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re:re:(final?):Reptilia



From: "Paleontology Columbus College, Georgia" 
<@lepomis.psych.upenn.edu,@uga.cc.uga.edu:Schwimm@uscn.bitnet>
 > ---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 reason I believe the term is obsolete (see also last para.).

Well, that is how *I* understand it, and I think more people would
understand that if the cladists hadn't ruled out paraphyletic taxa.

That definition follows fairly immediately from its scope, once
one admits that paraphyletic taxa are as valid as cladistically
(hyper-)monophyletic.

[Note, I prefer the older definition of monophyletic that includes
paraphyletic, but there is no chance now to return to that
definition.  Of course that means we need a new term for
"cladistically monophyletic"+paraphyletic, as there is currently
no term for that condition].
 > 
 > >>>>"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.

The issue is not whether we can come up with valid diagnostic
criteria.  The issue is whether the *scale* and *scope* of the
differences are comparable to the differences amoung the other
classes.  At any given taxonomic level, the subtaxa should be
more or less equally distinctive - that is they need comparable
levels of difference.

 > Of course dinos share a suite of diapsid
 > plesiomorphies as do all descendant groups.

And since they have the diapsid plesiomorhic state for over half of
the avian autapomorphies, the dinosaus are closer to the reptiles
than to the birds.

The largest additional suite of new synapomorphies is the set
associated with the evolution of flight in birds, centered around
the enlarged breast bone.
 > 
 > >>>>"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.
 >

Not entirely.  I am saying that birds are sufficiently distinct
that even *if* you seperate dinosaurs from the reptiles, the
birds *still* should be maintained as a seperate class.  This
is because birds are more distinct from dinos than dinos are from
diapsids.  Thus, on the basis of equivalence of distinctions,
the birds must be distinguished at at least as high a taxonomic
level as dinosaurs.

[Note, I would tend to define birds to be restricted to the
carinate forms, which may or may not include Archaeopteryx].
 > 
 > 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.

But, in terms of adaptive anatomy, they were effectively
"lizards".  That is the critical aspect.  High level taxa
should represent broad adaptive zones, or styles.  The "lizard"
style of life is the same whether it is a true lizard, a tuatara,
a stem diapsid or a stem synapsid.

Certainly they had distinctive aspects to their soft anatomy,
so does the tuatara.  The question is whether those differences
are "significant" enough to warrant a class level distinction.

I maintain they are not.  The life styles of each of these groups
are too similar for it to be useful to seperate them.

Now, for the *advanced* synapsids, the therapsids, the issue is
different.  There is good evidence that at least the later cynodonts
had hair.  This combined with heterodont dentition and other
mammalian apomorphies tends to make at least the cynodonts more
mammalian in overall style than reptilian.
 > 
 > >>>>"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.

Well, I would say that the turtles *do* fall within the amniote
stem group.  They lack sufficiently profound adaptive changes
to qualify them for a class level distinction.

For the dinos+pterosaurs, the matter is more difficult.

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

The peace of God be with you.