Re: Belly Ribs

["Steve Brusatte" <dinoland@lycos.com>]

On Sat, 30 Jun 2001 09:18:53
 Daniel Bensen wrote:
>I've been thinking about belly ribs (gastrulae, right?) lately.
>
>Tyrannosaurs had them, as evidenced by Sue, in the Field Museum.  I've
>seen reconstructions of various other predatory dinosaurs (Velociraptor,
>Ornithomimus, etc.) and even birds (Archaeopteryx, Confuciusornis) that
>have belly ribs.
>
>I didn't think anything of these bones until I was showing some skeletal
>reconstructions to a friend of mine who is a doctor (you know, one of
>those people who studies _humans_, god knows why).  He was very weirded
>out by the belly ribs.
>
>My question is this: How wide-spread are belly ribs?  Modern birds don't
>have them---when did they go away?  Do crocodiles have them?  What
>groups of dinosaurs have them?

Dan et al.,
Yeah, gastralia are definitely weird.  To be honest, I was thinking of 
beginning a gastralia thread after our digit discussion several weeks ago, 
since the function of gastralia is still debated.  But, I still haven't gotten 
around to it, so maybe Dan's post can start a nice discussion.

Gastralia are pleisomorphic for tetrapods, but only crocodiles and _Sphenodon_, 
among living animals, possess them (that's all I can think of).  In dinosaurs, 
they are known from theropods and prosauropods, and I believe I remember 
reading about some reports of them in theropods.

Crocodiles, as I said above, do indeed have them.  I've never dissected a 
crocodile, but have seen photos.  They are relatively small in crocodiles, but 
are indeed present.

Exactly what is their function?  This is something I would like to debate.  The 
answer probably lies in breathing, as Leon Claessens talked about at the Armour 
Symposium.  Since they are only known in saurischians, this must mean that 
theropods and sauropods had different breathing mechanisms.  Or, possibly 
gastralia were used to hold in the visceral mass, or had a purpose or function 
that nobody has even guessed at.

Short take: gastralia are very odd, rare in modern animals, and only seen in a 
few groups of dinosaurs.

I am open to anybody wanting to talk or debate about their function :-))

Steve

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