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Re: Evolution of tyrannosauroid bite power



On Mon, Dec 31, 2007 at 12:14:59PM +0100, David Marjanovic scripsit:
> ----- Original Message ----- From: "Graydon" <oak@uniserve.com>
>> So instead they have dinky forelimbs that clearly underwent
>> proportionately high loads.  Could be mating related (but then we'd
>> expect to see disproportionate development/injury in males) or it
>> could be a prey position sensor.  It more or less couldn't be a 'hold
>> the prey' mechanism because even a thousand pounds of force isn't
>> worth anything when a five ton Edmontonosaurs thinks it's about to
>> die.  It can generate *lots* more force than that with its legs.  But
>> it *could* be a 'where is the Edmontonosaur I've just crashed into,
>> bit once, and which is now a)under my head/out of my direct line of
>> vision and b)vigorously concerned to get away.
>
> But why would a mere sensor have this kind of claws that converge
> towards each other so that anything the claws have been stung into can
> only escape if it rips its own flesh apart, if it rips the
> tyrannosaur's hand apart, or if the tyrannosaur graciously decides to
> pull the claws out itself?  Wouldn't that be selected against?

I suspect the claw convergence is the ancestral condition, and hard for
selection processes to get rid of.

Since there is no way for even a juvenile T. rex to bite what it's
grabbed or for it to expect to kill what it's grabbed with its hands, it
doesn't seem plausible that the claws would be a primary prey capture
mechanism.

However, given that the prey wants to get away and the 'sensor' is
'where is this thing I've just bit once and which I'm standing over
going', it's not hard to see how there'd be benefit to a very firm grip
into flesh, especially since a five ton hadrosaur is going to have thick
scaly hide; a sensor that skitters off the surface returns less useful
information than one that's firmly attached to one spot.

The whole process would be hard on the arms, but it's not clear that the
arms aren't at least somewhat expendable, or that odds of permanent
disablement weren't low.

-- Graydon