RE: Nomina Dubia Part II: Rapator

[Michael Mortimer <mickey_mortimer111@msn.com>]

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 
> _Rapator_).

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 
you tell?"

Mickey Mortimer                                           
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