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RE: Nomina Dubia Part II: Rapator
Tim Williams wrote-
> I think what the authors are saying is that the characters used to
> distinguish _Rapator_ from _Australovenator_ might be accommodated by
> individual variation, and are therefore unlikely to be apomorphies. Or to put
> it another way, the authors are hesitant to embrace the features seen in the
> _Rapator_ holotype as apomorphies.
> Thus, if we had a dozen specimens of _Australovenator_, the amount of
> variation seen in metacarpal I might suggest that certain traits (such as the
> degree of development of the distal medial condyle) might vary from one
> specimen to the next. Therefore, the assumption (or presumption) is that such
> traits are not phylogenetically informative, and therefore inadequate to
> establish a genus or species on.
> It's true that we know nothing about morphological variation in the
> metacarpal bones of individual megaraptoran species. Nevertheless, we know
> from other species known from multiple specimens that certain traits can vary
> enormously from one specimen to another - such as the shape and orientation
> of horn cores; or the length and depth of a sulcus; or the degree of fusion
> of certain hindlimb elements. On that basis, I can understand why many
> paleontologists are reluctant to accept "subtle differences" in the shape of
> a wrist bone as diagnostic characters at the level of genus or species.
I agree the differences could be due to individual variation. While we don't
have any megaraptoran sample sizes large enough, I bet Allosaurus would be
useful in this regard.
>> Put bluntly, a taxon can't be a nomen dubium if it's only
>> undiagnostic compared to one other taxon! In that
>> case, it's a synonym. It has to be undiagnos
>> since then we couldn't tell which taxon it came from.
>> If Agnolin et al. didn't want to sink the more complete
>> Australovenator into Rapator, they could have relied on the
>> differences they noted. But if they don't think those
>> differences warrant such a separation, then to be honest
>> they'd have to synonymize the taxa. But you can't have
>> it both ways.
> I don't think the situation is quite as black-and-white as you make it out to
> be. Because we know so little about how much morphological variation can be
> sustained within a species, such as in the detailed anatomy of a metacarpal
> bone, I think this is a gray area when it comes to certain specimens (e.g.,
> the _Rapator_ holotype). The authors are obviously taking a conservative
> approach in regarding _Rapator_ as a nomen dubium.
My issue with that is that a species of uncertain validity vs. another is NOT a
nomen dubium. Or else we'd be calling Archaeopteryx seimensii, Wellnhoferia,
Cryptovolans, Microraptor gui, Nanotyrannus, etc. nomina dubia. But nobody
does this. They're merely labeled as junior synonyms or valid species,
depending on the author. Another point against calling Rapator a nomen dubium
because of this is when you have two or more taxa that may be synonyms, which
one gets to be the nomen dubium? Rapator is just as distinctive as
Australovenator according to Agnolin et al. and has decades of priority.
Shouldn't Australovenator be the questionably valid name since it was based on
a specimen that could not be definitely distinguished from a previously named
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