Nomina Dubia Part II: Rapator

[Michael Mortimer <mickey_mortimer111@msn.com>]

I just posted this to by blog 
(http://theropoddatabase.blogspot.com/2010/05/rapator-in-agnolin-et-al-2010-and.html),
 but it's quite relevent to our nomen dubium discussion here, so I'm 
cross-posting.
 
Agnolin et al.'s (2010) extensive new review of Australian and New Zealand 
dinosaurs provides an excellent example of someone labeling a taxon a nomen 
dubium incorrectly.  Remember, a nomen dubium is an undiagnostic specimen.  It 
does not preserve any apomorphies, so cannot be distinguished from other taxa.  
Here are the facts of the matter in regard to Rapator according to Agnolin et 
al.
 
- Rapator is easily distinguishable from most theropods- ceratosaurs, 
Tugulusaurus, Szechuanosaurus? zigongensis, Torvosaurus, Allosaurus, 
Acrocanthosaurus, and coelurosaurs including Ornitholestes and Nqwebasaurus.
- Rapator is most similar to Megaraptor and Australovenator, so is a 
megaraptoran.
- "Rapator and Australovenator differ from Megaraptor in 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. Thus, albeit being very similar in morphology, 
Rapator and Australovenator are clearly distinct from Megaraptor."
- "Hocknull et al. (2009) recognized subtle differences between Rapator and 
Australovenator (e.g. more subequal distal condyles, flat proximal articular 
surface, straight lateral distal condyle) and we add here the presence of a 
distal medial condyle ventrally extended in Rapator."
 
I have no actual opinion on the validity of these statements, as I haven't 
studied Rapator yet myself.  Let's assume they're all correct.  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."
 
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 undiagnostic 
compared to TWO other taxa in order to be undiagnostic, 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.
 
Another less explicit example is Timimus.  Agnolin et al. consider it Paraves 
indet., though they distinguish it from Troodontidae and eudromaeosaurs.  They 
end up viewing it as most similar to unenlagiines, but don't bother trying to 
distinguish it from any.  Yet Rahonavis is obviously different in having a 
trochanteric crest, while both Unenlagia and Buitreraptor have downturned 
femoral heads.  If Timimus is an unenlagiine, it's a valid taxon.
 
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).
These are all great examples of why I don't trust pronouncements of nomen 
dubium status by authors without them going through a rigorous anaysis.  
Agnolin et al. are actually much more detailed in their analysis than most 
authors when it comes to this, but they still end up declaring fragmentary taxa 
to be indeterminate even when their own written conclusions would indicate it 
cannot be so.
 
Reference- Agnolin, Ezcurra, Pais and Salisbury, 2010. A reappraisal of the 
Cretaceous non-avian dinosaur faunas from Australia and New Zealand: Evidence 
for their Gondwanan affinities. Journal of Systematic Palaeontology. 8(2), 
257-300. 
 
Mickey Mortimer                                           
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