Re: DINOSAUR digest 1272

[Dann Pigdon <dannj@alphalink.com.au>]

Martin Human wrote:
> 
> > From: Dann Pigdon <dannj@alphalink.com.au>
> 
> > I like the idea that they were used to help the beast get up. If 
> > tyrannosaurs
> > spent much time lying around (and that huge pubic boot is obviously there 
> > for
> > some reason), they may have had to rock forward to get up in order to begin
> > straightening their legs. Maybe the forelimbs were still there (rather than
> > being lost entirely, or vestigialised like in Carnotaurus) to stop them 
> > scooting
> > forward on their chests before they could pivot their front section up to
> 
> > I don't think much of the idea of the forelimbs being used during mating. 
> > Who'd
> > want to antagonise a 6 tonne female by jabbing her with meat hooks at the
> > critical moment? (please don't answer that question)
> 
> Doesn't the latter statement contradict the first? If you want to push
> yourself up, or stop yourself slipping forward when rising, why keep
> "meathooks" as appendages? A flat strucuture with toes to dig in would
> seem a better idea?

Providing that was all the limbs were used for. I can't think of what else they
could have done personally though. I've read that Tyrannosaur forelimbs couldn't
straighten out at the elbow more than about 90 degrees (that is, couldn't
straighten out at all). If this is not one of those Mesozoic myths (as opposed
to urban myths) that has been endlessly repeated until assumed true, it
certainly seems to fit in with the former of the two above mentioned theories.
Imagine a Tyrannosaur lying down with its weight supported by its folded legs,
pubic boot and chest. The position of the arms, and their c.90 degree angle
would probably put the forelimbs lying horizontally along the ground, with the
claws digging into the earth. Being heavily muscled, and unable to straighten
out any more than c.90 degrees (I'm still assuming this), they would seem to be
perfect for stopping the chest from scooting forward when the animal straightens
up its hind limbs before rocking back and using the counter balancing tail to
raise the front part of the body. Perhaps the large curved pubic boot functioned
as a pivot early on in the process before the legs were straightened enough for
it to leave the ground (that is, enough to allow the centre of balance to be
moved forward to take some of the stress off the hind limbs to allow them to
begin the process of straightening, when the chest would be baring a bit more
weight and perhaps require the forelimbs to prevent it scooting forward with the
change of weight/angle on the chest).

I suspect the whole process would have been similar to how a goat rises to its
feet (I'm assuming from memory that they rise back legs first). As to why it
retained curved claws, this could have been a holdover from earlier theropods
who had more use of their forelimbs, or perhaps they did other things with them
that we haven't thought of yet.

Conjecture at its best (arguably). 

-- 
____________________________________________________
        Dann Pigdon
        GIS Archaeologist
        Melbourne, Australia

        Australian Dinosaurs:
        http://www.geocities.com/capecanaveral/4459/
        http://www.alphalink.com.au/~dannj
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