Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull Author Christopher A. Brochu Department of Geology, Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605 text Journal of Vertebrate Paleontology 2003 2003-12-31 22 1 138 journal article 10.2307/3889334 6f85aa0a-0368-41a5-83d3-21c33f40845e 3739929 Daspletosaurus torosus Russell, 1970 — Russell (1970) based Daspletosaurus torosus on a partial skull and skeleton, NMC 8506, from the Campanian Dinosaur Park Formation (Oldman Formation in Russell, 1970 ) of Alberta. It had a somewhat longer rostrum than other tyrannosaurids, and the number of maxillary or dentary alveoli at maturity may exceed 15. Material from the Early Maastrichtian Two Medicine Formation, including an undescribed skeleton (MOR 590), is very similar to Daspletosaurus (Horner et al., 1992; Holtz, 2001a) and may pertain to it ( Varricchio, 2001 ). Daspletosaurus has also been reported from the Early Maastrichtian Horseshoe Canyon Formation of Alberta ( Ryan and Russell, 2001 ). The Dinosaur Park, Oldman, and Horseshoe Canyon Formations appear to be the only units in which two large tyrannosaurids ( Albertosaurus / Gorgosaurus and Daspletosaurus ) cooccur. This complicates our attempts to compare any tyrannosaurid skeleton with isolated fossils from either formation, as such bones could pertain to either taxon. There are several cranial characters that clearly separate the two, including number of teeth, size and orientation of the jugal foramen, and morphology of the promaxillary foramen, but incomplete (essentially postcranial) material may not be diagnosable to either taxon, and some may only be distinctive in mature individuals. An immature tyrannosaurid skull and skeleton from the Dinosaur Park Formation (RTMP 94.143.1) can be referred to Daspletosaurus on the basis of the number of teeth, but the morphology of the jugal and maxilla is Albertosaurus-like, in several respects.