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Re: Defining Ornithischia (was Re:)
On 9/19/07, David Marjanovic <email@example.com> wrote:
> > A taxonomic definition may include a character or suite, and is in fact
> > suggested in the PhyloCode for purposes of trait-based definitions:
> > "Wing-powered flight in *Vultur gryphus*," for example. Thus, "Loss of
> > calcaneal spur" can be argued to be a valid point in a definition of a
> > taxon
> > name, without _being_ the diagnosis, even if it were also part of it.
> It can be part of a phylogenetic definition, yes (and would automatically
> also be part of the diagnosis of the same clade).
Actually, it's not necessarily part of the diagnosis: spheniscids,
ratites, hesperornithids, etc. are avialans that do not possess
"wing-powered flight" (unless you accepted spheniscid swimming as
"flight", but that seems to be a stretch).
Also, just for the sake of argument, suppose someone defined
"Quadrupedes" as Clade("obligate quadrupedality" in _Testudo graeca_).
(This would presumably be equivalent to an apomorphy-based
_Tetrapoda_, or "_Apo-Tetrapoda_".) "Obligate quadrupedality" would
not be part of the diagnosis for this clade, not only because there
are members which are not obligate quadrupeds (e.g., present company),
but also because some members (sauropods, many ornithischians) do not
possess "obligate quadrupedality" synapomorphically with that in _T.
graeca_. In those clades, the trait re-evolved.
Or can a diagnosis include characters that are only present in some of