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RE: Another question about Tyrannosaurs

From: Christopher Pearsoll [mailto:c_pearsoll@hotmail.com]
Sent: Tuesday, May 29, 2001 5:35 PM

>I have a question concerning tyrannosaurs.  Are there one or two main branches of Tyrannosaur evolution, where one, in 
>Asia, led to the larger tyrannosaur genera like Tarbosaurus and Tyrannosaurus, and the other, in North America, leading to
>the smaller tyrannosaurs, like Albertosaurus?  For some reason, (maybe I half recall reading this somewhere) I have been
>thinking that this was the way the tyrannosaurs developed.  I have since noticed that the phylogenies of the tyrannosaur
>family do not appear to bear the same concensus.  If I am correct, please tell me.  If I am not, please tell me.
In actuality, most recent tyrannosaurid phylogenies are strongly converging, with _Alectrosaurus_ and _Alioramus_ outside the big guys, _Albertosaurus_ and _Gorgosaurus_ as sister taxa, and _Daspletosaurus_, _Tarbosaurus_, and _Tyrannosaurus_ forming a clade (most likely with _Tarb._ and _Tyr._ as sisters).
These results have been found by Currie, by myself, strongly suggested by Brochu (who does caution the possibility of _Daspletosaurus_ being closer to _Tyrannosaurus_ than is _Tarbosaurus_), and by a few others in press or in prep.
Thus, there is no compelling evidence at present for a primarily Asian branch vs. a primarily North American branch among the big guys.  However, it should be cautioned that we know so little about mesic (wetter) environments in Asia for most of the Late Cretaceous that there might very well have been many genera of giant tyrannosaurines there which are not yet discovered.  (_Tarbosaurus_, for example, is found in Nemegt beds, which are the "wettest" environment known of the typical late Late K of Mongolia).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742      
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796