RE: Tyrannosaur Evolution

[Tim Donovan <uwrk2@yahoo.com>]

--- "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
wrote:
> 
> 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)

  It's not even true. Western North America was then a
relatively narrow north south strip of land. The east
was considerably larger, yet Dryptosaurus was much
smaller than T. rex.



>       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.

  Exactly. The West had titanosaur immigrants like
Alamosaurus, which, as Starkov proposed, caused the
large T. rex to be selected for in inland environments
of the West.



>       3. As number 2, but factoring in the possibly
> greater effect of transgressive-regressive episodes
> promoting temporary isolation and
> speciation events in the West.

  The West had intermontane basins with somewhat
different faunas, including titanosaurs and
ankylosaurids which were rare or absent in lowlands. 


> 
> > 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.


  IIRC a  JVP article reconstrcted Dryptosaurus as a c
7m predator, smaller than Albertosaurus, let alone its
contemporary farther west.

> 
> >  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.
> 

  But Labocania was only of Campanian age. So
Bagarataan was a tyrannosauriod, not a troodont?

> 
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