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RE: Tyrannosaur Evolution
--- "Thomas R. Holtz, Jr." <firstname.lastname@example.org>
> 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: email@example.com
> [mailto:firstname.lastname@example.org]On Behalf Of
> > Mickey Mortimer
> > Keep in mind our record of Eastern North American
> > 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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