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Re: Tyrannosaur Evolution and Bagarataan



--- Tim Williams <twilliams_alpha@hotmail.com> wrote:

> Tim Donovan wrote:
> >   P. maximus is a possible ancestor of
> Saurolophus.
> >Are possible ancestors known from the Campanian of
> >Asia?
> 
> The comparison is a tad unfair. 

 Godefroit expressed a view similar to mine in the
June 2004 JVP. I also note that Saurolophus is found
in unit 3 as well as unit 4 of the Horseshoe Canyon,
indicating it was present in North America since about
the beginning of the Maastrichtian. In Asia it is
probably only known from the c late early or mid
Maastrichtian, since Tarbosaurus suggests an age
closer to the late Maastrichtian. That is also
consistent with the idea of Saurolophus originating in
NA and migrating to Asia.



> We know a lot more
> about the hadrosaur 
> fossil record in the Late Cretaceous of North
> America than we do about Asian 
> hadrosaurs living around the same time.


  AFAIK, saurolophines are unknown from the apparently
Djadokhtan equivalent beds at Laiyang. Godefroit
suggested a late Maastrichtian age for the Udurchukan
and Kerberosaurus. I think the Udurchukan is only
early Maastrichtian, and preceded the Saurolophus
radiation and Nemegtian. AFAIK the Udurchukan has no
taxa in common with NA units, which should be present
given the accessibility of more remote, central
Asia.(I also suggsted btw, that Aquillapollinites
subtilis and Wodehousia spinata, the sole evidence for
the putative late Maastrichtian age for the
Udurchukan, actually originated in Asia and spread to
America c mid Maastrichtian over the same Bering
bridge that enabled Saurolophus to cross, probably in
the other direction.Therefore the palynomorphs may
indicate a late Maastrichtian age only in NA.) It is
likely IMO that the sauroloph niche was already
occupied in Asia by Keberosaurus to almost mid
Maastrichtian, and the Tsagayan types were then
supplanted by American immigrants.

 Btw, getting back to Bagarataan, recently a theropod
of medium size and "unknown taxanomic position" was
found in the white beds of Khermeen Tsav I. Sounds
like it might be Bagarataan. If it is, its presence in
the white beds may indicate it did coexist with
Tarbosaurus. I note, though, that the type and this
possible new specimen both occur in stratigraphically 
very low beds indicating close proximity to
Barungoyotian environments. Mabe, like that particular
type of environment, it didn't last long.

 Tim


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