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Re: Coelurosaurs (was segnosaurs)



In a message dated 96-03-04 12:05:34 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes (replying to my post):

>> 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.

That's not exactly the definition you gave earlier, which is "_Allosaurus_
and all theropods closer to _Allosaurus_ than to birds." The latter is
stem-based, whereas the preceding definition involving the "more recent
common ancestor" is node-based. Anyway, that's certainly ONE way in which the
taxon Carnosauria could be rendered meaningless: by becoming a synonym of a
sole included member! This would happen if it were to turn out that
_Allosaurus fragilis_ were itself a member of the bird clade. Then there
would be NO taxa "closer to _Allosaurus_ than to birds" (even INCLUDING
_Allosaurus_ itself, which gets into the taxon by fiat only!). This is highly
unlikely in the case of _Allosaurus_ and birds, of course--but the
theoretical possibility exists.

Another way a phylogenetic-based ersion of Carnosauria could become
meaningless I if birds were (shudder, gasp) found not to be theropods after
all, but were a parallel radiation. Then "all theropods closer to
_Allosaurus_ than to birds" would literally cover ALL theropods, and
Carnosauria would be rendered a useless synonym of Theropoda. Again, I don't
think this is very likely(!), but the theoretical possibility exists.

But it's really no big deal. So a clade is meaningless. So what? Taxonomy is
littered with meaningless taxa. The point I want to stress is that these
"formal definitions" are quite contingent upon the phylogeny developed to
support them, and if the phylogeny is dumped, some of the definitions will go
with it.