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RE: Tyrannosaur Evolution



--- "Jaime A. Headden" <qilongia@yahoo.com> wrote:


> but we do not know
> the nature of the
> eastern geography to determine the extent of
> upland/lowland/shore/montane
> areas within which these animals would have been
> able to move in).

  Some maps give the impression the east was mostly
flat, with little or no mountain building or
intermontane basins conducive to isolation and
speciation.


> However, the predator/prey arms race certainly also
> has something to do
> with things, and it is possible in the east things
> had not escalated to
> the size and armament that they had in the west, 


  Possible? Definitely. Compare T. rex and
Dryptosaurus. The latter seems a frozen relic of the
pre-Baynshirenian stage of tyrannosaur evolution.



nor
> does it seem a faunal
> diversity had been reached that is comparable. (All
> based on erosional or
> collection biases, of course.)

  The east lacked big ceratopsids etc.
 
> <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.>
> 
>   Only in the south, 

  Alamosaurus occurs as far north as the Evanston and
North Horn. I suspect lack of more northerly specimens
may be due to absence of intermontane sediments of
late Maastrichian age in Canada and MT etc. AFAIK the
Scollard represents upper coastal floodplain habitat
not intermontane.




>and only with a select range of
> *Tyrannosaurus.* 


  Tyrannosaurus could have originally evolved in
response to titanosaurs, then spread to the lowlands,
compelling escalation among ornithiscians. Once the
ceratopsids, ankylosaurs and hadrosaurids etc had
become larger, there was equilibrium and T. rex could
not regress in size, even in environments lacking
titanosaurs.

 If Tyrannosaurus evolved in the lowlands, why aren't
there any close relatives in the upper Horseshoe
Canyon of c mid Maastrichtian age, when Tyrannosaurus
must have been evolving? It is noteworthy that there
is a close relative-Tarbosaurus-in a far inland
environment of about the same age, which harbored
titanosaurs. Tarbosaurus could have originated in
North America and spread to Asia, prior to giving rise
to Tyrannosaurus in NA. Note that Saurolophus, an
American immigrant, seems well adapted to the
predator.



>In
> fact, some of the southern *T. rex* specimens have
> even been questioned on
> their application to that species, much less that
> genus, given the maxilla
> variation.

  K. Carpenter doubted that TMM 41436-1 is a T. rex
but more recent research by C. Brochu suggests it is,
and T. rex is therefore present in the Javelina. It is
definitely present in the McRae, which is fairly
close, and other units.



> This doesn't explain the collection of
> the consistently largest
> animals in the north.


  The south had large hadrosaurs, T. rex and
Torosaurus besides Alamosaurus.
 

> <But Labocania was only of Campanian age. So
> Bagarataan was a
> tyrannosauroid, not a troodont?>
> 
>   Cranial anatomy indicates that the jaw derives
> from a tyrannosauroid.
> Postcranial features would be plesiomorphic or
> convergent under this idea.
> More than one phylogeny has come out with this,
> including Holtz (2004). I
> am sure either Mickey will supply the detials, or
> already has....


 What about Bagarataan?


    Tim
> 
> =====
> Jaime A. Headden
> 
>   Little steps are often the hardest to take.  We
> are too used to making leaps in the face of
> adversity, that a simple skip is so hard to do.  We
> should all learn to walk soft, walk small, see the
> world around us rather than zoom by it.
> 
> "Innocent, unbiased observation is a myth." --- P.B.
> Medawar (1969)
> 
> 
>               
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