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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.


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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