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.
JimA [Friend of ASA]

David Campbell wrote:
> 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.
>
>

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Received on Tue Oct 13 20:20:00 2009