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>Um, because gills don't work in air?<
Um, I wish things were so simple.
>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
gills)remain moist. The efficiency of that exchange, of course, changes
based on the characteristics of the fluid medium.
>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.
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