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Re: Archosauromorph classification & Thecodonts



Ken Kinman wrote,
>     I would be interested to know some paraphyletic groups you would
advocate.


     Within the Archosauriformes, probably none, although I don't yet know
the group in nearly as much detail as I would like.  Just glancing at
Parrish's (1993) cladogram (I haven't read the paper yet), perhaps something
like Suchia excluding the Crocodylomorpha  and Aetosauria, which would group
most of the traditional "popsaur-rauisuchians", mostly large bodied,
terrestrial, quadrupedal carnivores, together as a paraphyletic group.
Possibly even something like Archosauriformes excluding crocs and aetosaurs
as well as the proterosuchids, phytosaurs, and Ornithosuchia/Ornithodira.  A
grouping like that would be tracing a plesiomorphic lineage of mainly
quadrupedal terrestrial carnivores that gave rise to some derived bipedal,
herbivorous, and aquatic members.  Again, I don't yet know the archosaurs
well enough to start proposing morphologically and ecologically useful
paraphyletic groups, just to question the utility of one big lumpy one in
particular.
    Smaller paraphyetic groups are probably useful more commonly then big
ones, particularly paraphyletic genera.  One example might be Gordon Bell's
phylogenetic analyses of mosasaurs, which show several species which have
been traditionally grouped as a single genera forming paraphyletic groups.
One small form in particular, Clidastes, has its species occupying a basal
position to the Mosasaurinae as consecutive outgroups to a clade composed of
several very distinctly different genera, mostly distinctly larger forms
with more impressive skulls and dentition.  Clidastes has been suggested as
an ancester to some of these forms in older papers.  It seems a shame to
ignore the morphological and possibly ecological distinctness of the
paraphyletic group we call "Clidastes" just because it was unfortunate
enough to give rise to different forms.
     The argument is often made that cladistic analyses cannot show
ancester-descendant relationships, but I disagree.  I would argue that
cladograms can give ancestor-descendant relationships as well supported as
any other hypothesis on how its taxa are related, on the basis of paraphyly.
Code individual species, and a paraphyletic genus suggests an ancestral
relationship to the clade up the tree.  Coding as many individual specimens
as possible from the widest geographic and stratagraphic range possible, it
should even hypothetically be possible to show an ancestor-descendant
relationship at the species level.
     The problem is that your "ancestor" has to be a subjective grouping of
some kind.  You subjectively group your individuals into species, or species
into genera (depending on what you are using as your OTU) on the basis of
morphology before your analysis; but all cladistic analyses, except those
that code individual specimens, have to do that anyway or they wouldn't have
any OTUs.  The difference is that you don't break down these groupings just
for the sake of monophyly after you conduct the analysis and make your tree
(unless your group turns out to be POLYphyletic, as with Halisaurus
sternbergi being taken out of Clidastes in the case of mosasaurs).  The
genus level can be a pretty clear cut case of a paraphyletic group being far
more useful then a monophyletic one, in spite of the subjectivity involved
in deciding where to cap it off.
    The only non-subjective ancester-descendant relationship would be one
where the single individual that gave rise to a descendant clade could be
identified, something is unlikely in the extreme to ever be determined in
paleontology for obvious reasons.
      Aren't you sorry you asked?

LN Jeff

You have to study a great deal to know a little.
-Baron de Montesquieu

They may forget what you said, but they will never forget the way you made
them feel.
-Carl W. Buehner
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Jeffrey W. Martz
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