RE: Nomina Dubia Part II: Rapator

[Tim Williams <tijawi@yahoo.com>]

Michael Mortimer <mickey_mortimer111@msn.com> wrote:

> No, my point is that the situation is the same for these
> taxa as for Rapator.  Microraptor gui has a prominent
> biceps tubercle on the ulna and a fused sternum, etc. but
> are these enough to distinguish it from M. zhaoianus? 
> Archaeopteryx siemensii has no pedal flexor tubercles and no
> cuppedicus fossa, etc. but is that enough to distinguish it
> from A. lithographica?  Rapator has a ventrally
> extended medial condyle and straight lateral distal condyle,
> etc. but is that enough to distinguish it from
> Australovenator?  


In all three cases (_Microraptor, _Archaeopteryx_, _Rapator_) I would say 
"perhaps not".  However, for _Microraptor gui_ and _Archaeopteryx siemensii_ 
there are enough characters (apomorphies) elsewhere in the skeleton to justify 
referral to _M. zhaoianus_ and _A. lithographica_ respectively as junior 
synonyms.  


But if _M. gui_ was known *only* from an ulna, and its only diagnostic 
character was the more prominent biceps tubercle, _M. gui_ would probably be a 
nomen dubium.  


> These all have features that
> potentially let them be diagnosed as different
> species/genera.  I happen to think the features of M.
> gui and A. siemensii are just individual variation, but that
> doesn't make them nomina dubia to me, it makes them synonyms
> of M. zhaoianus and A. lithographica.  


Yes, exactly.  But as I said above, the rest of the skeleton provides 
phylogenetically informative characters that allow _M. gui_ to be referred to 
_M. zhaoianus_, and _A. siemensii_ to be referred to _A. lithographica_.  For 
example, the _A. siemensii_ and _A. lithographica_ specimens share enough 
characters that are diagnostic at the genus- or species-level for the two 
specimens to be united as a single genus and species.


> It's the same with Australovenator.  If its differences from Rapator
> turn out to be individual variation, then it would be a
> junior synonym of Rapator, not a nomen dubium.


No, it's different.  For _Rapator_ all we have is 
ay belong to _Australovenator_, or it may not.  With only a single bone to go 
by for _Rapator_, we can't know either way.  None of the features present in 
the _Rapator_ Mc I are diagnostic for the genus _Australovenator_.


> And they also said "albeit being very similar in
> morphology, Rapator and Australovenator are clearly distinct
> from Megaraptor", which makes their statements somewhat
> contradictory.  


Yes, it is contradictory.  Their reasoning is a little murky, I know.  But I 
think the moral of the story is that unless the morphology of metacarpal I can 
be used to diagnose individual megaraptoran genera, then we should not be using 
metacarpal I characters to diagnose _Rapator_ and/or _Australovenator_.  
Applying this logic, the latter cannot be referred to the former, nor can 
_Rapator_ be diagnosed to differentiate it from _Australovenator_.  Hence 
_Rapator_ is a nomen dubium.


> Just another reason not to trust
> general unquantified statements like that when it comes to
> how distinct taxa are.  My point is that the metacarpal
> characters that distinguish Australovenator from Megaraptor
> serve equally well to distinguish Rapator from Megaraptor-
> "the presence of a more dorsoventrally developed mediodistal
> condyle and a lateral facet for articulation with the
> metacarpal II lying in almost the same plane as the lateral
> margin of the shaft."  These are thus genus-level
> diagnostic characters of Rapator as much as they are
> genus-level diagnostic characters of Australovenator. 


No, I think you're reading too much into the qualitative descriptions given by 
Agnolin &c.  Never are these characters explicitly stated to be genus-level 
diagnostic characters.  


> To say they count for Australovenator but not Rapator is
> just a bias for the more complete specimen.


Not a bias, just a reality.  A more complete specimen of _Rapator_ might reveal 
characters that are diagnostic at the genus and species level, and can 
therefore be used to synonymize _Rapator_ and _Australovenator_ (or alternative
al I does not allow us to do this - irrespective of the handwaving (pun not 
intended) about how the metacarpal of one specimen "looks different" to another.


> Er... why don't the features that differ from Megaraptor
> qualify?
> As for Rapator being the more fragmentary specimen, that
> doesn't matter one bit for nomen dubium purposes.  A.
> lithographica's holotype (the skeleton, not the feather) is
> much less complete than A. siemensii's, but lithographica
> still gets priority because it was named first.


It is not _Rapator_ being fragmentary that is the issue per se.  After all, 
Agnolin &c regard _Kakuru_ (another fragmentary theropod taxon) as valid.  The 
issue is whether the material known for _Rapator_ has characters that allow it 
to be diagnosed at the genus level.  No argument has been presented to say that 
it can be.


Cheers

Tim