Siamo-what??

[NJPharris@aol.com]

Warning:  long post concerning theropod systematics.  (but no cladograms; I
promise!)

I hate to disappoint Messrs. Buffetaut, Suteethorn, and Tong, but (with all
due respect to Dr. Tom) they have not discovered the world's oldest
tyrannosaur.  They have discovered, AFAIK, the world's youngest sinraptorid.

The ilium of _Siamotyrannus isanensis_ is virtually identical to those of
_Allosaurus_ and _Sinraptor_, apart from a pair of vertical ridges in _S.
isanensis_, the distribution of which among the Theropoda seems rather
variable.  

The dorsal margin of the ilium is quite straight, as in allosauroids, and
squared off at either end in a manner quite unlike tyrannosaurs.  The cranial
end bears little resemblance to the prominent hook found in all tyrannosaurs
(and, for that matter, also in ornithomimosaurs).  

The pubic peduncle is long and points cranioventrally, as in allosauroids,
unlike the pub. ped. of tyrannosaurs (and all other coelurosaurs), which is
much stubbier and points ventrally.  The supra-acetabular ridge overhangs the
ischiadic peduncle quite strongly, as in allosauroids but unlike
tyrannosaurs, as far as I can tell.

The pubic boot somewhat resembles that in tyrannosaurs, although it resembles
nothing so much as the boot of _Deltadromeus_; it looks sort of like the boot
of _Sinraptor_ tilted forward.  

The pubic foramen is largely surrounded, but not completely enclosed, by a
large bony hook.  This resembles the situation in _Sinraptor_ and even more
so that in _Yangchuanosaurus_ (the pubic foramen in _Yangchuanosaurus_ is
often drawn as completely surrounded by a thin bony ring, the hook having
been interpreted as broken; however evidence from _Sinraptor_ and from
photographs of the type specimen [e.g. in Norman's _Illustrated
Encyclopedia_, p. 67] confirms that the pubic foramen in _Y. shangyouensis_
was not completely enclosed).  I can find no non-sinraptorid which shares
this character.

Oddly, the authors place _Siamotyrannus_ in a clade containing
Tyrannosauridae and Bullatosauria (wouldn't that be Arctometatarsalia?) based
on some characters of the pelvis.  They also conclude that _Siamotyrannus_
could not have evolved from the Allosauridae because of the hook around its
pubic foramen.  However, they apparently fail to realize that the presence of
this hook should also exclude _Siamotyrannus_ from membership in the
Coelurosauria, every known member of which, from _Ornitholestes_ to
_Gorgosaurus_, has a completely open pubic foramen!

The obturator process of the ischium is quite different in allosauroids and
coelurosaurs.  In allosaurs the old ischiadic plate has been cut away at both
ends, leaving a quadrangular (Allosauridae and Sinraptoridae) or "lobulate"
(Carcharodontosauridae) obturator flange.  The obturator process in
coelurosaurs is formed solely by the opening of the obturator foramen of the
ischium; the ventral edge of the obturator process is confluent with the
ventral edge of the shaft of the ischium.  The form of the ischiadic shaft
and the area of attachment for the obturator process (which has itself broken
off) suggest an allosauroid-like flange in _Siamotyrannus_ rather than a
coelurosaur-like process.

There is a prominent ridge running along the caudodorsal edge of the ischium.
 The authors compare this to the situation in arctomets but fail to note that
a very similar ridge is found in _Sinraptor_, _Yangchuanosaurus_, and
_Szechuanosaurus_.  The authors themselves admit that the caudal vertebrae
preserved along with the pelvis are more like those of allosauroids than
those of tyrannosaurs.

In short, there are enough specialized characteristics in the pelvis of
_Siamotyrannus isanensis_ to convince me that this animal is a member of the
Allosauroidea and not of the Tyrannosauridae as the describers advocate.
 Some of these characters (hook around pubic foramen, ridge on ischium),
along with biogeographical considerations, point towards a more specific
referral of this taxon to the family Sinraptoridae.

Check it out if you're interested; tell me what you think.

References:

E. Buffetaut, V. Suteethorn, and H. Tong.  "The earliest known tyrannosaur
from the Lower Cretaceous of Thailand."  _Nature_. 20 June, 1996.  pp.
689-691.

P. Currie and Zhao X.  "A new carnosaur (Dinosauria, Theropoda) from the
Jurassic of Xinjiang, People's Republic of China."  _Canadian Journal of
Earth Sciences_.  October-November, 1993.  pp. 2037-2081.

D. Norman.  _The Illustrated Encyclopedia of Dinosaurs_.  New York:
 Salamander Books, Ltd., 1985.

Thanks for your time.

Nick "he's at it again!" Pharris
Olympia, WA