Re: Nomina Dubia Part II: Rapator

[David Marjanovic <david.marjanovic@gmx.at>]

>  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
>  dubium.

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.