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Re: Stokesosaurus


On Monday, July 29, 2002, at 05:45 AM, Thomas R. Holtz, Jr. wrote:

From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
Williams, Tim

Stokesosaurus probably is (Holtz, 2001).

Unfortunately, the assembled material for _Stokesosaurus_ - pelvic bones
(including the type), part of an upper jaw, braincase and tail bones - may
not belong to one taxon. Although all the bones come from the Morrison
Formation, none of the material was found associated AFAIK. The
ilium (sort
of) and braincase (definitely) look tyrannosaurid-like,

Actually, as Rauhut and I (different papers in press) have shown, even the
ilium by itself (i.e., the type material) has derived features otherwise
found only in tyrannosaurids and Eotyrannus.

Agree with the braincase. Would be great if it were.

The ilium is pretty weird. It has the downcurved posterodorsal margin of a maniraptoran, for example. However, most coelurosaurs have a prominent ridge running posteriorly from the ventral margin of the anterior iliac blades onto the pubic peduncle. This ridge is poorly developed in Stokesosaurus, and looks more comparable to allosaurids its development than the long, well defined ridges seen in tyrannosaurids, ornithomimids, and dromaeosaurids for instance. This implies that Stokesosaurus occupies a more basal position in the tree than do tyrannosaurids. Additionally the assigned braincase has a groove running along the ventral surface of the parasphenoid, this sulcus is typical of primitive theropods like Coelophysis, Ceratosaurus, Sinraptor and Allosaurus, and also it seems to be present in Stegosaurus following Marsh's plates, but this trough is absent from the braincase of derived theropods like birds, Troodon, Erlikosaurus and Gallimimus; so unless this groove is also present in tyrannosaurids (I don't have any good figures handy here) the presence of this primitive feature in Stokesosaurus would argue against it being a tyrannosaurid.
Personally I'd like to see what the Liaoning beds present in the way of tyrannosaurids. We've got dromaeosaurids, troodontids, therizinosaurs, oviraptorosaurs, it will probably just be a matter of time until many of the other branches of the theropod family tree show up.

nL