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RE: Nomina Dubia Part II: Rapator
Michael Mortimer <firstname.lastname@example.org> wrote:
> On to the topic at hand... You replied to David that "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
> character." I would agree that is my view, that unless
> we have good reasons for thinking a character is
> undiagnostic, the default assumption should be that it is
Yes, and here we come to the crux of why we have different opinions. For the
vast majority of characters, I would tend to treat them as potentially
non-diagnostic. These include shape or length of horn cores, length and depth
of sulci, size and orientation of metacarpal processes, degree of fusion of
metatarsals, and so on.
So for our old friend _Ceratops_, I think we can safely assume (and I concede
that it is *just* an assumption) that because of the huge variation in the
length and orientation of postorbital horn cores known within *other*
ceratopsian species, that _Ceratops_ cannot be diagnosed based on the length
and orientation of its postorbital horn cores. I think we can say this even in
the absence of a specimen-level statistical analysis of variation in
ceratopsian horn cores.
> My issue with that is that the characters you view as
> diagnostic (such as those in Australoraptor's diagnosis)
> aren't backed up by any more data than the metacarpal
> proportions we've been arguing about.
Yes, I agree. It's all very warm-and-fuzzy, and so often in the eye of the
beholder. But I think at a qualitative level that we should diagnose
characters that we suspect may be prone to intraspecific (including
ontogenetic) variation. I know there is no standard operating procedure for
this; your _Australovenator_ is a perfect example of this.
> To say some char
can make (remember the folly of 'key
"Key characters"... yep, don't get me started on those @&%# "key characters".
> thought the frill differences between
> Torosaurus and Triceratops were obviously diagnostic, but it
> seems they're just ontogenetic variants.
I'm still keeping an open mind on that one...
> The presence
> of an obturator process on the ischium always seemed like a
> solid character, but then came Mirischia along with it on
> one side but not the other. We simply don't know the
> developmental and/or mutational processes needed to change
> one state to another for basically any Mesozoic dinosaurian
> character. So when you say a metacarpal process' size
> is less important than a deep femoral extensor groove or any
> other of Australovenator's listed diagnostic characters, I
> can't help but ask "how can you tell?"
That's an excellent question. If push comes to shove: you can't. Not for most
characters anyway. I suspect that a taxon based on a partial skeleton gets
treated far more leniently than a taxon based on just one or a few elements.
But given we have multiple elements for _Australovenator_, the sum total of
'diagnostic' characters at least offers the chance of a unique combination of
It's perhaps unfair that _Rapator_ is considered a nomen dubium when we haven't
actually tested the validity of its metacarpal characters. Instead we've just
*assumed* that its characters are not reliable for diagnosing a genus.
But let's say we did erect a new name for *every* fragmentary specimen that has
one or two characters that are possibly diagnostic. The result would be a
proliferation of new genera and species. Such a policy would lead to an
explosion of highly suspect taxa, and spawn a thousand arguments over whether
this-or-that genus had priority. Your question regarding whether
_Australovenator_ should be referred to _Rapator_ (based on shared characters
mentioned by Agnolin &c) would be repeated dozens of times over. This sounds
without us throwing more dodgy names into the mix.
Your approach of giving any and all characters the benefit of the doubt would
justify naming taxa such as _Serendipaceratops_, based on a single ulna that
was said to have unique proportions. I would say that _Serendipaceratops_
should probably not have been named, because the fact that the ulna is shaped a
little differently to all other known ornithischian ulnae does not stack up as
a 'good' diagnostic character.
I think the current approach, whereby names like _Rapator_ and _Ceratops_ and
_Serendipaceratops_ are regarded as nomina dubia even though nobody has
actually *tested* whether or not their characters are subject to intraspecific
variation, is an attempt at damage control. While it might seem unfair, from a
taxonomic perspective these names tend to cause more trouble than what their