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Re: Nomina Dubia Part II: Rapator
David Marjanovic <firstname.lastname@example.org> wrote:
> I'm not sure if that's what you mean, but... just to make
> sure: there is no such thing as a character that is
> inherently diagnostic at a particular level.
I'm afraid you've lost me here David. A specimen can be classified as
diagnostic at different taxonomic levels. For example, _Rapator_ is classified
as "Megaraptora indet." whereas _Walgettosuchus_ is classified as "Theropoda
indet.". This is because _Rapator_ (based on a metacarpal I) has characters
that allow it to be assigned to Megaraptora, whereas _Walgettosuchus_ (based on
a tailbone) can only be classified at the level of Theropoda.
> "A and B are different, but are they different enough to be
> classified as separate genera?" is not a scientific
Sure it is. It may be subjective, but it is certainly scientific.
> * I'm not saying this has been done; I lack an opinion on
> whether *Ceratops* should be considered a nomen dubium.
I don't. I think it's a nomen dubium. And I don't think I need a
specimen-level statistical analysis to tell me so. So let's bite the bullet,
and junk the family Ceratopsidae.
> Concerning the last sentence, a nomen dubium is one that
> pertains to a specimen which cannnot be reliably
> distinguished from _at least two_ other taxa.
Yes, exactly. What are we arguing about?
> "reliably" does not mean at any particular level, because
> the levels don't exist.
OK, now you've lost me again. These "levels" are hierarchial (increasingly
inclusive) clades. _Allosaurus_ is inside Allosauridae, which is inside
Carnosauria, which is inside Theropoda, and so on...
> *Australovenator* is the one that could be a junior
> synonym; *Rapator* is the one that could be a nomen dubium.
> *Rapator* is a nomen dubium if it cannot be distinguished
> from _both_ *Australovenator* _and_ *Megaraptor* by any
> characters other than those which can be shown to be due to
> individual (etc.) variation in... close relatives, the less
> close, the le
nd_ if furthermore
> *Australovenator* and *Megaraptor* _can_ be distinguished
> from each other by such characters. (If they can't be, all
> three must be lumped into *Rapator ornitholestoides*, unless
> you allow the geographical location of *Megaraptor
> namunhuaiquii* as a diagnostic character.)
I know what you're saying. You and Mickey are taking the view that any
character that is mentioned in a paper as being "different" between genera is,
by implication, a diagnostic character. Thus, you would argue that we need a
separate study to demonstrate that said character is prone to intraspecific or
ontogenetic variation in order to refute its validity as a diagnostic
I say this is fruitless in most cases, because we don't have enough data either
way - and probably never will. So I err on the side of caution, and think we
should avoid erecting new genera and species based on fragmentary material that
have diagnostic characters that are a little shady (like the shape or
orientation of a process on the metacarpal, or the length and orientation of a
horn core). This is current practice, BTW. So what I'm proposing is not
controversial. If discovered today, _Rapator_ and _Ceratops_ would probably
not be named.
> > Because _Rapator_ is known only from an isolated metacarpal I, it is a
> > nomen dubium.
> This alone is not correct.
I know. The context in which this sentence appeared made it very clear that I
think it's fine that we name genera or species based on a fragmentary specimen.
But such a specimen *must* have unambiguous autapomorphies or a unique
combination of characters.