pedal ungual II who practicly takes over the
function the hands had in predation. With this tool in place the hands have
time to re-evolve their mobility. If the hands had re-evolved that function,
there is no use in the specially adapted killer claw and therefore is
getting smaller. This is something that is known in several late Cretaceous
forms (Adasaurus (although possibly paravian), Velociraptor)
>
>> 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.
>
But not necesairry incorrect.
>
>> 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.
>
But these animals did not develop the big head as was the case in Carnosaurs
and Tyrannosaurs, they retained or "grew" the arms larger to suit their way
of life. True for the case of Allosaurus, but take into account that none of
it's supposed ancestors have been found. So no accurate comparison can be
made compared to it to see if the arms are proportionnally shorter or did
not change at all.
>
>It is interesting to note that, based on _Eotyrannus_, the craniodental
>specializations seen in tyrannosaurids preceded truncation of the
forelimbs.
>
Nasal rugosities don't count :)
>
>> 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.
>
Bird physiology is something that needs to be looked into when the time is
there.
>
>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.
>
Or they could not have had a function after all. When something get's
progressively shorter, that is done for a reason and when it is getting
shorter, it is not known for a fact that it had a function as time went by.
Take your own appendix for example, it is very reduced and serves no
function at all, but it is still there.
>
>> 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.)
>
Or the other way around