RE: Tyrannosaur Evolution

[Tim Williams <twilliams_alpha@hotmail.com>]

Tim Donovan wrote:

> It would be odd for a
> basal tyrannosauroid to coexist with a rather derived one.

Personally, I don't think this is really so odd.  There are other examples.  
We have basal birds (_Rahonavis_) coexisting with derived birds (_Vorona_).  
A basal neoceratopsian (_Leptoceratops_) coexisted with derived 
neoceratopsians (_Triceratops_, _Torosaurus_) at the end of the Cretaceous.  
Early sauropodomorph evolution is characterized by different 'grades' living 
in the same time and place (especially southern Africa).  If _Bagaraatan_ is 
a basal tyrannosauroid, then it presumably had a very different ecology to 
derived tyrannosaurines like _Tarbosaurus_.

> I note that all of the putative nontarbosaurid
> tyrannosaurs from the region-Gorgosaurus lancinator,
> Maleevosaurus, even Alioramus-are known or suspected
> to be tarbosaurs.

Most of these are suspected to be not just "tarbosaurs", but the same as 
_Tarbosaurus_.  The most likely exception is _Alioramus_, which appears to 
be distinct (although this is complicated by the immaturity of the 
specimen).


Tim