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RE: Nomina Dubia Part II: Rapator

Michael Mortimer <mickey_mortimer111@msn.com> wrote:

> So I must ask, when is a character that distinguishes a
> taxon from its closest relatives not diagnostic? 

In an ideal world, they would be one and the same.  However, a formal diagnosis 
should only include those characters that can be confidently used to 
differentiate the genus or species.  Usually authors this clear when they 
formally name a new genus or species.

For example, the description of _Australovenator_ by Hocknull et al. (2009) 
includes a formal diagnosis that makes no mention of metacarpal characters.  
Yet, Mc I of _Australovenator_ and _Rapator_ are discussed later in the paper, 
including differences between the two.  So I would baulk at using metacarpal 
characters to diagnose either taxon (_Australovenator_ or _Rapator_).

> Especially since you said you allowed unique combinations of
> characters to be diagnostic, so even a plesiomorphy could be
> diagnostic if it's present alongside a synapomorphy.

I think plesiomorphic characters are fine per se - such as absence of 
characters that are present in related taxa.  Case in point: _Lagosuchus_ might 
be a valid genus if were to use plesiomorphic characters to diagnose this taxon 
(including distinguishing it from _Marasuchus_).

> Well there you go.  You're saying that you're not
> confident one of Agnolin et al.'s statements is
> correct.  Yet my point required us to assume it was
> correct.  So really we agree conceptually about nomina
> dubia and that IF they were right in saying "Rapator and
> Australovenator are clearly distinct from Megaraptor" then
> they were wrong to call Rapator a nomen dubium. 

Yep.  IF they were right.  But I think context is important here, because 
Agnolin &c also make it clear that they do not think the metacarpal characters 
evident in _Rapator_ warrant its acceptance as a valid genus.  I don't think we 
should be mining this paper (or any other paper) for potentially diagnostic 
characters based on each and every statement to the effect of "character X i
all comes down to context.  

I agree that we should not be dismissing fragmentary or poorly preserved taxa 
as nomina dubia will-nilly.  I happen to think _Lagosuchus_ is valid, and maybe 
_Titanosaurus_ too.  But in the case of _Rapator_, I think that nomen dubium is 
the safer option.  Not because it is based on fragmentary material, but because 
in this case the fragmentary material is insufficient to establish a genus or 
species upon.