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D-shaped teeth: what are they, and whose got'em



Damn!! 163 messages when I logged in, and a number of them theropodan.

I'll try and get this one dealt with before going off to Historial Geol.

D-shaped teeth (as I use the term) might better be called U-shaped teeth.
They have parallel sides (VERY VERY different from the condition in
Allosaurus et al.), and the carinae are equidistant along the back.
Tyrannosauroid D-shaped premaxillary teeth are so different from typical
theropod pmax teeth that Marsh thought that they were the incisors of some
hitherto undiscovered large Mesozoic mammal.

This tooth form has been found in tyrannosaurids as traditionally defined,
in Eotyrannus, and in isolated teeth associated with the cf. Stokesosaurus
material from Guimarota and from the Morrison.  They are arguably present in
Pelecanimimus, but the fact that the skull is small, the jaws are closed,
and the specimen is still in the block means that one can't check this for
certain.

The cranial material currently associated with Stokesosaurus clevelandi has
eroded crowns with conical roots: however, it does seem that these teeth
have a more ancestral assymmetrical orientation.

Troodontids have symmetrical carinae 180 degrees off the ancestral carinal
condition (i.e., left & right rather than front & back), but they are not
placed on the back of the tooth nor are the sides of the premaxillary teeth
slab-sided.

Many avetheropods have assymmetrical carinae (i.e., the carinae are offset
from being directly front and back).  These teeth have sometimes
inaccurately been described by Jurassic theropod workers as "D-shaped", but
they are easily distingiushable from the tyrannosauroid condition.  More
basal theropods have conical pmax teeth, with the carinae up the front and
down the back.

As to wether this is a coherent signal: this can only be interepreted in the
light of the consilience of the information with the distribution of OTHER
derived characters.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796