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Re: Age of Gobi formations



Tim Donovon (sirius531@yahoo.com) wrote:

<I was under the impression that Ukhaa Tolgod, which yields Pinacosaurus,
underlies Khulsan, which has larger ankylosaurs, since the more E/NE localities
in the basin tend to be older. There is just no evidence for Pinacosaurus in
more westerly strata which clearly represent the top of the Barun Goyot.>

  Ukhaa Tolgod lies far to the west and south of Khulsan. There is no 1:1
corrolary. There are only faunal overlaps, and the overlying strata at Ukhaa
Tolgod resemble the sand and clay/mudstones of the Barun Goyot, rather than the
almost pure redder sandstones of the Djadokhta.

<Wodehousia spinata argues for an early Maastrichtian age for the Tsagayan. It
is hard to believe the Nemegt fauna, with Horseshoe Canyon aspect hadrosaurs, is
older i.e. was supplanted by, a Campanian aspect fauna with far more diverse
hadrosaurs-exactly the reverse of what occurred in NA. And Asia and NA had
biogeographic connections around then.>

  It is just as hard to believe that the Barun Goyot can be applied as a younger
formation when there is no real 1:1 correlation in one place as to their
relative age (and in sedimentology, a formation that is "younger" _can_ underlie
and interfinger an "older" formation, as occurs in the Judith River Wedge in
North America with respect to the Bearpaw Shale. The further east, the deeper
the shale, and the further "temporally" separated the two. Using the few site to
site corollaries of only the two "uppermost" formations to correlate the oldest
of them or all three as one, is rather far reaching. The above is an example
that one should not take it for granted. The Djadokhta has virtually _no_
corollary contacts. As noted below, however, the Nemegt and Horseshoe Canyton
compare well in some degrees, not in others.

  Also, to build on what others have stated before, and to repeat Jon Wagner to
some degree, in any one formation, relative faunal consistency can very
horizontally as easily as vertically, as evidenced by Lehman's research in
"upland"/"lowland" faunas in the Judithian/Edmontonian of NA (Lehman, 2001. Late
Cretaceous dinosaur provinciality; pg. 310-323, in Tanke and Carpenter (eds.)
Mesozoic Vertebrate Life. NRC/Indiana University Press.), where fauna vary in
their relation to the various incarnations of the inland sea and the orogens to
the west. They also vary north to south in the same formation. One tends to note
similar regional specialization in the Morrison Formation.

<All things considered, including size, isn't Tarbosaurus still most similar to
Tyrannosaurus, and isn't it at least more likely to be closer temporally than
Daspletosaurus?>

  How so, closer temporally? We have little but a scant bit of suggestive but
non-definitive information that leads to an age for the Nemegt, most of which is
comparative biostrat. As noted in previous posts, there are cranial features
that link *Tarbosaurus* to *Daspletosaurus* rather than to *Tyrannosaurus* and
the authors preclude their entirety from synonymy with *Tyrannosaurus* (sensu
Paul, 1988). If the Nemegt was lower Maastrichtian, and the Horseshoe Canyon was
dated on NA biostrat and using the marine standard, and was also lower
Maastrichtian, then it seems logical they should have similar taxa: rather, they
appear to share only some fairly "primitive" tyrannosaurines and *Saurolophus.*
Donovon choose to relate *Barsboldia* to *Hypacrosaurus* based on correlates in
strata, rather than anatomy, and the neural anatomy just doesn't seem to support
any referal but implicate that *B. sicinski* is an endemic hadrosaur and not
illustrative of this faunal comparison. Saurolophine phylogenies aside not
implying that *S. angustirostris* must be *Saurolophus* (no one really seems to
have tested this in print yet), the Judithian and Nemegt also share primitive
tyrannosaurines, *Elmisaurus,* extensive troodontids, *Mesodma* and *Cimexomys,*
several lacertilian species. But for all their comparability, and not to render
my own case bad (I am not arguing for a particular age) but to strengthen it (I
am arguing that biostrat in this case doesn't seem to resolve anything, and
should not be relied so much as the absolute iostope dating may reveal, incl.
margins of error):

  There are a complete absence of nodosaurids, ceratopsids, hadrosaurines from
the Asian formations: any comparability must be taken with a grain of salt. Jon
Wagner has earlier stated onlist his queries about the use of *Wodehousia*
dating in the Tsagayan to date the Amur hadrosaurs, because that is regional in
NA, and one should have more data at hand to support the theory if one assumes
that the NA and Asian flora both comprise *Wodehousia* at the same time. This is
also why I do not like far-reaching biostrat, as both today and likely then,
biogeographic dispersal of species tends to be limited without extraordinary
means, such as human and domestic faunal/floral dispersals.

<Altan Ula IV has yielded the same Nemegtian dinosaurs as other localities.
Maybe the Barun Goyot interfingers there too.>

  If the experts cannot agree, for any reason, I say we need third party
cooperation. Corroboration on date methods. As in the Judithian through
Edmontonian NALMA, some taxa cross formational boundaries, and it is not
unreasonable to say that though one taxon (*Pinacosaurus*) may not expand beyond
the Djadokhta, at least one (*Velociraptor*) has, and one (*Oviraptor*) crosses
from the Djadokhta to the Nemegt, however that someone has implied that it has a
new generic name, this seems mostly based on its formational occurence. Some
lizard species cross the Djadokhta to Nemegt, without occuring in the Barun
Goyot to date, and some lizards cross all three, whereas mammals tend to be a
tad more restrictive. Almost exclusively one's conclusions on biostrat seems a
bit tricky, given that relative dating is just as tricky.

  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)