Really short forelimbs

[Nick Longrich <longrich@phoenix.Princeton.EDU>]

        Having yet another look at the Carnotaurus, which has some
pretty robust humeri as far as I can tell (they look thicker,
absolutely, than the T. rex cast I saw), I'm beginning to have some
thoughts about an idea I cooked up a little while ago.
        There was a question on one of my vertebrate bio exam about
the feeding behavior in great whites: they bite into a sea lion from
below, pull it down into the water, carve a bite out, let it float
back to the surface, and repeat the behavior.
I think that this serves to improve the shark's pull agains the
sea-lion: it pulls against the animal's own floatation. I've heard
that's why they throw prey back and forth while it's in the jaws- the
pulling motion helps to cut through the meat. Birds do something
similar when they tug at food held in their feet. I'm guessing that
these serve to increase the tension of the fibers. A rock-climber has
told me that if you so much as breathe on a rope with a knife while
it's under tension, it will break; muscle fibers aren't much different
in that respect: they're easier to cut through under tension.
        So I throw this idea out: that the purpose of the T. rex and
Carnotaurus limbs could be to push, rather than to hold, against the
prey. Consider T. rex coming up to its prey, putting its jaws around
it, and biting down. Now consider instead T. rex coming up to its
prey, putting its jaws around it, and biting down while pulling back
with the head. Now consider T. rex coming up to its prey, biting down,
and instead of simply tugging back with its head, pulling back while
simultaneously pushing against the prey with the arms. Increased pull
might increase the tension of the muscle fibers and improve the
cutting action of the teeth, in the same way the great white pulling
the sea-lion down underwater or thrashing its head from side to side
improves the cutting action of the jaws.
        Tyrannosaur arm bones are quite robust and frequently damaged
(demonstrating their use and abuse during life), while Carnotaurus has
arms that appear quite robust for their size. So they could have been
strong enough to serve this purpose.  I'll have to take a look at
Horner's book and see if the rex forelimb reconstruction at all
matches what one would expect to see, though: a very large triceps, a
large deltoid as well, and probably less of a bicep.

        -nick L.