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Re: Nomina Dubia Part II: Rapator



Michael Mortimer <mickey_mortimer111@msn.com> wrote:
 
> What conclusion
> would we reach about the status of Rapator?  There are
> only two possibilities- it's diagnostic compared to
> Australovenator or it isn't.  If the former is true,
> it's a valid taxon.  If the latter is true,
> Australovenator is a junior synonym.  Yet Agnolin et
> al. confusingly state "However, due to the fragmentary
> condition of Rapator and the absence of autapomorphies and
> clear differences with Australovenator, we consider the
> taxon to be a nomen dubium."


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.

  
> 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
c,
> 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.


But I agree with you that the presence of autapomorphies should not be the only 
criterion for regarding a specimen as diagnostic.  A unique combination of 
characters (apomorphies) should be sufficient (e.g., _Genyodectes_).


> In general, the paper seems confused in regard to "indet."
> and "nomen dubium".  In their Table 1, the terms are
> listed separately, with some taxa they consider determinable
> being labeled as indeterminate (e.g. Kakuru,
> Muttaburrasaurus).


Yes - these are obviously typos.  Perhaps the authors mistakenly wrote "indet." 
instead of "incertae sedis".




Cheers

Tim