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RE: Torosaurus NOT Triceratops
One way to test this might be to see whether morphological changes that occur
in Triceratops over time are also seen in specimens referred to Torosaurus:
In the Scannella and Fowler (2009) abstract cited above, morphologic changes
occur in Triceratops populations in progressively younger strata of the Hell
Creek Formation, so that the two morphotypes of Triceratops that have been
traditionally grouped as species (T. horridus, T. prorsus) do not appear to
have overlapping stratigraphic ranges. Instead, they seem to represent a
single lineage which changes over time. If these changes (increase in the size
of the nasal horn, rostral elongation, closing of the frontal fontanelle) are
also seen in stratigraphically younger specimens of Torosaurus, this would be
consistent with the two representing the same taxon. If Torosaurus does not
also exhibit these morphologic changes over time, it would be consistent with
Torosaurus and Triceratops representing separate taxa.
> Date: Thu, 1 Mar 2012 21:28:25 +0000
> From: firstname.lastname@example.org
> To: email@example.com
> Subject: 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
> 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?