Thoughts on endothermy (long)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

Splicing together a couple of comments from the endothermy thread:

Larry Febo:
>OK Dr. Holtz. I`m looking for negative evidence here. The only thing I come
>up with is some kind of comparison with the anapsida as an intermediate
>link. If diapsids did occur later, from anapsid ancestors, then there has to
>be a closer link between them and the anapsids than between them and
>synapsids. The only thing I can think of would be  a DNA comparison. Surely
>someone has performed this already, no? If so, is it admissable evidence???
>Maybe some other physiological comparisons can be made as well.

Hmmm, a method by which you could evaluate which of various relationships is
better supported by the data?  A method of analysis which would be capable
of yielding results other than those you favor at first?  A method which
could be run as an analysis not only by you, but by any other worker using
your data?

Could it be... cladistics?

It may come as a suprise to some of the folks on the list, but a real
cladistic analysis is not "I'll draw up a family tree of a group of
organisms I like based on one or two characters or resemblances I see".
This is precisely analogous (an odd concept...) to saying that "I see that
this group of animal seems at first glance to be longer legged than that
group" is a morphometric analysis.

A real cladistic analysis is done with a bunch of data (DNA or genes,
morphology, behavior, etc.), run through a computer (or done by hand, if you
have a couple of million years to spare), and produces not only a cladogram
but also various metrics by which to describe and evaluate it.  (Similarly,
a morphometric analysis starts with a bunch of measurements, is run through
a computer (or done by hand), and produces not only the distribution of
points of the data but also various metrics by which to describe and
evaluate it).

Currently, the molecular and morphological evidence of the precise position
of turtles within Amniota is ambiguous, but I don't know of recent trees
which place mammals as closer to lepidosaurs and archosaurs than to turtles.
(Turtles are winding up as outside a lepidosaur-archosaur group or,
occasionally, closer to lepidosaurs than to archosaurs).

Also, please be careful (and look up some of the old archives) of using the
term "anapsid".  It can refer to the condition of not having temporal
fenestrae, or it can refer to a monophyletic group (sometimes alternatively
called Parareptilia) of turtles and various extinct relatives not ancestral
to any other group.  While all paleontological models suggest that,
ultimately, the ancestors of diapsids (and synapsids, for that matter) were
anapsid (the primitive condition for tetrapods), most modern models suggest
than none of the groups in "Anapsida" (aka "Parareptilia") are ancestral to
either diapsids or synapsids.

Also from Larry:
>There`s only one other option that I would find acceptable thats still in
>accord with my theory of one time development of endothermy. I don`t know
>why I overlooked it, except that I was concentrating on present day
>warm-blooded mammals. and birds, figuring anapsids represented by the turtle
>clan to be pure cold bloods. (Guess my mind on that matter is still 25 years
>behind the times) That would be that ANAPSIDS themselves once had the warm
>blooded ability to brood their eggs, BEFORE either the synapsid or diapsid
>split. Now, Dr. Holtz, you may not agree with that either, but it`s the only
>thing that makes sense to me at this point.

It doesn't matter so much if I agree with it, or that it is the only thing
that makes sense to you: what matters is the evidence.

>At any rate, I think it deserves
>a closer look. ( I still don`t think  diapsids could have developed warm
>from a cold-blooded condition, if the synapsids already had it, and
>dominated the landscape).

Again, why not?  Do you consider the possibility that endotherms might not
dominate in all climate conditions, for example; or the possibility that a
mass extinction of therapsids might have "opened up" the "endothermic niche"
for diapsids; or that some other physiological difference between therapsids
and archosaurian diapsids may have been favored?  There are many
possibilities here.

And from Tom Hopp:

>    By the way -- to Tom Holtz -- wouldn't it be a shock if skunk cabbage DID
>use a homologous uncoupling protein to generate its heat?

That would indeed!

>    That suggests another unknowable idea -- what if some form of endothermy
>truly is very ancient?  Would there ever be reasons for endothermic creatures
>to evolve TOWARD an ectothermic condition? I'll bet there are.  Maybe
>endo/ectothermies are just options that come and go depending on lifestyle
>requirements.

I would agree with this whole-heartedly: the idea that endothermy is in all
ways superior to ectothermy doesn't seem to be as strong as some might
think.  However, *demonstrating* that such a transition has actually
occurred in Earth history is another matter entirely.

Hope this helps, and keep on looking.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661