Re: bauplan convergence

[Ronald Orenstein <ornstn@home.com>]

This "bauplan" stuff brings up some very interesting possibilities. 
However, I think something has to be said first about the whole idea of 
"re-evolving" a quadrupedal herbivorous dinosaur body form.  If you are 
talking about birds, then it is perhaps relevant that birds could not 
"re-evolve" such a form because, strictly speaking, the theropod lineage 
never evolved it in the first place - are they not primitively bipedal 
within dinosaurs?

Anyway, I think you can use aspects of the "bauplan" argument without 
invoking hypothetical genetic constraints, at least in this case.  Remember 
that we are not just talking about folivory here, but specifically 
graminivory (something, for obvious reasons, that the dinosaurs never 
evolved at all).  Even quadrupedal herbivorous mammals had to undergo a 
number of evolutionary shifts (eg hypsodont dentition) to be able to handle 
grass as a major portion of their diet.  Folivory is comparatively rare in 
birds of any kind (possibly because the need to eat large amounts of grass 
and macerate the remains in some way adds a weight constraint that is 
counter-selective for flying birds); geese are perhaps the best examples, 
along with the hoatzin and the kokako of New Zealand.  The suite of 
adaptations a bird would need to become a really large-bodied full-time 
grazer would probably require a much greater number of evolutionary shifts 
than would be required of (say) a browsing ungulate.

Thus the bauplan argument may be sort of correct here not because birds, 
absent any competitive constraints, were physically incapable of becoming 
grass-eaters in a big way, but because the mammals (who were, if I can use 
the term, already possessed of a number of useful pre-adaptations and 
therefore had an evolutionary head start on the birds) got there first, and 
occupied the available grazing niches.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@home.com