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Re: My apologies, but... physiology revisited



From: "Mickey P. Rowe" <mrowe@indiana.edu>
 > Stan wrote:
 > 
 > > One can conclude that RT's imply endothermy.  That is, the
 > > *presence* of RT's is good evidence *for* endotermy.  However, the
 > > converse *cannot* be derived conclusively from this data.
 > 
 > Using Stan's reasoning it would be impossible to show that *any* trait
 > is necessary for endothermy.

Well, I would accept as conclusive any trait tht was present in 100%
of known endotherms.

 > a) RT's aren't as important as the OSU crowd claims they
 > are with respect to retaining respiratory water and heat losses,

Which is what I claim the exceptions do in fact show, at least for
all practical purposes.

 > or b)
 > dinosaurs must be shown to have some other mechanism for compensating
 > for respiratory water and heat losses.

At present this is indeterminable (see below).  Still, given the extant
exceptions, an alternative mechamism is quite *plausible*, and cannot
simply be assumed not to exist.
 > 
 > Stan is lapsing into unfairness again.  The authors do not claim that
 > the exceptions don't matter.  They say that the exceptions can be
 > accounted for because they appear to have other mechanisms for
 > retaining (whales) or compensating for the loss of (Pelecaniformes)
 > respiratory water.

I seem to have been unclear. (That happens when I am typing fast).

What I meant was the authors are claiming that the exceptions do
not substantially weaken their case for ectothermy in dinosaurs
on the grounds that they are accounted for.

I disagree.  This is, at least, partly a matter of relative weights.
I see two *different* alternative water-loss control mechanisms in
the two cited exceptions.  The way I see it is: if there are two,
why not more? Unless and until we can make a more or less complete
list of alternative mechanisms we simply cannot say with *any* certainty
whether a given fossil animal did or did not have a water-loss control
mechanism of some sort.

That is, the absence of RT's *by* *itself* is insufficient to establish
an animal as ectothermic (or rather bradymetabolic).  Just as desert
animals have many *different* specializations to resist dessication,
so could dinosaurs and other basal endotherms.  In fact many of the
same adaptations seen in desert animals might well serve in place of
RT's.

 > Other compensatory mechanisms are possible, of course.  ...
 >  But the point stands that a good case has been made that
 > respiratory water loss is an important problem for endotherms, and if
 > dinosaurs were endotherms then they must have found a way around the
 > problem.  At present there is no evidence that they did.

What sort of evidence would there be for the way a kangaroo rat deals
with desert aridity??  As far as I know most known mechamisms for water
conservation simply do not leave much in the way of traces in the
hard parts.  This makes this very hard to evaluate in dinosaurs, or
any other extinct group.  Merely assuming that because RT's are perhaps
the most efficient such mechanism, their absence shows the absence of
any such mechamism is not really valid, IMHO.

[And the convergent evolution of RT's can be explained quite well if
they are the most efficient, in cost/benefit terms, of the available
mechanism for water conservation; this applies even if the basal endo-
therms in each lineage lacked RT's, but used some other mechanism].

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.