Re: Belly Ribs

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

On Sat, 30 Jun 2001 13:36:10
 christopher robert noto wrote:
>>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.
>
>I'm wondering if "holding in the visceral mass" is a good argument.  If
>that was their primary function and Ornithischians did not possess them,
>how did Ornithischians hold their innards in?  I am much more partial to
>the respiratory system argument.  Could it be assumed (if direct evidence
>doesn't exist) that the most basal Saurischians had gastralia and if so,
>were these gastralia used as part of a more efficient cardiovascular
>system that aided these creatures in their carnivorous predatory
>lifestyle? Sauropods then would have retained them and birds could have
>eventually lost them.

Oh, I definitely agree.  I don't know why the visceral mass would be any 
different in ornithischians.  I believe the visceral mass theory was what was 
originally hypothesized, but in my opinion (and obviously yours, Chris) it is 
very weak.  I definitely point to breathing as their main function.  Birds, 
with their highly advanced respiratory system, probably had no use for them.  
However, I wonder what this means in terms of theropod respiratory systems.  It 
has been hypothesized many times (for instance, Pete Larson) that theropods may 
have had a very avian respiratory system.  Would gastralia have held that avian 
respiratory system back in theropods??

Are gastralia known from the feathered dinosaurs?  I really don't know, and 
their presence or absence may say something about the respiratory system of 
theropods and perhaps early birds.

Steve

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