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Re: Nomina Dubia Part II: Rapator

 This is because the respective holotypes of these taxa preserve
 characters that allow them to be diagnosed at the level of genus or
 species.  So they cannot be nomina dubia.

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. Indeed, the levels themselves don't exist in the first place. A character that consistently distinguishes two large clades can be individual variation in a single population in some other clade, and so on.

"A and B are different, but are they different enough to be classified as separate genera?" is not a scientific question.

 Well, this is not exactly what Agnolin et al. said, or even implied.
 They actually state: "In particular, the morphology of metacarpal I
 in _Rapator_, _Australovenator_ and _Megaraptor_ is almost
 identical." True, what follows is a description in which the Agnolin
 et al. distinguish _Rapator_ and _Australovenator_ on the one hand
 (so to speak) from _Megaraptor_. But Agnolin et al. never actually
 recognize any genus- or species-level diagnostic characters in the
 metacarpal I of _Rapator_. That's what makes _Rapator_ a nomen

Cut "genus- or species-level" out of the second-to-last sentence. Any characters will suffice unless there's evidence that they're due to individual (or ontogenetic or sexual or ecomorphological...) variation. Such evidence can come from close relatives; for instance, if* we can make a statistical argument that the direction and degree of curvature of the horns is variable enough in at least one other ceratopsids that the condition in *Ceratops* cannot be considered to lie outside all of that variation at a high significance level, these character states should not be considered diagnostic of *Ceratops*.

* I'm not saying this has been done; I lack an opinion on whether *Ceratops* should be considered a nomen dubium.

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. Again, "reliably" does not mean at any particular level, because the levels don't exist.

> Shouldn't Australovenator be the questionably valid name since it
> was based on a specimen that could not be definitely distinguished
> from a previously named taxon?

 No, because in the opinion of Agnolin et al. the holotype of
 _Rapator_ has no genus- or species-level diagnostic characters.
 Thus, the specimen can only be assigned to the level of Megaraptora
 (i.e., it is "Megaraptora indet.").

*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 less sure the inference -- _and_ 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.)

 Because _Rapator_ is known only from an isolated
 metacarpal I, it is a nomen dubium.

This alone is not correct.