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RE: Tyrannosaur Evolution



--- "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
wrote:

> > From: Tim Donovan [mailto:uwrk2@yahoo.com]
> > Sent: Sunday, February 13, 2005 7:31 AM
> >
> > --- "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
> > wrote:
> > >
> > > B) That being said, what limited we know of the
> > > eastern North American dinosaurian fauna
> suggests
> > > rather smaller animals in general.

 By the late Maastrichtian.


> > > Some possible contributing factors include:
> > >   1. Smaller geographic ranges (some hypotheses
> > > suggest that the largest animal size correlates
> with
> > > the available geographic range.
> > > I don't entirely buy that, but I'd thought I'd
> throw
> > > that out)
> >
> >   It's not even true. Western North America was
> then a
> > relatively narrow north south strip of land. The
> east
> > was considerably larger, yet Dryptosaurus was much
> > smaller than T. rex.
> 
> Well, only if you limit the continent involved to
> "western North America" rather than "Asiamerica".


  It has been suggested that there was only an island
chain between Asia and NA but that may not be credible
if ankylosaurids radiated across.



 
> Still, as I said, I don't buy
> it...
> 
> > >
> > > >  So I don't
> > > > think we have a good idea of the variation
> that
> > > existed there and then.
> > > > Western North America had its own smaller
> basal
> > > tyrannosauroid too
> > > > (Labocania), and the Nemegt had Bagaraatan.
> > >
> >
> >   But Labocania was only of Campanian age. So
> > Bagarataan was a tyrannosauroid, not a troodont?
> 
> Bagarataan's position is far from certain, but it
> does come out as a basal tyrannosauroid in my latest
> analyses (see Dinosauria II)


  Exactly where was Bagarataan found? I think it was
from the type locality but I don't know exactly where.
Do field notes indicate it was found in the white beds
or the Barungoyotian beds there? It would be odd for a
basal tyrannosauroid to coexist with a rather derived
one. I note that all of the putative nontarbosaurid
tyrannosaurs from the region-Gorgosaurus lancinator,
Maleevosaurus, even Alioramus-are known or suspected
to be tarbosaurs.





>               Vertebrate Paleontologist
> Department of Geology         Director, Earth, Life & Time
> Program
> University of Maryland                College Park Scholars
>       Mailing Address:
>               Building 237, Room 1117
>               College Park, MD  20742
> 
> http://www.geol.umd.edu/~tholtz/
> http://www.geol.umd.edu/~jmerck/eltsite
> Phone:        301-405-4084    Email:  tholtz@geol.umd.edu
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> 301-405-0796
> 
> 



                
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