Re: [asa] Extinction/Extant

The modern lungfish retain a combination of features of the
transitional form that are not found in most other extant descendants,
but they are not exactly the true transitional form. Part of the
difficulty relates to how narrowly you define a transitional form. It
is very difficult to be absolutely certain that a given fossil is the
actual transitional ancestor as opposed to a third cousin twice
removed or the like. The Australian lungfish has fairly
well-developed lobe fins, but these are very reduced in the African
and South American species.

Lungs are actually fairly common among primitive bony fish, but most
modern bony fish have transformed them into a swim bladder or lost
them entirely. Also, other moist, well-vascularized surfaces besides
the insides of lungs can serve for gas exchange, so fish and
amphibians can generally get oxygen through their skin and the lining
of the mouth. Thus, vertebrates can have gills, lungs, both, or
neither-the gill to lung transition is not hard.

One could argue that we do have plenty of transitional forms living
today; just wait a few million years and you'll see what they are
transitioning to.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.