The phylogeny and taxonomy of the Tyrannosauridae Author Holtz, T. R. text 2001 2001-12-31 Indiana University Press Bloomington Editor Tanke D. H. Editor Carpenter K. Mesozoic Vertebrate Life 64 83 book chapter 10.5281/zenodo.3245327 9bf3a9f5-cd36-4be6-bba9-49607815bcd0 3245327 Tyrannosaurus rex Osborn 1905 : The last and largest known tyrannosaurid, T. rex is represented by numerous skulls and postcrania from the late Maastrichtian Hell Creek Formation of Montana, Wyoming, and South Dakota, the Lance Formation of Wyoming, and equivalent beds in Saskatchewan, Alberta, and other localities in the North American West. This species is characterized by numerous autapomorphies. Gilmore (1946) described CMNH 7541 , a 572 mm long skull from the Hell Creek Formation of Montana, as a new species of Gorgosaurus , G. lancensis . This taxon was later ( Bakker et al. 1988 ) referred to its own genus, Nanotyrannus , Because of some similarities with adult Tyrannosaurus rex , these authors and others ( Russell 1970 ; Carpenter 1992 ) have voiced suspicion that this skull might represent a juvenile of that larger sympatric species. Carr (1999) documents the presence of juvenile striated cortical bone over most of the skull's surface, and cannot verify the presence of cranial fusions previously used to indicate the adult nature of this skull. Additionally, the changes in lateral tooth shape and maxillary tooth number used to distinguish " Nanotyrannus " from Tyrannosaurus also occur in the growth series of Gorgosaurus . Furthermore, the skull of " Nanotyrannus " demonstrates several T. rex autapomorphies: 103.1, 104.1, 105.1, 106.1, 108.1, and 109.1. In light of this, and pending the discovery of a skull of different morphology which can be more clearly demonstrated to be a juvenile T. rex , " Nanotyrannus " is here considered to be a young individual of Tyrannosaurus and not a distinct taxon.