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RE: Andrew McDonald response re: European iguanodonts



Jaime Headden wrote-

>   1. *Pectinodon bakkeri* is named for a crown with mesial denticulation. It 
> is from the Maastrichtian Lance Formation, and is thus almost 10ma younger 
> than *Troodon formosus* (75mya). Prove to me it is the same taxon as *Troodon 
> formosus.*
>   2. The position in the dental series of ANSP 9259 has been 
speculation, most recently and following that of Currie (1987) as a 
premaxillary crown, but this is not based on any material association 
with a jaw bone and _in situ_ crowns: >snip<
>   3. Absence of mesial denticulation in premaxillary crowns of 
virtually all other troodontids named to date (Norell et al. 2009 
confirming Mickey's assertion "Not Saurornithoides, Zanabazar (contra 
Barsbold, 1974)" and its likely but uncited source) does not mean only 
one taxon with mesial denticulation exists. >snip<

Actually, the holotype of Pectinodon (UCM 38445; Carpenter, 1982 fig. 3a) lacks 
mesial serrations, though some referred specimens have them.  But the larger 
point is I don't need to prove Pectinodon, Polyodontosaurus or Stenonychosaurus 
are synonyms of Troodon formosus for the latter to be valid.  Nor do I have to 
prove Troodon formosus is based on a premaxillary tooth (though the D-shaped 
section is only known in premaxillary teeth, and found in Zanabazar's first 
two, for instance).  Nor do I have to prove only one troodontid had teeth with 
mesial serrations and serrated premaxillary teeth.

Troodon cannot be compared to Polyodontosaurus (based on a dentary) or 
Stenonychosaurus (based on postcrania), so those taxa are simply not an issue 
when it comes to Troodon's validity.  They may be synonyms, sure, but if so are 
junior synonyms and Troodon formosus remains valid.  Similarly, Pectinodon may 
end up differing only due to positional variation, but even if that's true it 
just becomes a junior synonym of Troodon formosus and the latter remains 
valid.  The key here is that until we find two distinct taxa which both have 
Troodon's characters, it can't be a nomen dubium.  But until that happens, 
Troodon formosus is a valid species based on a diagnosable holotype, and the 
number of other proposed troodontid species that are correctly or incorrectly 
referred to it won't change that.

We have plenty of other cases where multiple taxa are based on non-comparable 
holotypes but are both considered valid since they each differ from other 
comparable taxa.  Enigmosaurus vs. Erlikosaurus, for instance (well, Zanno 2010 
reported a distal humerus from the Enigmosaurus material, but it remains 
undescribed).  Or Shixinggia and Elmisaurus (besides some referred pedal 
phalanges of the latter).  Or Therizinosaurus and Nanshiungosaurus, Zanabazar 
and Borogovia (besides distal tibiotarsi, which have not been distinguished), 
Luanchuanraptor and Velociraptor osmolskae, Banji and Machairasaurus, etc.  And 
that's just among maniraptorans.  Sometimes these are synonymized by some 
authors, as in Neuquenraptor and Unenlagia paynemili and of course the famous 
Chirostenotes, Macrophalangia and Caenagnathus.  But what doesn't happen is 
declaring one of the taxa a nomen dubium.  

>   4. If I were to assume that *Pectinodon bakkeri* has as much "right" to 
> exist alongside *Troodon formosus* as *Zanabazar junior* does to 
> *Saurornithoides mongoliensis*, then it matters strongly how many species 
> might exist and by what "level" of nomenclature we recognize their taxonomy. 
> I do not think you can argue correctly that *Troodon* should be the 
> appropriate container, because there is no means of distinguishing this 
> result from one placing all species into their own equivalent containers 
> (thereby permitting *Pectinodon* alongside *Troodon*). I would then reject 
> the notion that *Troodon* would be a useful container at all for any quanta 
> of material broader than a morphology precisely identical to the holotype 
> specimen, and almost certainly from only the Judith River Formation of 
> Montana (that is, also not from the Two Medicine, Dinosaur Park, etc.). Is 
> this a ludicrous and over-conservative opinion? You betcha. But these are 
> _shed and broken teeth_, and using them to validate variation of an unknown 
> quality (especially given their uniqueness) is itself ludicrous.

It's an unwarranted assumption that Pectinodon has as much a right to exist as 
the other taxa.  I can't tell its tooth from Zanabazar or (the admittedly worn) 
Saurornithoides, so would provisionally have no problem with it being a nomen 
dubium among derived troodontids.

Mickey Mortimer