Re: TYRANNOSAURIAN IMPLOSION [long; part 2 of 2]

["Tim Williams" <twilliams_alpha@hotmail.com>]

> Stokesosaurus clevelandi from the Late Jurassic Morrison Formation has 
> >been described in several papers as a candidate for tyrannosaurian 
> >ancestry. The material is pretty meager but nonetheless suggestive. >the 
> holotype ilium has a tyrannosaurian shape and a prominent supraacetabular 
> ridge, but a >referred premaxilla does not have characteristic 
> tyrannosaurian teeth.

The presence of tyrannosaurids (or a close relative) in the Late Jurassic is 
supported by an unusual basicranium (UUVP 2455) found in the same site 
(Cleveland-Lloyd Quarry, Utah) as the _Stokesosaurus clevelandi_ type 
material (pelvis).  Chure and Madsen (1998) tentatively referred the 
braincase to _S. clevelandi_.  UUVP 2455 is very derived (it is, for 
example, very anteroposteriorly compressed) and shows features in common 
with _Itemirus_ (dromaeosaurid or tyrannosaurid, take your pick) and Late 
Cretaceous tyrannosaurids.  Its referral to _Stokesosaurus_ is based on 
comparable size to the hip-bones and jaw, and tyrannosaurid-like characters 
in the type ilium.

As George says, the ilium does have a vague tyrannosaurid shape and a 
prominent vertical ridge over the acetabulum (also seen in _Iliosuchus_, and 
of uncertain phylogenetic significance among theropods).  It was on the 
basis of the morphology of the ilium that Madsen (1974) initially referred 
_Stokesosaurus_ to the Tyrannosauridae - but no one was really enthusiastic 
about the idea until the braincase turned up.

George is correct about the referred premaxilla being un-tyrannosaurid-like. 
 The teeth (though very poorly-preserved) are unlike the D-shaped (in 
cross-section) premaxillary teeth of tyrannosaurids.  A tall, rectangular 
premaxilla is also observed in _Ceratosaurus_ (which, unlike _Stokesosaurus_ 
has only three teeth per premaxilla).  None of these features, however, 
preclude _Stokesosaurus_ from being ancestral to later tyrannosaurids.

The pelvis, braincase and premaxilla may belong to three separate taxa of 
Morrison theropod, or to just one.  Time will tell.


Tim





------------------------------------------------------------

Dr Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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