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Re: Torosaurus NOT Triceratops
>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
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
>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?