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Re: Tyrannosaur Evolution



--- Phil Bigelow <bigelowp@juno.com> wrote:

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


 Yes, only intermittent, and the faunas remained
largely different. There were apparently no
ceratopsids in Asia. Nor nodosaurs, except possibly in
the far eastern part.
> 
> 
> 
> > >   2. Possible higher diversity of dinosaurs in
> the
> > > West (poor sampling in the East may bias this)
> may
> > > favor evolutionary "racheti
> 
> 
> >   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,

 Daspletosaurus was only Albertosaurus sized.


 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.
> 

    Starkov attributed the large size of Tarbosaurus,
probably closest to the ancestry of T. rex, to the
presence of titanosaurs in its well inland Asian
environment.

> 
> > 
> > >   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. 

  Inland or lowland?


> 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.


 Starkov suggested T. rex originated in the inland
habitats but then spread to the lowlands, causing
escalation among ornithiscians there such as the
ceratopsids. It is noteworthy that Albertosaurus was
still the top predator in lowland environments
represented by the Horseshoe Canyon down to about mid
Maastrichtian, which suggests Tyrannosaurus evolved
elsewhere, farther inland.



> 
> <pb>
> --
> 
> 
> 
> 
> 
> 
> 



                
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