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re:re: "Reptilia"



From: "Paleontology Columbus College, Georgia" 
<@lepomis.psych.upenn.edu,@uga.cc.uga.edu:Schwimm@uscn.bitnet>
 > Responding to Stan Friesen:
 > 
 > >>>"Reptilia is a perfect valid paraphyletic taxon.<<<
 > 
 >     No, I don't agree.

Well, if not, it is not because it is paraphyletic.
 > 
 >     First, I'm by no means a rabid cladist.  In fact, by pure
 > cladistic logic, allowing no paraphyly, we're crossopterygian
 > fish.
 > 
 >   But, the original concept of Reptilia as generally understood
 > is in no way the modern group.

That can be said of most groups originally defined on modern forms
and then expanded by fossil discoveries (and the discovery of
relictual living forms).

>From toothed birds to egg-laying mammals, we have greatly changed
the scope of many groups.

 >  We all can agree that the
 > Synapida are far removed from all the other basic amniotes, as
 > are the turtles and (probably their ancestors) the procolophonids
 > and pareiasaurs.

In what sense?

In terms of sequence of branching?  yes, I agree, but I attach
no taxonomic significance to branching sequence.

In terms of ecological, physiological and morphological divergence?
Then I would only say that of the advanced synapsids, the
therapsids, which might be better placed in Mammalia.

My approach is to routinely use paraphyletic taxa, *especially*
for stem groups.  Thus, I would treat Reptilia as the stem-group
of Amniota, and try to split the amniotes ina  way that more or
less equalizes the range of variation within each subgroup; placing
the boundaries at points of major ecological or adaptive shifts.

This is generally necessary to keep the crown groups well
charaterized ecologically and morphologically.  Otherwise
the groups tend to be distinguished solely by a small number
of basal synapomorphies.

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.
 > 
 >   More so, the dinosaur/avian clade may be as far away
 > phyletically _and_  morphologically from the remaining "reptiles"
 > (i.e. minus turtles, other "anapsids" and synapsids) as are any
 > other amniotes.  So the term "reptile" hardly applies to them.

Well, the avian portion, certainly.

For the dinosaurian portion, this is less clear.  Dinosaurs
share many general features with other diapsid reptiles,
especially other archosauromorphs.  The question becomes whether
the improved, erect, gait of the dinosaurian lineage is a sufficient
adaptive shift to warrant recognition at the same level as the
Reptilia/Aves/Mammalia split.

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.

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.
 > 
 >   My point is, our remaining more-or-less-related core of
 > "reptiles" is so different from the original usage of the term,
 > that "Reptilia" becomes virtually meaningless.

I maintain it 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.

 > >>>"Let us take an example - birds.....How to do this?  In fact
 > how to seperate Aves at *any* taxonomic level?"<<<
 > 
 > 
 > Easy, define autapomorphies, such as presence of pygostyles vs.
 > tails, feathers, carinae or carinae as a plesiomorphy, etc. and
 > you've separated avian from non-avian dinos.  

That wasn't the intent of this question.

I was pointing out the difficulties of doing so from within
the cladistic approach.

 > The problem is
 > simply reduced to whether or not they are a Class. I think the
 > evidence suggests not.  Dinosaurias should include Aves, but as a
 > subclass; or betterm Dinosauria could be a superclass with
 > Dinosaurxxxx and Aves as Classes.  They do it all the time with
 > Aganathan "fish" (check any recent work).

The difference is that I do not believe that the agnathans
span such a wide range of adaptive types as the dinosaurs+birds.

The problem even with a superclass containing both is that this
leaves us a choice of either having two other superclasses each
having one class (a 2+1+1 distribution), or grouping mammals and
stem amniotes (aka reptiles) in another superclass.  I do not feel
that having a single non-monotypic taxon at a given level is useful,
and I see little useful commonality between reptiles and mammals to
justify grouping them against dinosaurs and birds.

 > Again, my basic argument was that "Reptile" was garbage can term
 > and it, alone, needs to be dumped as a vertebrate class.
 >     A point I make to my VP classes is that any primitive
 > (whatever that means) amniote will basically look lizard-like in
 > overall body plan, and hence "reptilian."

Yep, that is right - stem groups of major groups tend to be
morpholically similar to one another.  That is why then to make
decent paraphyletic taxa.

 > This is the basic
 > amniote gross shape: tetrapodal, elongate, long tail,terminal
 > head, etc.  This plesiomorphy of amniotes led to the concept of
 > Reptilia, which we now know is grossly paraphyletic.
 > 
So?  Any stem group will be "grossly paraphyletic".  As long
as they are well distinghuished from the derived, specialized
crown groups, such stem groups are not only acceptible taxa, they
are, in my opinion, necessary in order to place the boundaries of
the crown groups in useful places - at the point of the major
adaptive shift that produced the new specialization.
[At least unless one wants to also have lots of small, fairly
conservative taxa at the same rank as the major taxa].

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

The peace of God be with you.