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

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.



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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