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At 01:59 PM 4/11/99 -0400, Patrick Norton wrote:
> I wrote:
>>In a sense, yes, but it is unable to extract oxygen from *air*. A gill,
>for some reason, can only exchange gases effectively with water.<
>Actually, gills can extract oxygen from air as long as they (the
Ah - that may be the key difference. Lungs, being internal, and protected
by a long mucous-lined passage, can remain moist in air, while the less
protected gills cannot.
>The efficiency of that exchange, of course, changes
>based on the characteristics of the fluid medium.
Though this may also matter - for larger animals, at least, reduced
efficiency may be an unacceptable cost.
>>The difference here is in prior anatomy. The digestive system was a tube
>with two valves at opposite ends already in the ancestral vertebrate. The
>lungs of early fish were blind sacks from the start, thus requiring two-way
>flow at least at the entry/exit point,<
>Arguments about prior anatomy only go so far; after all, the condition prior
>to primitive lungs in vertebrates was gills.
Not so. The oldest known jawed vertebrates had *both* structures!
(Besides, I was discussing the difference between lungs and the *digestive*
tract, which is even older than either gills *or* lungs)
>My thought on my original post was only that birds (aka: derived theropods)
>seem to be the only vertebrates that successfully re-acquired some degree of
>counterflow respiratory physiology which vertebrates gave up when becoming
Yes, there is that. I suspect that this may be due to luck as well as
stronger selection for gas exchange efficiency. That is, it is possible
that the requisite structural modifications may only have occurred within
the diapsid lineage. The key factor may be the widespread occurrence of
lung diverticulae in diapsids, and the lack of such in synapsids (including
mammal). (Note, there is reason to suspect that pterosaurs may have had a
system similar to that of avians).
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