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Re: Age of Gobi Formations



Tim Donovon (sirius531@yahoo.com) wrote:

<I have a question about that recent study which suggests Daspletosaurus is more
closely related to T. rex than Tarbosaurus is, based on braincase/occiput
features. PIN 551-1 lacks the posterior portion of the skull as well as
postcrania. Presumably the only tarbosaur specimens with the posterior portion
preserved are subadults, in which certain features aren't fully expressed and
therefore may appear deceptively primitive. Is it possible that adult
Daspletoaurus material resembles T. rex more due to the lack of maturity in
available tarbosaur material?>

  The skull studied by Hurum and Sabath (2003) for *Tarbosaurus bataar* was that
of ZPAL MgD-I/4, whereas that of *Daspletosaurus* sp. I relayed on was RTMP
94.143.1, considered a subadult. The skull of *Tarbosaurus* is considered an
adult, and the authors felt the braincase matched those of other particularly
large *Tarbosaurus* as adults or near-adults (such as GI 107/2); while the
paroccipital process is not inflated internally, the base of the opisthotic
appears to be, and the pneumatic inflation may have been limited but present.
Hurum and Sabath identify several skulls and skeletons as belonging to adults or
"probably represents a fully adult individual" (pg. 182), so conclusions to
identy only the type as adult seems to be pretty premature based on the rather
large number of specimens available (ZPAL n=16, GI n=8, PIN n=8, plus material
collected in the 90s by the Mongolian/Canadian and Japanese/Mongolian teams will
bring this number up, total n=32); addition of Chinese material will raise this
even more, as Currie and Dong (2001) consider *Shanshanosaurus* a juvenile
*Tarbosaurus* or something like it, and it is treated as such in Currie et al.
(2003).

  Distinct features uniting *Daspletosaurus* and *Tarbosaurus* are, apart from
the *Tarbosaurus* + *Tyrannosaurus* node, the possession of a lateral nasal
surface that participates in the antorbital fenestra, which would not appear to
be ontogenetic. However, as must be ntoed again, this is a skull-only analysis,
and can be taken with a grain of salt. Currie et al. (2003) seem to keep
themselves open by suggesting that this topology is suggestive that *D.* may be
basal to tyrannosaurines in general, but essentially still a tyrannosaurine.
This was stated onlist before.

  Cheers,

  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps in
the face of adversity, that a simple skip is so hard to do.  We should all learn
to walk soft, walk small, see the world around us rather than zoom by it.

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