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RE: Tyrannosaur Evolution

Now THAT'S a subject line guaranteed to get my attention... :-)

> Joe Cantrell wrote-
> > Why did tyrannosaurs become so large and advanced in western North America
> while the east harbored a > frozen relic of the
> pre-Baynshirenian-Dryptosaurus- down to the late Maastrichtian?

A) There is more to the tyrant record in eastern North America than has been 
published so far; keep reading the journals...

B) That being said, what limited we know of the eastern North American 
dinosaurian fauna suggests rather smaller animals in general.
Some possible contributing factors include:
        1. Smaller geographic ranges (some hypotheses suggest that the largest 
animal size correlates with the available geographic range.
I don't entirely buy that, but I'd thought I'd throw that out)
        2. Possible higher diversity of dinosaurs in the West (poor sampling in 
the East may bias this) may favor evolutionary "racheting"
(such as arms races) selecting for more advanced, larger forms.
        3. As number 2, but factoring in the possibly greater effect of 
transgressive-regressive episodes promoting temporary isolation and
speciation events in the West.

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Mickey Mortimer
> Keep in mind our record of Eastern North American Late
> Campanian-Maastrichtian tyrannosauroids basically consists of one specimen
> (Dryptosaurus holotype), six teeth, a partial maxilla and three femora.

And more to come.

>  I
> don't know how big any of the fragmentary specimens is, do you?

Not terribly big, and not all are necessarily really tyrannosauroids. On the 
other hand, Dryptosaurus itself has a 787 mm long
femur, making it comparable in size to Neovenator, Afrovenator, and the bigger 
Ceratosaurus specimens.

>  So I don't
> think we have a good idea of the variation that existed there and then.
> Western North America had its own smaller basal tyrannosauroid too
> (Labocania), and the Nemegt had Bagaraatan.


                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796