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Re: Ceratosauria vs. Neotheropoda?

2009/11/22 Tim Williams <tijawi@yahoo.com>:
> Mike Taylor <mike@indexdata.com> wrote:
>> In short: taste, judgement and an
>> understanding of the historical
>> background are required.  Any fool can define a clade,
>> but doing it
>> well is not as easy as it looks.  Caveat author!
> Well said.  By the time PhyloCode is enacted, clade definitions should have 
> arrived at the point that they ensure stability, as well as reflect 
> historical usage.
> However, sometimes compromises have to be made: Dinosauria now includes birds 
> (Aves), even though this would make Richard Owen spin in his grave.  But the 
> nesting of birds inside Dinosauria is a phylogenetic reality, and so 
> overrides historical usage.  Ditto for nesting tetrapods inside Osteichthyes, 
> or putting mammals inside Therapsida.

Sure.  These are just the inevitable consequences of defining these
things as clades.  (I know I don't need to tell you this but) there is
NO clade that includes ornithischian and saurischian Mesozoic
dinosaurs but doesn't also include birds.  In situations like this,
the only options are to expand the understanding of the taxon (as with
Dinosauria, now all-but-universally understood to include birds), or
to draw back and not assign a clade definition of the name at all
(which, happily, sees to be the way we're headed with the paraphyletic
group Reptilia).

Oh, and of course option #3: define the groups as paraphyletic using a
formal phylogenetic definition.  I know a lot of people have religious
objections to this, but I'm not one of them.  For example, I'd be
perfectly happy with Proasauropoda = (Sauropodomorpha - Sauropoda)
where Sauropodomorpha = (Saltasaurus not Allosaurus) and Sauropoda =
(Saltasaurus not Melanorosaurus).