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Re: Belly Ribs



> I've been thinking about belly ribs (gastrulae, right?) lately.

Gastralia, singular: gastrale.

> 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.

*Confuciusornis*? That's news to me. Did it really have gastralia?

> 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.

Because no mammal has them.

> 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?

Seem to be a synapomorphy of Amniota, IIRC; sauropods, ornithischians and
birds among dinosaurs lost them, the former two maybe to expand the belly
for longer guts, the latter because the belly became shorter and shorter
while the sternum grew and apparently took over their functions, whatever
these are.

Protection and support of the belly and a function in respiration (to
ventilate the posterior air sacs?) have been argued, probably the latter
applies to the dinosaurs that had them, while the former two apply to
plesiosaurs, placodonts and others with strongly developed gastralia.

Contrary to early reports, *Sinornis* does not have gastralia, and the bones
identified as such in *Eobrontosaurus* may be sternal ribs.

Thin and delicate in dinosaurs and therefore often not preserved, so their
absence is usually difficult to ascertain.