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Re: taxonomy is not stratigraphy (was Re: JVP 25(2): New Dinos, Birds, Discoveries)




--- Tim Williams <twilliams_alpha@hotmail.com> wrote:

> Tim Donovan wrote:
> 
> >   Brochu said TMM 41436-1 was T. rex or a close
> relative. The latter is 
> >probably true given the age of the Javelina and the
> relatively small size 
> >of TMM 41436-1. (Carpenter also mentioned a short
> face compared to T. rex.)
> 
> A more recent paper dealt with this.  TMM 41436-1 is
> probably _T. rex_.


  He said or a close relative. We shouldn't ignore the
earlier study, which noted that TMM 41436-1 does not
closely resemble any T. rex maxilla, in addition to
being far smaller. 69 Ma seems too old to be T. rex.
That was probably the heyday of Tarbosaurus.



> 
> >It must have been a smaller ancestor of T. rex.
> 
> Why "must"?

  It does appear closely related, but different and
smaller, and older. The appearance of an apparent T.
rex ancestor in a titanosaur dominated environment
just prior to the Lancian and T. rex virtually
clinches Starkov's case.It now does appear that T. rex
evolved in response to titanosaurs.The Javelina was
roughly coeval with the top of unit 4, where
Albertosaurus was still the top predator in an
environment lacking titanosaurs.It was also coeval
with the Nemegt, where Tarbosaurus, fairly close to
the ancestry of Tyrannosaurus, may have preyed on
titanosaurs.

 
> >No wonder its tyrannosaurs were rather rare and
> Alamosaurus so common.
> 
> Actually, as pointed out in previous postings,
> _Alamosaurus_ is a 
> wastebasket/dustbin for LK titanosaur material from
> North America.  Several 
> titanosaur taxa may in fact be represented across
> the southern U.S.A.


 I was only referring to Javelina titanosaurs, which,
according to Lehman, made up about 70% of Javelina
dinosaur specimens-even before more recent
discoveries- whereas they were about 20% of
Naashoibito dinosaurs, suggesting a decline by the
time of the Lancian and T. rex, which could more
effectively cull them.

 
> >They hadn't yet evolved the size necessary to take
> on the titanosaurs in an
> >environment dominated by them.
> 
> Even the largest of titanosaurs were small when they
> were young. 

  But they probably derived protection from parents or
other adult herd members.


> BTW, it's 
> my personal opinion that theropod predators
> (including tyrannosaurs) 
> typically targeted the smaller or weaker members of
> a sauropod herd anyway, 
> and avoided the larger and stronger individuals. 

 Maybe. But, as Starkov noted, there is a correlation
between the rise of the largest sauropods and largest 
theropods, suggesting the latter became bigger to
overcome more massive prey.




> This would be more in 
> keeping with the behavior of modern predators.

 I don't think they are quite as large in relation to
prey as the biggest theropods.

 Tim
 
 


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