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Re: Horner and T-rex



In a message dated 2/7/01 11:21:02 AM EST, Chapman.Ralph@NMNH.SI.EDU writes 
(quoting Jack Horner):

<< and tore into its prey with 50 serrated teeth, each 6 inches long >>

This statement is only approximately correct (paleo license, I suppose). The 
tooth count for Tyrannosaurus rex was usually 4 premaxillary teeth, 12 
maxillary teeth, and 13 dentary teeth, for a total of 58. Of these, the two 
caudalmost maxillary and dentary teeth were short and stubby, hardly 6 inches 
long, perhaps used for crushing bone rather than tearing meat away from bone. 
The premaxillary teeth were also much smaller than the huge primary maxillary 
and dentary teeth. Some T. rex individuals had a small first dentary tooth, 
others had a large one--a distinction that Bob Bakker once asserted 
distinguished two species of Tyrannosaurus. This is probably individual 
variation or ontogenetic, since we know the teeth of tyrannosaurids changed 
size (of course) and shape somewhat throughout the animals' growth. Finally, 
at any one time there were always a couple of teeth missing from the maxillae 
and dentaries and in the process of being replaced, in all tyrannosaurids. So 
if we count just the big 6-inch-plus maxillary and dentary teeth, an adult T. 
rex probably had no more than 40 in its jaws at any one time, and more 
usually 36 or so.

Premaxillary tooth count seems to have been remarkably stable at four (total 
eight, for two premaxillae) throughout tyrannosaurid evolution. Maxillary 
tooth count also doesn't seem to have decreased by more than one or two. But 
dentary tooth count decreased significantly: from a maximum as high as 17 (in 
Albertosaurus sp.) down to 13 (in Tyrannosaurus rex). Fifteen is typical of 
Albertosaurus libratus and also Tarbosaurus; 14 is typical of Albertosaurus 
torosus. Maybe adult dentary tooth count has taxonomic/evolutionary 
significance. Tooth count seems to decrease during tyrannosaurid ontogeny, 
too; perhaps this is phylogenetic recapitulation. In some tyrannosaurids with 
maxillary tooth count of 13 (e.g., Albertosaurus libratus), the first 
maxillary tooth is transitional in form between the D-shaped-cross-section 
premaxillary teeth and the more bladelike lateral maxillary teeth.