Message 2004-06-0042: Re: First International Phylogenetic Nomenclature Meeting

Wed, 16 Jun 2004 14:07:58 -0700

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Date: Wed, 16 Jun 2004 14:07:58 -0700
From: Mickey Mortimer <>
Subject: Re: First International Phylogenetic Nomenclature Meeting

David Marjanovic wrote-

> ----- Original Message -----
> From: "Mickey Mortimer" <>
> Sent: Wednesday, June 16, 2004 4:45 AM
> > > > Diapsida Osborn 1903 = Apomorphy (1st reptile with
> > > > Caiman crocodilus' two temporal arches/fenestra).
> > >
> > > That name _really_ cries for an apomorphy-based definition.
> > > The apomorphy is very unambiguous (unlike "powered flight"
> > > or "feathers"), and fossils around the base of Diapsida often
> > > include skulls, so I don't see a serious problem here.
> >
> > M[]ller (2003) suggests the lower temporal arch of some saurians
> > (rhynchocephalians, turtles, placodonts, choristoderes,
> > rhynchosaurs+archosauriformes, trilophosaurs) is not homologous with
> of
> > basal diapsids like Petrolacosaurus and Youngina.  If this is true, only
> the
> > Archosauriformes + Rhynchosauria node would be Diapsida according to
> > Gauthier et al.'s definition, the lower arch in other clades being
> > convergent.
> > M[]ller, J., 2003, Early loss and multiple return of the lower temporal
> > arcade in diapsid reptiles: Naturwissenschaften, v. 90, p. 473-476.
> Well. The lower temporal arch of (Archosauriformes + Rhynchosauria) is not
> homologous with those of the basalmost diapsids. The fenestrae are,
> even though the lower one was open for a long part of Permian diapsid
> history (and still is in over 1/3 of living diapsids) due to the absence
> the arch ventral to the fenestra.

And that's why we can't use apomorphy-based definitions.  They'll just
require constant revision (Gauthier's definition includes 2 temporal
arches), and we never can be sure of homology.  If we have a case where an
apomorphy is present in taxa A and B, but not C or outgroups, and the
topology is (A(B,C)), it's a subjective choice where to place the clade.
Say the apomorphy-based definition states it has to be homologous with B.
Under DELayed TRANsformation in PAUP, it would be optimized as convergent in
A.  So A wouldn't be a member of that clade.  Under ACCelerated
TRANsformation in PAUP, it would be optimized as reversing in C.  So A and C
would be members of that clade.  This is what Phylocode was designed to stop
people!  Ambiguous and subjective placement of clades.  WHAT is the
rationale for allowing this to continue, just so that the basalmost diapsid
is assured to have a diapsid temporal arch?  Names are just names.  They
don't need to describe the clade perfectly.  Lord knows most don't already.

> > > > Rhynchocephalia Guenther 1867 = Apomorphy (1st lepidosaur with
> Sphenodon
> > > > punctatus' premaxillary chisels).
> > >
> > > Probably similarly unambiguous. I just hope we can use *Sphenodontida*
> or
> > > suchlike for the stem. :-)
> >
> > I'm guessing they are derived from fused teeth, right?  Somewhere, a
> > existed with slightly fused teeth, resembling Sphenodon's morphology
> > somewhat.  We'll just need to keep altering the definition if we keep it
> > apomorphy-based.
> Not if the "premaxillary chisels" are sufficiently described &

It's going to be ambiguous at some point.  Characters develop gradually.
Look at the sternal keel in Confuciusornis sanctus- it's polymorphic in the

> > > Oh, they aren't even the problem. If he's right, then *Pterosauria*
> _does
> > > not exist_. Why? Because *Pterosauromorpha* is defined as a part of
> > > *Archosauria* -- and at least *Longisquama* is most likely not an
> > > archosaur.
> >
> > So according to Phylocode, if you start a definition by saying "members
> > clade x that are...", and if the clade you define isn't a member of
> x,
> > then the taxon is invalid?
> This is not regulated. In this case, I applied logic. If pterosaurs are
> archosaurs, then there are no archosaurs that are closer to *Pterosauria*
> than to *Dinosauria*, which means that *Pterosauromorpha* self-destructs.
> And *Pterosauria* is defined as a part of *Pterosauromorpha*; if the
> doesn't exist, the former can't exist either.

I think it would be a good thing to regulate, IF people were smart enough to
use it well.  If you want Ornithodira to only be an archosaurian clade, and
not expand if pterosaurs were more basal archosauromorphs, for instance.
Then you could say "Ornithodira = (Archosauria which fall under
Pterodactylus antiquus + Megalosaurus bucklandii)".  But if people are going
to go assuming their phylogeny is right (*cough* Sereno *cough*), we'll end
up with a series of self-destructing clades.

>> Of course, this is also probably a
>> transformational suite for which the series is not well documented.
> Either there is a hole, or there is none. Or so I think.

You're not thinking ahead.  Eventually, we may find basal forms with a slit,
or depression that sometimes perforates the bone.  Maybe polymorphically in
a species, or even an individual.

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database -


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