Re: Eponymous Taxa [was: RE: new _Scleromochlus_ ref]

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

John R. Hutchinson wrote:
>Yes, Adam is correct (oops, I missed his earlier post on this).  Defines
>Avesuchia as "a node-based taxon consisting of Avemetatarsalia and
>Crurotarsi and all their descendants."
        OK, AFAIK, Crurotarsi is a node-based taxon currently believed to be
within Pseudosuchia, and Avemetatarsalia is a stem-based taxon. As far as
the practice of phylogenetic taxonomy goes, this definition is... well...
unpleasant. Use of higher taxa in phylogenetic definitions may potentially
result in logical contradiction (except node-in-node definitions and
node-stem triplets, but in the both cases, synonomy may alter the
definitions of reference taxa and cause the taxon to become contradictory).
In this case I don't forsee that occurring. Then again, if the definitions
of Crurotarsi or Avemetatarsalia were to be changed for some (presumably
good) reason, or these taxa were affected by synonymy, a logical
contradiction might develop.
        Given the currently accepted phylogeny, Avesuchia is, of course, a
subjective junior synonym of Archosauria (definition of Gauthier accepted by
priority, since attempts to more fully capture the content of the
typological taxon Archosauria will inevitably fail when more basal taxa
fitting the original taxon concept are recovered).

>Avemetatarsalia = "all avesuchians closer to Dinosauria than to Crocodylia."
>(p.1440)
        I sure wish folks wouldn't use the "all members of taxon X closer to
blah blah blah" formulation. If read literally, it seems to contstrain a
taxon with an if-then statement which is not a part of PT as originally
concieved (and arguably should never be a part of it, since the clade exists
regardless of what other clades it is a member of). This does work for
node-stem triplets, but only if synonymy doesn't disrupt your NST or its
definitions. In general, it is unnecessary, and potentially confusing,
especially so if people decide (as some have on this list) that a change in
phylogeny which makes such a definition nonsensical obviates the definition
entirely.
        Again, use of higher taxa as reference taxa not only may result in
logical contradictions, but makes the taxon vulnerable to the vagaries of
phylogenetic synonymy (that they might otherwise be vulnerable to the
vagaries of traditional synonymy is bad enough...).

>I'm not a big fan of Ornithosuchia as the name for the sister stem to
>Pseudosuchia within Archosauria either.
        Actually, I'm pleased with the name itself, but the association with
a previously named genus which may be outside of the definition bothers me.

>However it is worth pointing out
>that the phylogenetic position of Ornithosuchidae is far from resolved. [...]
>I'd say that a "consensus" placement for Ornithosuchidae is in a trichotomy
>with Crurotarsi + Ornithodira/Avemetatarsalia.
        In other words, Archosauria incerta sedis, in a trichotomy with
Pseudosuchia and Ornithosuchia. What a bloody headache! Anyway, given
DeQuiroz and Gauthier 1992, I'd say there is clear cause for redefining
Ornithosuchia, perhaps as "_Ornithosuchus_ and all taxa sharing a more
recent common ancestor with _Ornithosuchus_ than with _Crocodylus_." This
would make Avemetatarsalia a potential subjective junior synonym of
Ornithosuchia, or it would be a valid taxon name and Ornithosuchia would
become a more exclusive taxon within Pseudosuchia.

>Avemetatarsalia: nope, no other published PT name available (except our
>whipping boy Ornithosuchia).
        Actually, I was hoping for a non-PT name which it could be argued
was better established than Ornithosuchia. Then one could pull a "Padian,
Hutchinson and Holtz", saying that there was no cause for the naming of a
new taxon, and be rid of Avemetatarsalia. Oh well...

        Summary (* = nodes, no * = stems):

        *Archosauria (= Avesuchia)
          Pseudosuchia
          Ornithosuchia (as redefined above, _incerta sedis_)
          Avemetatarsalia
                *Ornithodira
                  Pterosauromorpha (I think this is the stem here...)
                  Dinosauromorpha (or is it -iformes?)
                        *Dinosauria
                          Ornithischia
                          Saurischia
                                *Eusaurischia
                                  Sauropodomorpha
                                  Theropoda
                                        *Avetheropoda (= Neotetanurae)
                                           Carnosauria
                                           Coelurosauria
                                                *Eumaniraptora
                                                  Deinonychosauria
                                                  Avialae
                                                        *Aves (stems unclear)
                                                                *(node unclear)
                                                                  Enantiornithes
                                                                  Euornithes
        Wagner

        P.S. The above is not meant to imply that I am the sister taxon to
the Archosauria! :)


--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi