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RE: Nomina Dubia Part II: Rapator
Michael Mortimer <firstname.lastname@example.org> 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