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Re: Tyrannosaur Evolution



--- David Marjanovic <david.marjanovic@gmx.at> wrote:

> ----- Original Message -----
> From: "Tim Donovan" <uwrk2@yahoo.com>
> Sent: Monday, February 14, 2005 5:41 PM
> 
> >> 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.
> 
> Research on the paleogeography of Alaska and
> Chukotka still being missing, I 
> can only offer another speculation: perhaps climate
> acted as a filter. 

Of course. Nodosaurs are another example. They may be
present in the Amur area but not in Mongolia.



The 
> mammal faunas of Montana and Alberta differ, as do
> the vegetations... the 
> land bridge was (if at all) much farther north
> still, with a long polar 
> night and all. But perhaps the differences are not
> Asia/NA in the first 
> place, but highland/lowland. Ceratopsomorphs rule in
> western NA, but are 
> absent from Asia... except its western coastal plain
> (Özbekiston). The same 
> holds for spalacother..oid mammals. Hadrosaurs are
> common in NA and rare in 
> Mongolia, 

  ?? Hadrosaurs are abundant in Nemegtian beds, and
seem common in Baynshirenian beds and even in some of
Djadokhtan age.




>but the Amur region seems to be stuffed
> with them. There are more 
> examples like that, and they've been discussed
> onlist.
> 
> > It would be odd for a basal tyrannosauroid to
> > coexist with a rather derived one.
> 
> Only if their ecological niches were sufficiently
> similar to necessitate (!) 
> competition. A casual glance at their body sizes
> suggests to me that this 
> was not the case.


  The idea of pygmy tyrannosaurs seems less credible
lately.
 
> > I note that all of the putative nontarbosaurid
> > tyrannosaurs from the region-Gorgosaurus
> lancinator,
> > Maleevosaurus, even Alioramus-are known or
> suspected
> > to be tarbosaurs.
> 
> *Tarbosaurus* itself is one of the few things
> *Bagaraatan* is certainly not. 
> :o) 
> 
> 


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