[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Andrew McDonald response re: European iguanodonts



  I don't think Mickey understands my point. *Troodon formosus* is not the same 
as, say *Ornithomimus velox*. There is a point at which the limitation of 
material itself renders the issue moot, such that a taxon is "dubious" when it 
lacks appropriate distinguishing characteristics. Tooth-based taxa have an 
additional hurdle to jump, as they may not simply be comparing two third 
metatarsals. Instead, tooth-based taxa have a positional complex that may or 
may not vary in closely related taxa, and may or may not be convergent across a 
wide breadth of phylogenetic (and morphologic) space when considering 
non-dental material. This issue goes beyond the concept of simply comparing 
morphology across taxa and trying to elucidate some form of species variation; 
it requires us to assume that a single tooth is _useful_ for taxonomy, 
regardless of the greater taxonomy used, without any grounding _whatsoever_ in 
resolving the validity of said argument.

  I could extend this very self-same argument to non-dental material, and allow 
convergent morphology of the third metatarsal to _not_ be instructive of 
phylogeny, and thus useless, but teeth still deserve an extra conditional 
because isolated crowns are not accurately locatable in a series without a 
comparative series to draw upon (while third metatarsals may be convergent, but 
unite taxa nonetheless). Smith designed a method that allows him to 
discriminate taxa with known dental series, and managed to discriminate teeth 
into taxa based on shape analysis alone, but this has been successful only for 
tyrannosaurids, and even then only so far: D-shaped crowns extend into the 
mesial maxillary teeth, may occur in the mesial dentary teeth, while strong 
curvature of the mesial carinae is characteristic in some tyrannosaurid distal 
maxillary and dentary dentition, but not all of them. The process works better 
in a mesial-distal axis, but less so in a upper-lower axis. How do we know 
troodontids are even moderately applicable?

  In the philosophy of Mickey (and Tim), any level of distinguishing features 
is useful for diagnosing a taxon, bar none. This is done on an "either/or" 
modus in which the mere presence of an autapomorphy or autapomorphic suite 
allows a taxonomist to distinguish a form; in this philosophy, failure to do so 
results in a _nomen dubium_. In this argument, *Troodon formosus* is a valid 
taxon (it is also distinguishable from *Pectinodon bakkeri* by the larger 
denticulation on the distal carina, at _least_). The presence of a 
distinguishing feature must be useful to the level of differentiating taxa, 
regardless of any fundamental aspect of the material. If we argue ontogeny, or 
general size (*Zanabazar junior* premaxillary crowns are about 5mm high, ANSP 
9259 is about 3mm, 3/5 the size) we can use this to validate taxonomy as well 
(as was done) by ignoring the ontogenetic change in the fossils and make a 
blanket statement in regards to their comparative differences. If we argue 
ontogeny _does_ influence differentiation, then it must influence diagnosis 
(again, in this philosophy), and we must account for, or be dismissive, of 
insufficient ontogeny -- a specimen may not be diagnostic merely because it is 
a juvenile, regardless of its unique morphologies.

  We do NOT know the role ontogeny has on troodont dentition, and we do not 
know the concise levels of variation in troodont dentition, even in known 
specimens, because a systematic survey has never been done, even by Norell et 
al. (2009) -- the most recent treatment. Currie (1987) positively compared 
Judith River's ANSP 9259 to tooth-bearing dentaries from the Dinosaur Park 
Formation which preserved only germ teeth, while the presence of germ teeth in 
Mongolian troodontid dentitions with erupted crowns is unknown. While Mickey 
notes that various workers recognize supposed variation dichotomies in tooth 
samples referred to any sort of troodontid entity, none of these are concrete. 
The best example of issues with this includes the highly distinctive and 
diagnosable *Paronychodon lacustris* which, by curious parallel, is also a 
complex of referred dentition that is almost certainly assumed to belong to 
multiple taxa, including some teeth now regarded as *Troodon formosus,* 
*Dromaeosaurus albertensis,* *Richardoestesia gilmorei* et *isoceles,* 
*Saurornitholestes langstoni* et al. The unspoken issue here is that theropods 
seems to be regarded as relatively isodont, or with only minor shape variation 
along the series, but heavily heterodont dentitions (the D-shaped mesial 
crowns, for example, of various unrelated taxa) imply that isolating samples to 
taxonomy can be fruitless and useless. This all serves as junk taxonomy. It is 
abetted, as with the issue of *Chirostenotes* synonymies, there there are 
multiple dentary morphologies present in the Dinosaur Park Formation, along 
with a minimum of two frontoparietal morphologies (I think there are three), 
suggesting at least DPFm troodontids were not limited to just one taxon. So 
there is doubt as to singular troodontid taxonomy, and applying a modicum of 
reason implies that lumping material into *Troodon formosus* is a problem.

  These issues, all of them long and rambling, present difficulties in how we 
regard *Troodon formosus*. My ultimate argument will not be presented here, 
because I keep getting pestered to publish my ideas instead, although the 
ultimate argument is a doozy in itself. Note that in their philosophy, I would 
regard *Troodon formosus* valid (by the strict sense of the modus used), but 
only just. Variation in the collections render this concept problematic on its 
face, and I think we should toss it and start over again, but that seems so 
difficult to grasp for some.

Cheers,

Jaime A. Headden
The Bite Stuff (site v2)
http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)





----------------------------------------
> Date: Mon, 4 Apr 2011 19:54:38 -0700
> From: mickey_mortimer111@msn.com
> To: dinosaur@usc.edu
> Subject: RE: Andrew McDonald response re: European iguanodonts
>
>
> 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.
>
> Mickey Mortimer
>
>