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Re: Theropod limbs - how mobile?



>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.
>
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? 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.
>
>Welcome to the world of paleobiology.
>
It has a sort of John Hammond feeling to it :)
>
>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.
>
True
>
>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.
>
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. Nothing new you would say, but the immobility in it's
hands causes me to think that it didn't use it's hands when hunting. Later
genera were perhaps able to do so, they could have lost this restriction in
hand movement, but this is all speculation. Again, my sources on
Velociraptor are not as good, especially on the area of hand and wrist
movement.
Don't see Deinonychus as an animal that could hang from trees, big trees
would be needed than :) Although we can not say this for sure, lack of time
machine is the major problem...
>
>> 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.
>
I believe HP Greg Paul has a wonderfull word for it: neoflightlessness.
Powerfull forelimbs are needed for air propulsion, the reason animals like
Baptornis are not considered flightless. Dromies are very likely secondary
flightless, but Deinonychus for example is very close to this event in it's
general evolution. Have to check again if Velociraptor has less developed
forelimbs though.
>
>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?  :-)
>
Sorry, have to dissapoint you, would be nice though. 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. This must be done for a reason, when something is useless in nature,
in becomes lost, birds did the same thing with their tails, as did apes. How
does this apply to large theropods? Forelimbs began to lose their function
as evolution progressed in the large theropod groups, perhaps resulting in a
total loss if the meteor didn't make an end to their reign.
>
>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.
>
And this is where HP John Conways post steps in. He clearly points out that
the use of the forelimbs in, for example, holding chunks of meat is not
usefull. T.rex' forelimbs are way to short to even reach the mouth of the
animal. Second, I don't think that 300 Lbs would be sufficient to hold any
animal down in the Late Cretaceous, except for the Troodontids maybe. 300
Lbs is perhaps somewhat around 150 kg, hadrosaurs and ceratopsids are
dinosaur groups that weigh around 3 tons, do the math. ;)
>
>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.

Thanks for clearing this up.