Re: Tyrannosaur Evolution

[David Marjanovic <david.marjanovic@gmx.at>]

----- 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. 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, 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.

> 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)