RE: Coelurosaur phylogeny

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Mickey Mortimer
>
> While I agree it is preferrable to have a written definition for a clade
> (diagnoses are comparatively useless due to their lack of stability), it's
> not an official rule that this must occur before a clade name is used.  I
> fail to see the difference between indication of content in a
> figure versus
> in text.  Surely one is as good as the other.

Not as such.  Specifically, a label on a figure does not state the
definition, but leaves open several different alternatives.  It does not
specify if that name is node-based, stem-based, apomorphy-based, or
content-based.  It does not specify the anchor taxa or "anchor characters"
or required content.

>  Though I don't have the
> reference in question, I'm betting Naish et al. (2001) indicate that
> segnosaurs and oviraptorosaurs are in the Enigmosauria, while
> tyrannosauroids, ornithomimosaurs, troodontids, dromaeosaurs and birds are
> not.  Further definition is not imperative at this point, the
> only agreed on
> "stem enigmosaur" being YPM 1996/1997.  If Enigmosauria is
> node-based, it's
> their sister group.  If it's stem-based, it's the basalmost member.

But such differences ARE imperative, as the stem-based and node-based taxa
are different taxa with different ancestors.  Okay, they KNOWN membership
might be the same, but the goal of phylogenetic taxonomy (itself a more
inclusive term than just the PhyloCode: I know of many PT practitioners who
are not pleased with various aspects of the PhyloCode, and dread it as the
potential downfall of PT) is a phylogeny based on statements of ancestry.
As such, a stem-based and node-based name are necessarily different taxa, as
they have different ancestors.

Futhermore, without a text statement of definition, how would we know what
happens to "Enigmosauria" when (for a hypothetical example) it turns out
that oviraptorosaurs are closer to eumaniraptorans than to
therizinosauroids?  If it is a node-based name anchored on _Oviraptor_ and
_Therizinosaurus_ (for examples) then Enigmosauria also includes
Eumaniraptora; if it is a stem-based name anchored on _Oviraptor_ (say "all
taxa closer to _Ovi._ than to _Passer_") then it excludes therizinosauroids
in this topology; if anchored on _Therizinosaurus_ then it excludes
oviraptorosaurs.

This is why text matters.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796