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Re: Nomina Dubia Part II: Rapator
Sorry I bowed out of this discussion. Between procrastinating elsewhere
and writing my thesis (I'm supposed to submit the finished thesis June
30th or earlier), I neglected it...
Tim Williams wrote on May 26th (or perhaps 25th for Americans):
David Marjanovic <email@example.com> 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.
Sorry. I had thought that by "level" you meant Linnaean ranks, because
you keep talking about diagnosing genera.
> "A and B are different, but are they different enough to be
> classified as separate genera?" is not a scientific question.
Sure it is. It may be subjective, but it is certainly scientific.
As Mike Keesey has pointed out, I regard this as a contradiction in
terms. As soon as you can't answer the question "if I were wrong, how
would I know?" any longer, you're not doing science.
> *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
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.
No. We're taking the view that any character that _is_ consistently
different between taxa is diagnostic. Whether they're mentioned in any
paper is irrelevant; as Mickey pointed out 5 hours ago, original
diagnoses are often wrong (and the ICZN has not the slightest problem
with that, BTW).
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'd say it cuts both ways. Ideally, _every_ potentially diagnostic
character should be tested that way.
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
Whether a character is shady depends on the clade. In most limbed
vertebrates, the number of gastralia (if different from 0) is about as
diagnostic as the number of hairs on a mammal -- except that AFAIK all
pterosaurs have six pairs of fused gastralia, not five, not seven.
I've heard a rumor about a molecular phylogenetic study that found that
in whatever Neotropical rainforest birds beak shape, traditionally used
for classification of that group, was utterly labile evolutionarily,
while plumage color, traditionally considered completely irrelevant,
carried a strong phylogenetic signal! Color! In rainforest birds! WTF!