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Re: Tyrannosaur Evolution

On Sun, 13 Feb 2005 04:31:24 -0800 (PST) Tim Donovan <uwrk2@yahoo.com>
> --- "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
> wrote:

> >     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.

Wasn't there a land bridge (albeit intermittent) between western North
America and Asia during the latest Campanian-early Maastrichtian?  If
Asia is included, then the "western North American landmass" was huge.

> >     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.

It also depends on whether the most recent common ancestor of
Tyrannosaurus, Daspletosaurus, and Tarbosaurus is Asian in origin or is
instead North American in origin.  All three genera are huge, and their
most  recent common ancestor was also probably large-ish.  If their most
recent common ancestor (who perhaps lived during the Santonian? or
earlier?) originated in Asia, then the sudden appearance of Alamosaurus
later in southern N. America probably played little to no role in causing
the size inflation of T. rex.

> >     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. 

Alamosaurus apparently never made it further north than Wyoming.  Yet
large Tyannosaurid teeth (probably belonging to T. rex) have been found
in Maastrictian seds. of  Alaska.  Alamosaurus appears to have been a
geographically provincial species (at least in N. America), while T. rex
was more cosmopolitan.  T. rex undoubtably preyed on the titanosaur, but
only the southern T. rex population(s) would have had that opportunity.