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RE: Andrew McDonald response re: European iguanodonts
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- Subject: RE: Andrew McDonald response re: European iguanodonts
- From: Mickey Mortimer <email@example.com>
- Date: Mon, 4 Apr 2011 19:54:38 -0700
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Tim Williams wrote-
> > Troodon looks valid to me, regardless of how many species are encompassed
> > by what we call Troodon formosus.
> This strikes me as a contradiction. If several species share the same
> dental characters that are characteristic of the _T. formosus_
> holotype, then these characters can no longer be used to diagnose a
> species. Therefore, _T. formosus_ cannot be diagnosed at the species
> level. Ergo, it is a nomen dubium.
> I say "if"... So far, ALL Campanian and Maastrichtian troodontid
> material from North America has been assigned to _Troodon_. This
> includes teeth found as far north as Alaska and as far south as Texas
> and New Mexico. Now if every one of these specimens is conspecific,
> then the dental characters used to diagnose _Troodon_ should be OK.
> But... if the skeletal material indicates that more than one
> troodontid species is represented in Campano-Maastrichtian North
> America, AND they share the same kind of teeth as the _Troodon_
> holotype, then I'm afraid _T. formosus_ is screwed. It's _Iguanodon
> anglicus_ all over again.
I was speaking only of currently named species and described materials. MOR
430 includes a premaxilla and caudal and hindlimb material, but it has not been
described and whether it preserves teeth is not known. Besides that and any
additional undescribed specimens, we have nothing overlapping Troodon,
Stenonychosaurus and Polyodontosaurus. So at this moment, whether you want to
place all, some or none of these taxa into the same species, Troodon formosus
remains the valid name for one of them.
There certainly is variation among American Campanian-Maastrichtian
troodontids. Currie et al. (1990) noted that though Horseshoe Canyon Formation
troodontid teeth are essentially identical to those from the Judith River
Group/Formation, teeth from the Frenchman, Hell Creek, Lance, Prince Creek and
Scollard Formations are different and may prove to be separate species. Baszio
(1997) elaborated, describing differences between teeth from the Judith River,
Horseshoe Canyon and Scollard Formations. Sankey et al. (2002) have split
Dinosaur Park Formation troodontid teeth into Troodon formosus and cf.
Troodontidae indet., with the latter being a rare form only present in some
formations and distinguished by various dental characters. If they are correct,
most of the non-dental specimens (including the Polyodontosaurus and
Stenonychosaurus holotypes and CMN 12340 of Russell, 1969) could not be
definitively assigned to either taxon, and many poorly- or undescribed Troodon
teeth could end up not being referrable to that genus. Also I've noticed
myself that e.g. the skull roof AMNH 6174 lacks the sigmoid supratemporal fossa
of famous posterior skull RTMP 82.19.23 (of Currie, 1985). When the Two
Medicine adult material is described by Varricchio, I wouldn't be surprised to
find differences from the Dinosaur Park material.
So if we find two non-congeneric specimens with Troodon's morphology, then yes
it will be a nomen dubium. Then we can decide if we want to restrict Troodon
to the type tooth, pick a neotype for stability, etc.. For now, we're fine
using Troodon. If we were to abandon Troodon now we'd be in the same position
with Stenonychosaurus or Polyodontosaurus. Polyodontosaurus has a heavier
symphysis than Zanabazar, but can't be compared to Saurornithoides or any of
the American braincase or postcranial specimens. Stenonychosaurus is scarcely
better off- it has distal caudals and a distal tibiotarsus to compare with
Zanabazar, and a pes to compare with Saurornithoides, though at least CMN 12340
provides overlap with it and the cranial American specimens. More importantly,
no diagnostic characters have ever been noted in Stenonychosaurus' holotype
compared to Saurornithoides or Zanabazar (contra Osmolska and Barsbold, 1990
metatarsals III and IV have the same transverse proportions in Saurornithoides
and Stenonychosaurus as far as can be determined, and the poorly preserved
second digit of the former could have the same ratio between II-1 and II-2).
So without depending on a restudy of American troodontid material to search for
autapomorphies, you'd have to pick CMN 12340, RTMP 82.19.23 or one of MOR
specimens and create a new taxon based on it. Even after all of that, you'd be
in the same position we are now. Your new taxon would be diagnosable compared
to other troodontids (like Troodon is now), the other names would still be
floating around as probably/maybe synonyms, and you'd still have most specimens
not definitely referrable to the chosen taxon (no teeth could be certainly
referred to a CMN 12340 or RTMP 82.19.23 based taxon, for instance).
> The prognosis is not good for ANY dinosaur species that is based
> solely on teeth. I can't see how _Troodon_ would be exceptional in
> this regard. Like _Deinodon_ and _Trachodon_ before it, I think it's
> inevitable that the _Troodon_ type will be shown to be non-diagnosable
> at the species level.
It's largely due to laziness. With Trachodon and Deinodon, people chose to
take the easy way out and start using more complete specimens as the basis for
their taxon names, instead of comparing Deinodon in depth with Gorgosaurus and
Daspletosaurus, or Trachodon with Centrosaurus, Styracosaurus, Chasmosaurus,
and whatever other valid ceratopsid taxa are relevent. The "dinosaur teeth are
generally undiagnostic" meme is as bad as the "ceratopsid/hadrosaurid
postcrania don't vary between taxa" meme or the "braincase characters are more
reliable for deducing phylogeny" one. People trust these things without
testing them and our nomenclature changes due to sayings instead of science.