Re: Dinofest Report #2 (and final)

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

Somebody (pardon me for losing the name) wrote:
>In his Saturday talk on North American oviraptorosaurs, Hans-Dieter Sues said
>his research implied that arctometatarsaly arose multiple times,
        This, of course, agrees with many workers' current interpretations
of theropod phylogeny. Various hypotheses seem to disagree on the "where and
when", varying mostly it seems with the position of Troodontidae and
"elmisaurids".

>and that Arctometatarsalia is not a valid taxon.
        Note that I assume here that the comments quoted above are an
accurate reflection of Dr. Suess' talk.
        As originally formulated, the taxon is problematic. However, given
the current phylogenetic definition (Holtz 1996), Arctometarsalia will
*always* be a valid taxon. True, the original members of the taxon may no
longer form a phylogenetic group with respect to other theropods. However,
the advent of phylogenetic taxonomy eliminates the possibility of "unnatural
groupings" (at least in theory). This is perhaps unfortunate, as taxa are
now monophyletic by definition rather than by discovery (tip of the hat here
to Dr. Holtz on the wording). However, as the process of integrating
phylogeny and taxonomy proceeds, the instructive value of the discovery of
paraphyly and polyphyly will gradually become less valuable than the
nomenclatural (although perhaps not compositionally) stability of naturally
monophyletic taxa.
        Dr. Seuss is applying a standard of evaluation which is
inappropriate given the current practice of PT. While he may perhaps choose
to retain old concepts of "taxon concept" pertaining to lower taxa included
within a taxon of higher rank, or the strict literal meaning of a taxon
name, his assertion that Arctometatarsalia is not valid is not consistant
with the intent of the namer and is thus perhaps inappropriate.
Arctometarsalia remains "valid" although the concept of all taxa possessing
an arctometarsus forming a monophyletic group exclusive of all other taxa
may not.
        His point, as he originally intended, is a good one however. It is
quite possible that the "arctometarsus" evolved more than once.

Jaime Headden wrote:

>Where arctometatarsaly is the condition of pinching mt III between II 
>and IV,occluding it from the proximal surface, and Tom's other points, 
>is indicative of an Arctometatarsalia taxon,
        However the arctometatarsus was orignially defined, it is clear that
there is a complex of characters in the coelurosaurian foot which may or may
not have evolved several times and which together shape the metatarsus of
arctomets sensu stricto. Indeed, while Dr. Holtz should be applauded for his
conservative coding of the arctometatarsus as one character (Holtz 1994), it
seems that several of these characters developed in non-arctomet taxa, and
re-examination may show that there are several different "types" of
arctometatarsi consisting of various combinations of these characters and
representing homoplastic solutions to the same functional "problem" (several
authors, hearsay).
        As to the taxon itself, as noted above nothing in the definition of
Arctometarsalia requires that included taxa possess an arctometatarsus
(although at least one, _Ornithomimus_ always will).


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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek