Re: Tyrannosaurs and Carnosaurs in Morrison

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 08:48 AM 2/11/99 -0800, Larry Dunn wrote:
>---tsg94001@uconnvm.uconn.edu wrote:
>> _Stokesosaurus_ of the Morrison
>> Formation of Utah (Cleveland-Lloyd Quarry I believe) is a possible
>> tyrannosaurid.  Madsen thought _Stokesosaurus_ was a tyrannosaurid
>back in
>> 1974 based upon the design of the ilium.  More recently (1998) Dan
>Chure
>> tentatively referred to _Stokesosaurus_ a partial braincase from the
>same
>> locality which is strikingly tyrannosaurid-like (UUVP 2455).

I **really** don't want to say anything in particular at this time, but
there is more to this rumor than just _Stokes._...

>All this talk of Morrison tyrannosaurids is really interesting.  It
>raises some questions, at least in my amateur's mind.
>
>I had assumed that carnosaurs declined at the end of the Jurassic
>either because:

What decline?  Carnosaurs remain among the most common big predators in at
least North America, Europe, northern Africa, and South America (and present
in Asia and Australia) through the Albian (c. 100 Ma) and into the
Cenomanian in Africa and South America.

That's a big long time from the J-K boundary.

>1) prey they were adapted to hunt became extinct in Asiamerica due to
>changes in flora; or
>
>2) (a more recent assumption), because of the intervention of one of
>those bolide things or some similar malevolent deus ex machina, which
>killed off larger animals and allowed radiation by "marginalized"
>groups.
>
>Well, *if* there were tyrannosaurids in the Morrison, what does that
>mean to this analysis?  
>
>Could it be that tyrannosaurids ultimately out-competed carnosaurs?  I
>mean, there were no carnosaurs in Asiamerica in the Cretaceous, were
>there?  Just coelurosaurs of various types, unless I'm forgetting
>someone.

_Stokesosaurus_ and other purported pre-Late K tyrannosaurid material are
all from relatively small animals (smaller than _Utahraptor_, and much
smaller than contemporaneous carnosaurs).  It isn't until the Campanian that
really big tyrant dinos are known for certain to have been around.

>And how does one explain the survival of tyrannosaurids when most of
>the other large groups go into decline in Asiamerica at the end of the
>Jurassic?  (OK, so ankylosaurs survived.)

One thing: there is no evidence for an "Asiamerica" at the end of the
Jurassic.  Asiamerica is strictly a Late K phenomenon.  Preliminary studies
show that Asia is faunally the most distinct region in the Late Jurassic and
Early K among well studied assemblages, and that North America was more
similar to Africa and Europe during the Jurassic, to just Europe in the
early part of the Early K, and basically on its own during the late Early K.

>Hmmmm.  Sure wish there were some theropod experts on this list (hint
>hint) ....

Hey, some of us have symposia to prepare for!  That takes time, folks... :-)

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661