Re: Torosaurus NOT Triceratops

[Denver Fowler <df9465@yahoo.co.uk>]

>The more immature specimens are 

skewed toward Triceratops while the most mature are skewed 
toward Toro. I know that doesn't prove anything, but it's 
statistical. 


I understand that there are reasons why the data come out the way they do: this 
analysis doesn't present anything unexpected given the data available for the 
specimens. Hadrosaur growth stages were named as many different taxa for a long 
time, until ontogeny and stratigraphy helped separate them out into growth 
stages of chronospecies.


>As Denver says (i think), it's unfortunate that many of 
the older but well preserved skulls don't have detailed 
stratigraphic data with them, but they are still useful. 
They're not just heads floating in space or whatever. 


But that's exactly what they are. You can plug them into a cladogram maybe, but 
the issue is when you try to compare them to specimens with known strat data. 
Lets take Raptorex as an example. All evidence indicates that the holotype (LH 
PV18, Sereno et al., 2009) is a young tyrannosaur from Mongolia (Fowler et al., 
2011, plus another paper pending that places it more precisely). It seems to be 
morphologically a little bit different from the Japanese specimen (MPC-D 107/7; 
Tsuhiji et al. 2011), so this could be used to justify that it is a unique 
taxon. Assuming that this is indeed the case, then it could come from 
stratigraphically above, the same, or below the Japanese specimen. How can we 
know? We need to find specimen morphologically identical to Raptorex of the 
same growth stage and then make the assumption that the original Raptorex 
derived from the same stratigraphic level (or perhaps it represents true 
diversity). However, it could be that
 morphological variation between the Raptorex holotype and the Japanese 
specimen is just representative of individual variation within the taxon, but 
we can't know this if we don;t know they come from the same stratigraphic level 
(at least roughly; ie within, say, 200ka). Also, linking the Japanese specimen 
to Tarbosaurus was pretty tenuous. Is it possible that the Japanese specimen is 
actually not Tarbosaurus (ie, the same as the holotype), but instead is 
something new (having come from a slightly different stratigraphic horizon) and 
that Raptorex actually turns out to be T. bataar? All possible. If you have 
strat data then you can at least rule out possibilities, without it... it's 
morphologies without context.


(note the locality/strat of the Japanese specimen is known; I don't want to 
suggest the authors did a bad job: quite the opposite. I do not know about 
holotype of T. bataar, hence the potential problem).


I know people will argue that taxa are based on morphology, not strat, but when 
you start to look at taxa with fine-level stratigraphy you see that subtle 
variation is often stratigraphic in origin. If you don't have strat data then 
you might be misinterpreting stratigraphic variation with some other form 
(taxonomic, gender, ontogeny), leading to all sorts of taxonomic problems, and 
worse; you won't know it. It's happened time and time again.


>There is a lot of circularity 
in trying to separate individual variation from ontogeny 
from taxonomy from dimorphism in similar taxa. 


Ontogeny can be ascertained most precisely by cutting limb bones and counting 
lags. The histological methods for relative maturity used by Scannella & Horner 
(and dismissed somewhat conveniently by Longrich & Field) do not give an age in 
years, but they work and are (currently) the best you can do with isolated 
skulls. Individual variation is what you are left with when you rule out 
stratigraphy (first) then ontogeny (followed by other potential factors: 
geography, taphonomy, etc). I do not agree with the notion that ontog
dual variation.


>You can 
>apply the tools, but in the end, it's largely statistical. 
I think in a year or two you will see someone arguing 
that Toro is the mature form only of *male* Trikes. 
Despite Horner and Padian on dimorphism, that seems to 
me a more likely 

But how would you falsify that idea? you can infer from 50:50 ratios maybe, of 
a certain maturity stage? If Toros are the females, then you might be lucky and 
find medullary bone in all toro postcrania (of which I think there is one 
specimen). but if toros are the males, then this doesn't work. Opinions are not 
testable hypotheses, by definition: right?