


Matthew Troutman wrote:
>Most authors come to the conclusion that pulmonary air sacs in birds
>evolved  for respiration and I concur.  However, I cannot see that they
>could have evolved for lightening weight.  Many birds lack postcranial 
>pnematization but still retain functional air sacs.  This argues against 
>the "lightened weight" hypothesis.  Moreover, many flightless birds 
>still retain large air sacs an postcranial pnematization.
        I refer to my earlier post titled "Re: ARTHROLOGY AND FLIGHT
CONTROL", in which I said: "It is, IMHO, very dangerous to ascribe limited
capabilities to fossil taxa which lack complex refinements seen in modern
forms." I would continue that it is very dangerous to use the morphology and
variability of modern forms to attempt to deduce the "original function" of
a feature.
        We must always remain aware that animals are whole organisms which
evolve. Sometimes evolutionary novelties aid in a function but are not
necessary to that function. "Helpful" does not equal "necessary". The
novelty may also serve some other function, or may simply be a result of the
animal's heritage, a result whose contribution to differential survival is
just enough to maintain itself. An animal's morphology may change for a
number of reasons, reasons which may have nothing to do with the trait you
are looking at. Somewhere along the line, the trait may change, even if the
animals continue doing whatever it is the trait aided. Recall that some
animals do things very well without showing any morphological adaptations to
that task.
        It may be instructive, when assessing the original "purpose" of a
character, to compare closely related taxa which show different character
states. However, we should be aware that, unlike in the world of computers,
these characters are not individual entities, they are part of a larger
integrated system with a distinct evolutionary history.
        Pneumaticity may have evolved as an aid to the development of air
sacks, or to lighten weight, or something else. We know it DOES lighten the
bones, and apparently (here's where my avian anatomy gives out) also DOES
help the air sacks. However, we do not know (well, I do not know) why the
birds you cite above "lost" their pneumaticization. Remember, loss or
reversal is an *adaptation*. Without an hypothesis concerning that, all we
are left with is the knowledge that the two do not *have* to coexist.
        If most everyone carries pistols in holsters, but Bucky has a
holster for his hairspray, what did pistols and holsters evolve for? The
holster evolved from the scabbard and was exapted to hold a pistol. The
holster would not have evolved without the pistol. The pistol would have
evolved without the holster. Yet it looks like the holster came first, since
it can be used without a pistol.
Sure, Lenny carries his pistol in his hand, so maybe that's good evidence.
But then it looks like the pistol and the holster evolved indpendantly. In a
sense, they almost did. Except the holster is exapted from the scabbared,
which is helpful (although not necessary) for the sword, which the pistol
evolved to compete with. Frustrating, no?
        Still this sort fo evidfence may be all you have. You certainly
should not ignore the distribution and manifestation of characters in modern
forms (if I said that, I would be doing far more than pissing in the
porridge of comparitive anatomy...). However, you should certainly be
cautious about the weight you give to such arguments.
        All that having been said, I have read the above quoted paragraph
several times, and I still cannot follow your logic. According to your
example, it seems that you are trying to demonstrate that pneumaticity
evolved FOR weight reduction, since you apparently don't need pneumaticity
to have air sacks (accoring to your example, which I just tried to weaken).
                    "...To fight legends." - Kosh Naranek