Re: Skulls and arms...

[Mike Taylor <mike@tecc.co.uk>]

> Date: Wed, 22 Aug 2001 23:55:01 -0300
> From: "Marcel Bertolucci" <mbertol@terra.com.br>
> 
> A friend of mine asked me something I've never realized and that
> it's so obvious that I may have not realized in my entire life:
> "Let's get three dinosaurs: _Spinosaurus_, _Allosaurus_ and
> _Carnotaurus_.  We can find something like a relationship between
> the skull and the arms: as longer the skull is, the bigger are the
> arms...", its what he said, and I just got wordless at the moment.
> So I am asking if anyone knows anything about it.

This makes no sense to me.  You'd expect to see an _inverse_
relationship between skull size and arm length, so that their total
weight is constant as a proportion of body weight -- an animal with
big head _and_ big arms would topple over!

And sure enough, that inverse relationship is what we see if instead
of your three taxa, we look at _Tyrannosaurus_, _Allosaurus_ and
_Ornithomimus_.  (Though it may be that all this proves is that you
can can show anything by picking the right data sets :-)

Surely the selection pressure on tyrannosaurids to develop smaller
arms was that it enabled them to develop larger and more robust heads?
Why else would it happen?

The fly in the ointment is, of course, _Carnotaurus_, with its bizzare
short snout and ridiculous truncated arms.  But that's just a weird
animal, and there's no legislating for it :-)  It did occur to me that
the short snout might be pathological (am I right in thinking that
there's only one, very well preserved, _Carnotaurus_ specimen?) but we
also the skull from _Majungatholus_ which is similar.

So what kind of selection pressure could produce a small head _and_
small forearms, together with that distinctive but presumably useless
long thorax?

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