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Re: The Unbearable Consequences of Undiscovered Phylogeny [was: Re: "The X Digit"...a Pteroid bone??]

On Mon, 25 Jan 1999, Jonathan R. Wagner wrote:

> T. Mike Keesey wrote:
> >Man, this would wreak havoc with the content of some clades -- Ornithodira
> >{Pterosauria + Dinosauria} would include pseudosuchians, _Euparkeria_,
> >etc. (not to mention prolacertiforms). Dinosauromorpha 
> >{Dinosauria > Pterosauria} would be come virtually identical  to
> >Archosauriformes.
>         C'est la vis. At least one of these taxa, Ornithodira was first
> named as a phylogenetic taxon (AFAIK). Thus, while *you personally* may have
> some preconcieved notions about what it should contain, there is no problem
> with access to the literature, because there is no pre-PT literature on the
> taxon. Once there is no problem with pre-PT literature, we all lose the
> right to bellyache. As George makes clear (from his own way), if you can't
> deal with this sort of problem, you might want to drop PT. ;)
I suppose you're right.
>         BTW: I see you use the new Buchholz-Wagner shorthand.
Yes, I finally caved in. See

> >and refinement of the definition of Dinosauromorpha {Dinosauria > 
> >Pterosauria,
> >Pseudosuchia} might be a good idea. 
>         Well, you know my feelings on this. I'm always up for multiple
> exclusive anchor taxa for stem-based definitions.
> Unfortunately, we have no means currentlyu available to alter an
> established definition.
There should be. If only the ICZN would expand to cover phylogenetic
taxa... appeals could be made to refine definitions, much as they are made
to appoint new type specimens. Really, though, definitions should be made
very explicit to begin with. All too often a definition rests on the
assumption that the cladogram accompanying it is the correct one, and when
that turns out to be false... ugh. The person defining the clade should
take every (reasonably) possible phylogeny into account.

> I'm not sure if Dinosauromorpha is pre-PT or not 
> (I doubt it). If it were, you could at least make a case for this. 
I don't think it is.

Another thing: I know it is against the "rules" (not that there is a
formal list of rules for phylogenetic taxa, TMK) to define a stem-based
taxon with multiple included taxa {A, B > C}, but mightn't this style of
definition be a good thing? For example, imagine that Ornithodira were
defined as {_Corvus_, _Pterodactylus_ > _Crocodylus_}. It would remain a
valid clade if the topology were (_Cr._, (_Co._, _P._)) (as is now
commonly held). But, if Peters is right, and the topology is 
(_P._, (_Cr._, Co._)), Ornithodira would become an invalid clade. What's
wrong with that? (Note: I'm not trying to propose anything as radical as
changing the definition of Ornithodira to this. I'm just saying it would
be nice if this were the way it had originally been defined.)

Perhaps a better example: I'd like to see Prosauropoda defined as
{_Anchisaurus, _Plateosaurus_, _Melanorosaurus_ > _Diplodocus_}. Thus, if
the people who say prosauropods and sauropods are sister groups are right,
it is a valid clade. If, instead, "prosauropods" are a parphyletic
assemblage ancestral to sauropods, "Prosauropoda" becomes invalid. I don't
see a downside.

--T. Mike Keesey                                   <tkeese1@gl.umbc.edu>
THE DINOSAURICON                http://www.gl.umbc.edu/~tkeese1/dinosaur