Re: [asa] Extinction/Extant

> Probably no way to answer this question, but I wonder if these lungs and
> gills were more in the nature of supplementary capabilities when they
> emerged, necessary to survive under adverse conditions, then not so
> necessary in combination at later dates.

Most modern fish that make significant use of lungs or other non-gill
breathing live in settings where they are likely to encounter stagnant
water or spend time out of water-mudskippers, lungfish, eels, walking
catfish, etc. Most of these probably can manage with gills if they
are in ordinary well-oxygenated water; apparently to get a photo of a
lungfish breathing air at the surface of a water-filled barrel it was
necessary to keep chasing it with a stick until it panted. An
exception is the tarpon, which lives in ordinary marine settings but
will drown if kept under water.

I think that the fish closest to amphibians were generally in
nearshore settings, where stagnant pools are not uncommon.
Air-breathing would facilitate getting into shallow temporary pools
with tasty bugs and less competition. Air has lots more breatheable
oxygen than even well-aerated water, so a fairly rudimentary
air-breating ability would still be quite useful.

Activity levels are a major factor, too-hibernating turtles can
survive underwater in the cold by absorbing what little oxygen they
need through the lining of the cloaca, but the same turtles can drown
if trapped underwater on a warm summer day.

On lungs, the avian lung is not an example of irreducible complexity
related to flight; non-flying dinosaurs had them long before as a way
to lighten the skeleton.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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