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RE: Tyrannosaur Evolution

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).