Learn Your D-shapes! (was Re: ORNITHOMIMID THERIZINOSAURS)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 12:27 AM 7/29/98 -0700, Jaime Headden wrote:
>  I will say that with a basal dentition like d-shaped unserrated
>premaxillaries, at least several taxa form the base of the group with
>a few out-flung branches (they're derived, in other words).
>
>  *Pelecanimimus*, representing the ostrich-mimics
>  *Sinornithoides*, representing the "troodonts"
>  "Aublysodon" and *Shanshanosaurus*, representing the
>     tyrants and allies
>  *Compsognathus*, representing the "little fellows"
>     (and you all know who you, all you dinos taken
>      for granted when you could very well
>      revolutionize dinosaur phyllogeny! Didn't Thomas
>      Huxley say this?)
>  *Ornitholestes*, arguably representing basal
>      Maniraptora

A lot of people are unfamiliar with how "D-shaped" true "D-shaped"
premaxillary teeth are.  Tyrannosaurids have D-shaped incisiform
premaxillaries in which both carina lie along the same plane (they are
equally far back posteriorally), the sides of the teeth are completely flat,
and the front a bit rounded.  In truth, calling them "U-shaped" is a lot
closer to the truth!

Among other theropods, only _Pelecanimimus_ approaches this condition.
Troodontid premaxillary teeth are symmetrical, too, but the carinae go down
both sides rather than the back.

Many other theropods (dromaeosaurs, _Ornitholestes_, carnosaurs, etc.) have
asymmetrical premaxillary tooth condition, not the symmetrical teeth of
tyrannosaurids.

The premaxillary teeth of compsognathids are more conical than asymmetrical.

For a good view of the premaxillary teeth of Judith River theropods
(dromaeosaurids, troodontids, and tyrannosaurids), see Fig. 8.6, p. 120 in:

Currie, P.J., K. Rigby, Jr. & R.E. Sloan. 1990. Theropod teeth from the
Judith River Formation of southern Alberta, Canada. pp. 107-125 in K.
Carpenter and P.J. Currie (eds.) Dinosaur Systematics: Perspectives and
Approaches. Cambridge Univ. Press.

For carnosaurs, check out Madsen's _Allosaurus_ monograph, or the
_Sinraptor_ paper in CJES.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661