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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
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
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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