RE: Sereno's (2005) new definitions

["Jaime A. Headden" <qilongia@yahoo.com>]

Tim Williams (twilliams_alpha@hotmail.com) wrote:

<Here's the problem...  If a taxon is deemed to be a nomen dubium it becomes an
invalid OTU, and so must be excluded from phylogenetic analyses.  Therefore,
its exact position in a phylogeny cannot be determined.  I would think this
would mean that the taxon in question would be invalidated as a specifier. 
Let's look at _Deinodon_.  Although the hypodigm for _Deinodon_  is limited to
the type teeth, we can still be pretty sure that _Deinodon_ is a derived
tyrannosaurid.  But we cannot demonstrate this hypothesis phylogenetically.  To
me, the usefulness of _Deinodon_ as a specifier is invalidated as a result. 
The same would be true for _Troodon_ if the hypodigm is limited to the type
teeth.  This problem could be sidestepped by designating an individual specimen
of _Troodon_ as the specifier - like the type for _Stenonychosaurus inequalis_
(currently referred to _T. formosus_).>

  I am with Tim on this. I have specific data that questions the validity of
referring the postcrania, but these are specific and related to in prep work.
However, the fact is, if two type species lack corresponding bones, one whould
not consider them synonymous, and this may be compounded by variation among
similar taxa that would relate any variation to specific variation, but it is
yet impossible to use individual variation to test this. Some taxa, unlike say,
bred dogs and even humans, exhibit remarkable consistency of form, especially
for a given area. Monitor lizards of the genus *Varanus* have been noted to
exhibit so much variation there are several generally accepted "subgenera", and
the "genus" *Megalania* is likely a synonym of the same species group that
includes large Australian monitors and the Komodo dragon, but is not synonymous
with the group containing the Savannah or Nile monitors, odd as it may sound.
This argues that many taxa generally used to refer to species lumping, may in
fact be over lumped, and the taxon *Varanus* would not reasonably correspond to
a "genus". Invention of the subgenus to help resolve large species groups seems
annoying when these additional names are ignored in favor of "genus/species"
clarity. It's all in the resolution, mates. Specimens referred to North
American troodontids appear to exhibit much variation in the cranium, enough
perhaps to indicate species variation, and this may be reflected in the
taxonomy, for which many names are used. As Tim said, the holotype tooth of
*Troodon formosus* is not directly comparable to the types of any other North
American troodontid except for *Pectinodon bakkeri*, which is very distinct in
form to *T. formosus*. Contra Marjanovic's statement about correspondence of
the teeth with those of *Saurornithoides*, only *P. bakkeri* is closely
corespondent, that of *T. formosus* is not, arguing we would need to find _in
situ_ teeth identical to the type to more directly infer it's nature. This is
currently not possible. Only two mandibles with _in situ_ teeth are known from
Dinosaur Provincial Park, and I would argue that of these, only one is similar
enough to suggest similarity to *T. formosus*, while the other has distinct
teeth and distinct mandibular morphology. Thus I would argue that *T. formosus*
is a nomen dubium, and is not a viable anchor taxon for phylogenetic
definitions. In the interests of stability, I would use the most comprehensive
taxon known now for which other names have been proposed to include it and
allies, and that is *Saurornithoides mongoliensis*.

  I actually favor the classical correspondence of the use of Troodontidae,
Dromaeosauridae, their anchoring except as noted above, and except to use birds
as external specifiers for each. It is not neccessarily to get mammalian on
topology-based definitions. Tetanurae should be used only to include birds but
exclude carnosaurs in the broad sense (i.e., that of Padian, Hutchinson and
Holtz, 1999), rather than get pedantic about their exclusion. I agree that
*Carnotaurus* should be used in Ceratosauria, Neoceratosauria, Coelophysoidea,
Abelisauria, Abelisauroidea, and Abelisauridae definitions, since it's use is
less ambiguous than the eponymous *Abelisaurus*, and it's position is more
secure than the latter, which may actually be arguably questionable (and was
suggested with affinities with carcharodontosaurs, though that was to actually
infer carchs were abelisaurs...). While we should try to use eponyms as taxa,
this may actually be problematic for some taxa, as the original taxa for which
the names are formed are often less well known than is ideal: there is only 1
specimen each of *Ornithomimus velox,* *Oviraptor philoceratops*,
*Dromaeosaurus albertensis*, and each is hardly complete or comprehensive in
the material known (I may be wrong about *O. velox*, though if I recall the
additional specimens may be referred from other lumped taxa). Since
Deinonychosauria was not named for *Deinonychus* it is certainly reasonable to
allow it to be based on *Velociraptor*, which may also be a senior synonym
(though I would argue their separation).

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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