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On Sat, 10 Apr 1999, Patrick Norton wrote:
> If it turns out
> that the more primitive unidirectional system is more efficient, it raises
> the question as to why terrestrial vertebrates abandoned this system in
> favor of the "in and out" breathing of birds, mammals and reptiles
Um, because gills don't work in air?
The water respiration and air respiration systems in vertebrates basically
have nothing to do with each other--and were apparently *both in place* in
the ancestors of tetrapods. The first land vertebrates merely continued
to use the lungs which their ancestors had already developed while losing
their now-useless gills. It's just that no vertebrate group had hit upon
the "idea" of developing a unidirectional air respiration system until
birds (or theropods, or dinosaurs, or maybe ornithodires) came along.
I hope this helps.
- From: "Patrick Norton" <firstname.lastname@example.org>