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RE: Nomina Dubia Part II: Rapator
Tim Williams wrote-
>> So I must ask, when is a character that distinguishes a
>> taxon from its closest relatives not diagnostic?
> In an ideal world, they would be one and the same. However, a formal
> diagnosis should only include those characters that can be confidently used
> to differentiate the genus or species. Usually authors this clear when they
> formally name a new genus or species.
> For example, the description of _Australovenator_ by Hocknull et al. (2009)
> includes a formal diagnosis that makes no mention of metacarpal characters.
> Yet, Mc I of _Australovenator_ and _Rapator_ are discussed later in the
> paper, including differences between the two. So I would baulk at using
> metacarpal characters to diagnose either taxon (_Australovenator_ or
First, I want to thank you Tim for sticking with this debate. I think we've
made some very good progress in our understandings.
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 diagnostic. You continue... "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)."
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. "Gracile dentary" and
"metatarsals elongate and gracile" are not only vague, but known to vary
ontogenetically in tyrannosaurs. "Gastralia unfused" is a plesiomorphy down to
the level of Archosauria at least. "Astragalus with tall ascending process" is
shared with Fukuiraptor. "Eighteen dentary teeth" is not determinable for any
other megaraptoran. When has there ever been a study on the individual or
ontogenetic variation of "olecranon process inflated in proximal view"? The
truth is we don't know if megaraptorans exhibit ontogenetic or individual
variation for any of Australovenator's so-called diagnostic characters, and
most have never been examined that way for any theropod. To say some
characters are "shady" is a value judgement I don't think we can make (remember
the folly of 'key characters'). We thought the frill differences between
Torosaurus and Triceratops were obviously diagnostic, but it seems they're just
ontogenetic variants. 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
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