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Re: Vestigial Arms (was: Theropod limbs - how mobile?)



----- Original Message -----
From: "John Conway" <john_conway@mac.com>
Sent: Saturday, May 18, 2002 6:36 PM

> > "Maximum working range" = 199 kg for the biceps alone.
>
> Haven't we got to be a little skeptical of such estimates? I wasn't
> aware that muscle strengths could be estimated with any degree of
> precision.

Not estimates, calculations :-) The math is spread over several pages, so I
won't explain it here (hm... sounds as if I'd understand it :-> )

> Even if they were that strong, they would still be ripped off by a
> struggling six tonne animal. I cannot envision tyrannosaurids struggling
> with their prey, it seems to risky for such large animals.

With such jaws in addition...

> To get their
> hand claws lodged in their prey would require tyrannosaurids to get very
> close, with wide areas of contact with their prey.

Were they so broad?

> At such close
> quarters, a simple shift in weight from the herbivore would throw the
> tyrannosaurid off balance, because its movement would be severely
> restricted.

Should depend on the tyrannosaur's leg muscles...

> And if they were using their arms for such purposes, why were they so
> short?

_Precisely_ because force alone was the issue, while velocity was not. Just
like the "can opener" arms of the mononykines. :-)

> It has been suggested before (sorry, I can't remember who it was) that
> they used their arms for intraspecific contests. The arms were small to
> keep them out of the way, and save energy growing them, yet they
> remained muscular for arm-wrestling with rivals.
>
> You know it makes sense. ;-)

Wouldn't explain the strong pulling muscles, I think, and the fingers:
"Finally, to ensure that the struggling prey not escape while the mouth is
attempting to kill it, the two ungual claws [sic] point somewhat inward
(fig. 9.13C) so that they do not slip out of the prey easily.

> P.S.   Show me a convincing drawing showing a tyrannosaurid using its
> arms to subdue prey, and I will reconsider.

Maybe this description is enough:
        "We envision that *T. rex* stalked or ambushed prey, mostly subadult
or young adult hadrosaurs (see Carpenter 2000). As with most extant
predators, the mouth was used to grasp the prey. Then the short, powerful
arms were used to grasp or clutch the prey against the body to prevent its
escape while the teeth were disengaged and repeated bites made to kill the
prey."
        :-)