Re: Age of the Barun Goyot, Djadok(ht)a and Nemegt Formations

["Jaime A. Headden" <qilongia@yahoo.com>]

Tim Donovan <uwrk2@yahoo.com> wrote:

<OK but AFAIK no researcher ever considered them different genera, which is
understandable if they're "virtually indistinguishable" as Norman wrote.  Btw I
note Williamson, Lehman and others long considered the Naashoibito, McRae etc
coeval with the Lance/Hell Creek based on just Essonodon, Torosaurus and T. rex
even though the latter two are represented by very fragmentary remains in the
southern units. The latter were generally assumed to be Lancian equivalents
even though Sullivan considered the T. rex etc fragments to be misidentified.>

  Isn't it just totally wicked that we only need ONE taxon to prove the age of
a formation, sans other data?

  BTW, thanks to Tim Williams, I have just read Norman and Kuzanov (1997) on
*Arstanosaurus*, which includes a figure (fig.7, pg. 198) of a juvenile
lambeosaurine skull (with nearly complete skeleton, unfigured, PIN 3458/5).
Now, given even that this skull is only about 14-15cm in length and thus
younger than a teenager, it is developed enough that the premaxilla has
expanded caudally over the frontal and is in the process of apparently
"pulling" the frontal dorsally as it expands into a crest. Now, unless I'm
GRAVELY mistaken about lambeosaur cranial crest ontogenetic series for
*Hypacrosaurus*, *Corythosaurus* or *Lambeosaurus*, which avoid expansion of
the frontal dorsally as the crest grows dorsally anterior to the orbits before
further expansion, this skull appears to parallel the expansion of the cranial
crest of *Parasaurolophus* in what is essentially a "parasaurolophine"
apomorphy. Guess what else? The skull is from Baynshin (or Baynshiin [Bayan
Shiin, Bayanshiyn]) Tasv (or Tsab). Yes, argued to be coeval with the Iren
Dabasu Formation. Now, if this IS a parasaurolophine, we can argue two
comparable theories on dispersal and speciation:

  1. It as with other parasaurolophines are index taxa relative to the
Maastrichtian, thus constraining their appearance to that stage regardless of
formation. Based on the number of *Parasaurolophus* specimens (RARE) and even
the "parasaurolophine" *Charonosaurus*, we are given to doubt a lot of index
categorization without ample fossil material to express to ourselves the ages
of these taxa. Similarly, reports of Campanian *Parasaurolophus* will need to
be confirmed, but since *Parasaurolophus* is an upper Maastrichtian fossil,
it's unlikely there ARE Campanian *Parasaurolophus* just as it is unlikely the
formation, if it IS *Parasaurolophus*, is Campanian but is in fact
Maastrichtian. We might also just call *Charonosaurus* a species of
*Parasaurolophus* since their time period can now be nailed down as the very
same, and can now use *Parasaurolophus* as an even better index genus.
Brilliant!

  2. Parasaurolophines evolved in Asia and dispersed to North America, so that
Mongolian PIN 3458/5 represents the earliest parasaurolophine, and
*Charonosaurus* the next in lower Maastrichtian Tsagayan deposits in the Amür
region, and finally Maastrichtian *Parasaurolophus*. If this is true, then its
possible the older specimens of *Parasaurolophus* represent possible
transistory taxa or even *Charonosaurus* in North America. Gasp! Given this, we
may now have a span of the entire Djadochta, Baruun Goyot and Nemegt Formations
to "never preserve a parasaurolophine". But since the fossil record is
currently complete enough to counter this ridiculous theory, it is highly
unlikely that any such scenario is true. Is it?

  And finally, a synthesis:

  3. Parasaurolophinae is a taxon that spans Campano-Maastrichtian times, from
an apparently middle Campanian or even Santonian Baynshiin Tsab (Which has been
associated with the Djadochta Formation by some) into the late Maastrichtian.
During this time, parasaurolophines are diversifying from essential basal Asian
stock, such as *Tsintaosaurus* or *Jaxartosaurus* or something completely more
odd, appearing in Djadochta or earlier formations, and then expanding in size
and latitude, eventually arriving in the Amür Region some time at the beginning
of the Maastrichtian stage or even earlier, and then crossing a Beringian
exposure into North America where it would become found from Mexico to Canada
and spawn even more species. The relative dearth of recovery in Asia can then
be lain at the feet of the more productive investigations in North America by
American, Canadian and Mexican scientists.

  So which, I ask, of all these scenarios, seems more likely given the fossils?

  Cheers,

Jaime A. Headden

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


                
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