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RE: Theropod limbs - how mobile?



 
Rutger Jansma wrote:

>How can someone say without or with a slight doubt when the forelimbs
>are powerfull, that the hands must be strong? 

What you say is true.  But biomechanical studies can shed light on the
mobility of individual limb elements.  This can tell us how they *might*
have worked together, especially when other parts of the skeleton are taken
into account.

> Most of this is based on speculation, since we are not able to step 
> into a time machine, shoot a poor Dromie and see how it's forelimb 
> would have worked. 

Welcome to the world of paleobiology.

You're right - to a degree.  However, your argument applies to *any* aspect
of a fossil species' inferred behavior - including (for example) the
proposed carnivorous and predatory nature of _Deinonychus_.  A suite of
dental and skeletal features indicate that this extinct creature was
probably a predator.  But unless you can witness the creature's behavior in
real life, courtesy of a trip back in time, this too is in the realm of
speculation.  

Nevertheless, when it comes to speculaton, there's a big difference in
restoring _Deinonychus_ as a ferocious predator that used its hands and
sickle-claw to grasp and kill prey compared to restoring _Deinonychus_ as a
lazy, sloth-like herbivore that used its sharp claws and powerful limbs to
hang from trees.  The former interpretation is congruent with the
combination of features seen in _Deinonychus_, with reference to modern
animals.

Plus, for dromaeosaurids, birds (which have a very similar forelimb
osteology to dromaeosaurids, especially in the forearm/antebrachium) offer a
template for reconstructing the possible forelimb movement for dromies.  Not
exact, but similar.


> My opinion is that strong forelimbs do not immediatly advocate a 
> powerfull grip. 

For an obligately bipedal carnivore like _Deinonychus_ I'm open to
alternative suggestions for why it had strong forelimbs.

> T.rex' forelimbs were powerfull, as is shown by extensive studies on 
> the musculature, but it's hands are capable of almost nothing.

You've just painted yourself into a corner.  By your own statement above,
how do you *know* that the forelimbs of _T. rex_ were "capable of almost
nothing"?  Do you, in fact, have a time machine?  :-)  

One hypothesis is that tyrannosaurids grasped and held their prey in their
jaws, and the forelimbs were used to clasp the prey against the chest.

>From another post...

> But now that Mike has mentioned this genus, is it a trully diagnostic 
> genus  or is it referable to Pelorosaurus? And what is the status of 
> Macrurosaurus  and Dinodocus? Obscure Wealden sauropods, give me a lot 
> of headaches... 

It's Migraine City where British sauropods are concerned.  My information is
that the type material for most (and perhaps all - _Pelorosaurus_ included)
of the above taxa is probably non-diagostic at the genus level.
_Macrurosaurus_ (which is not Wealden but from the Cambridge Upper
Greensand) is based on material from more than one sauropod species (at
least according to Le Loeuff [1993]).  Some British brachiosaurs have been
diagnosed on the basis of the shape of the cavities and pleurocoels in the
vertebrae (e.g. _Ornithopsis_, _Eucamerotus_); but these traits appear to be
subject to individual variation within sauropod species.



Tim