Abstract

Individual variation for the large theropod Tyrannosaurus rex may be seen in the maxilla, dentary and ischium. The maxilla is variable in its depth, the size and shape of the maxillary and antorbital fenestrae, and the size and shape of the lacrimal and jugal processes. Even the left and right maxillae of the same skull show variation. Sexual dimorphism is suggested by the presence of two morphs, one more robust than the other. The angle between the ischia and caudals of the robust morph is greater than in the slender morph, and would provide ample space for the passage of eggs. On this basis, the robust morph is considered the female.

Introduction

The large theropod, Tyrannosaurus rex, was named by Osborn in 1905 on the basis of a partial skull and skeleton from the Hell Creek Formation of eastern Montana. The holotype (AMNH 973) was later transferred to the Carnegie Museum of Natural History (CM 9380) where it is presently on display. A second specimen (AMNH 5866) from the Lance Formation of eastern Wyoming, was named Dynamosaurus imperiosus by Osborn (1905) but later synonymized with T. rex (Osborn, 1906). This specimen [BM(NH) R7994 and R7995] is now mounted at the British Museum (Natural History). Numerous additional specimens have since been recovered from the Scollard and Willow Creek formations of Alberta, the Hell Creek Formation of North Dakota, South Dakota, and Montana, and the Laramie Formation of Colorado. These specimens are presently under study by Robert Bakker, Philip Currie, Ralph Molnar, Greg Paul, and myself.