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Re: References



In a message dated 97-05-01 01:38:25 EDT, enrique@sn.no (DinosØMP) writes:

<< > Could I get references for Stygivenator and Siamotyrannus please.  They
are
 > new enough that I have nothing that refers to them.  I know that they have
 > been on the list before, but I could not find them in the archives.  Well
all i know is that _Siamotyrannus isanensis_ is a very primitive 
 tyrannosaurid, mabye as primitive as _Itermirus medularis_. There are 
 three species of _Stygivenator_: _S. amplus_, _S. cristatus_ and _S. 
 molnari_. One of these species has replaced _Aublysodon_. As you may 
 understand, _Stygivenator_ is an aublysodontid (or allso called 
 stygivenatorid, or shanshanosaurid). >>

Because the tooth that is the type specimen of _Aublysodon mirandus_ is
generically and specifically indeterminate, it is necessary to create new
genera for some of the determinate material that has been referred to that
genus. One such specimen is the rostral portion of a skull and mandible with
teeth from the Hell Creek formation that Greg Paul made the type specimen of
the species _Aublysodon molnari_. It was described by Ralph Molnar in 1978,
but he didn't name it. Later he had second thoughts and suggested calling it
_Stygivenator_, so when the opportunity to name the genus arose, I used this
name.

The _Aublysodon mirandus_ tooth comes from the Judith River Formation of
Montana, which is of Campanian age. Since it is more likely that
"aublysodon-like" teeth from the Lance and Hell Creek belong to a Lance/Hell
Creek genus than to a Judith River genus, I tentatively referred the two
Lance/Hell Creek species _Aublysodon cristatus_ and _Aublysodon amplus_ to
the genus _Stygivenator_. The genus _Aublysodon_ itself must remain isolated
at least until someone finds some good material in which there is a
premaxillary tooth that exactly matches the type tooth.

_Stygivenator_ shows what some interesting specializations for carnivory,
namely, oversize teeth that seem adapted to scraping meat off bones. Perhaps
it was some kind of scavenger?

Having premaxillary teeth with D-shaped cross-section is a tyrannosaurid
apomorphy, so _Stygivenator_ and similar genera probably represent a sister
group to the subfamily Tyrannosaurinae within the family Tyrannosauridae. In
this sister group, the premaxillary teeth have unserrated carinae; in
Tyrannosaurinae the premaxillary teeth are almost always serrated. The name
Aublysodontinae for this subfamily cannot be used because _Aublysodon_ is
indeterminate, so the next available name based on a determinate specimen,
Shanshanosaurinae, must be used.

It's entirely possible that some "aublysodon-like" premaxillary teeth belong
to juvenile tyrannosaurines, but given how different _Stygivenator_ is from
the usual tyrannosaurines, I presently doubt this. It is really bad luck that
not a single North American specimen is known in which an "aublysodon-like"
premaxillary tooth is actually found inserted into the jaw. Even in
_Stygivenator_, the premaxillary tooth was found not in the premaxilla but
just outside it. This despite the fair abundance of "aublysodon-like" teeth
in the fossil record. From Mongolia, however, I believe there now is a good
specimen of _Alectrosaurus_ that shows an "aublysodon-like" tooth in the
premaxilla and that therefore strongly suggests referral of that genus to
Shanshanosaurinae. And from China, the type specimen of _Shanshanosaurus_
shows the _impressions_ of "aublysodon-like" teeth in the matrix near the
premaxillae, hence referral of that genus to the "aublysodon-like" subfamily.
Pretty skimpy evidence, all in all, leading to a taxonomy strongly subject to
modification with the discovery of new material.