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Re: bauplan convergence



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.
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Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
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