Phylogenetic taxonomy

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

>These formal definitions and such are all well and good, I suppose, but we
>should always keep in mind that the ICZN (International Commission on
>Zoological Nomenclature) does not govern taxonomic categories at levels
>higher than those with the term "family" in their names (i.e., superfamily,
>family, subfamily; and for that matter, it doesn't address such terms as
>gigafamily, megafamily, hyperfamily, etc., either). There is as yet no
>consensus within the group (that I know of) about how to deal with the
>multitude of names and levels that cladistic taxonomies have presented us
>with, so using a term like "formally defined" for such groups seems a bit
>inappropriate. It carries an aura of officiality that is in reality still
>absent.

Okay, insert "(within the principles of phylogenetic taxonomy)" after
"formally defined".  These rules HAVE been spelled out, and within that
system (phylogenetic taxonomy) are formal. As stated above, the ICZN does
not govern these.

>There was a time when Carnosauria was defined in such a way as to include
>_Tyrannosaurus_ as well as _Allosaurus_, but we are now pretty sure that that
>definition gave us a polyphyletic group. So that particular definition of
>Carnosauria is no longer acceptable. In the future, it may turn out that
>_Allosaurus_ is related to birds in a different way from what we now think,
>which could likewise render the above "formal definition" of Carnosauria
>meaningless.

No, a phylogenetic definition can never be rendered "meaningless", so long as
the definition is based on monophyletic points. If we except evolution, then
(except in the cases of polytomies greater than dichotomies) the three taxon
statement will always be true: A shares a more recent common ancestor with B
than either does with C. Always.

The content of a phylogenetic-based version of Carnosauria will change, the
synapomorphies will change, but there will always be a clade composed of all
taxa sharing a more recent common ancestor with _Allosaurus_ than with
birds, even if that taxon is just _Allosaurus fragilis_ itself.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661