re:re:(final?):Reptilia

[@uga.cc.uga.edu:Schwimm@USCN.BITNET (Paleontology Columbus College, Georgia)]

---Just a few additional comments addressed to Stan Friesen: I
believe, in general, we should amicably agree to disagree on the
amount of paraphyly reasonable within a vertebrate class.
                     _____________

>>>>"Thus, a Reptilia composed of "unimproved" basic stem
amniotes is a good taxon.  It also implies that Mammalia and Aves
should be distinguished at the same taxonomic level as
Reptilia."<<<<

This would be OK if the term "reptile" was generally understood
as a synonym for "stem amniote." It is not, and that is the main
reason I believe the term is obsolete (see also last para.).


>>>>"For the dinosaurian portion, this is less clear (that they
are far removed from stem amniotes).  Dinosaurs
share many general features with other diapsid reptiles,
especially other archosauromorphs."<<<<

Well, they have diapsid temporal fenestrae, but there are so many
dinosaurian/avian autapomorphies (vs., say, lepidosaurs, and most
Permian diapsids) that we can easily come up with valid
diagnostic criteria.  Of course dinos share a suite of diapsid
plesiomorphies as do all descendant groups.
   Remember, I'm not pleading for pure monophyly but for
_reasonable_ paraphyly when necessary.


>>>>"If I did split off the Dinosauria as a class, I would
include the advanced meso-tarsal thecodonts (Lagerpeton,
Lagosuchus, and so on), as well as the pterosaurs.

That wouldn't bother me.  I'd probably expect the crocodile-
normal "archosauromorphs" to remain with the Diapsida: that's how
I present it to my classes now.  Accepting paraphyly requires a
line to be drawn somewhere;  we're just debating the place.


>>>>"Trying to set up a narrower amniote stem-group than Reptilia
would tend to make for a large number of rather small classes
with only rather minor differences.  For instance, typical
pelycosaurs (stem synapsids) are generally very lizard-like
in morphology.  Only the more specialized forms, like
Pelycosaurus and Dimetrodon, vary much from this core
morphology."<<<<

I'm no expert on primitive synapsids, but I believe even the base
of the group shows the characteristic temporal fenestra and
differentiated teeth.  Given that these are old fossils, we have
no reason to assume they didn't have loads of soft-tissue
differences. Again, we agree they have common ancestry with the
diapsids and we're only arguing the point of establishing a
taxonomic break.



>>>>"I maintain it (Reptilia) makes a very good amniote stem
group, to be defined as "unimproved" amniotes - forms with a low
metabolism, generally sprawling or semi-improved gait, and so on
- a group adapted to habitats with limited food availability,
where energy budgets are critical.<<<<

Here's the crux: if your definition above were generally
understood and accepted, I would be in complete agreement.  But
we have most people (amateurs and paleontologists too) calling
dinosaurs, advanced synapsids, pterosaurs, and turtles
"reptiles," the same term applied to your "unimproved aminiotes;"
and these forms don't have those characteristics.
                   _____________________

I guess we've reached the limit of discussion on this topic.
Let's move on to something else worth a debate.

   Anyone care to share opinions on whether or not mosasaurs were
viviparous?

David Schwimmer
schwimm@uscn.cc.uga.edu