Stokesosaurus (was RE: Cleveland / Loyd fauna was Re: Cleveland Lloyd...)

[Tim Williams <twilliams_alpha@hotmail.com>]

Cliff Green wrote:

>     According to Dan Chure et. al, Stokesosaurus is a primitive 
> tyrannosaur,
> based on the brain case. Not much of this animal is known, a premax, illium
> and of course the brain case.

Thanks for the reply Cliff.  The type material for _Stokesosaurus_ is an 
ilium found in the Cleveland-Lloyd Quarry.  The jaw fragment (premaxilla) 
was referred to _Stokesosaurus_ by Madsen (1974), who first described this 
theropod.  The braincase (basicranium; UUVP 2455) is another CLQ find, and 
it was tentatively referred to _Stokesosaurus_ by Chure and Madsen (1998).

However, unless something has changed, there is no positive evidence that 
these three elements (ilium, premaxilla, braincase) all belong to the same 
species.  (Of course, by the same token, there is no evidence that they do 
not belong in the same taxon, and there seems no reason to think that Dan is 
wrong.)  The holotype ilium does have a vaguely tyrannosaurid shape and a 
prominent vertical ridge over the acetabulum (also seen in _Iliosuchus_ and 
_Aviatyrannis_).  It was on the basis of the ilium's morphology that Madsen 
(1974) initially referred _Stokesosaurus_ to the Tyrannosauridae - but no 
one was really enthusiastic about the idea until the braincase turned up.  
Foster and Chure (2000) referred a smaller ilium (presumably from a 
juvenile) from South Dakota to _Stokesosaurus_.

The braincase is very derived (e.g., very short anteroposteriorly) and shows 
features in common with with both _Itemirus_ and Late Cretaceous 
tyrannosaurids.  Interestingly, the braincase of _Dilong_ (Lower Cretaceous) 
is anteroposteriorly long - a primitive trait.  The referral of UUVP 2455 to 
_Stokesosaurus_ is based on size, and on the tyrannosaurid-like characters 
in the type ilium (also seen in _Aviatyrannis_).

The referred premaxilla is rather un-tyrannosaurid-like. It is tall and 
subrectangular, as seen in _Ceratosaurus_ (which, unlike _Stokesosaurus_, 
has only three teeth per premaxilla), but not tyrannosauroids (_Dilong_ 
included).  The teeth in the premaxilla are poorly preserved, but they don't 
appear to be D-shaped (unlike tyrannosauroids, even _Dilong_).  None of 
these features, however, preclude _Stokesosaurus_ from being a 
tyrannosauroid - although if the braincase is correctly referred, it is 
unlikely that _Stokesosaurus_ was ancestral to _Dilong_.

> IMHO, it probably looked alot like the primitive
> tyrannosaurs coming out of Great Britain and China, small, three fingered,
> and feathered to its toenails.

No argument there.  Poor _Stokesosaurus_ would have been dwarfed by the big 
megalosaurs and allosaurs of the Morrison.  But the tyrannosaurs would have 
the last laugh.



Tim