Tyrannosaurus rex, Osborn, 1905

Holtz, TR jr., 2004, Tyrannosauroidea, The Dinosauria, University of California Press, pp. 111-136 : 2

Tyrannosaurus rex

is the last and largest known tyrannosaurid (Osborn 1905), having a premaxilla-occipital condyle length of 153 cm and a femur length of 138 cm. It was widespread throughout the North American West during the last few million years of the Cretaceous. Tyrannosaurus demonstrates the greatest constriction of the nasals between the lacrimals (less than one-sixth the maximum nasal thickness). The rostralmost point of the quadratojugal ventral process is rostral to the infratemporal fenestra. As in Daspletosaurus , the distal margin of metatarsal III is sigmoid with a concavity in the distolateral margin in cranial view; and as in Albertosaurus , the scapula is expanded caudally. Tyrannosaurus shows the greatest lateral expansion of the skull found in tyrannosaurids (Holtz 2001b; Currie 2003a), with the transverse width of the rostrum at the caudal end of the maxillary tooth row approximately three times the maximum width of the nasals and the maximum postorbital skull width more than two-thirds the length of the premaxillaoccipital condyle. Consequently, the orbits of Tyrannosaurus face more rostrally than laterally (Molnar 1991).

Some small tyrannosaurids from the Hell Creek Formation of Montana previously treated as Albertosaurus or Aublysodon (Molnar 1978, 1980a; Currie 1987a; Paul 1988b; Molnar and Carpenter 1989; Holtz 1994, 2001b) almost certainly document juveniles of Tyrannosaurus (Carr and Williamson, in press; Currie 2003a). Of special note is a 57 cm long skull from the Hell Creek Formation of Montana described by Gilmore (1946a) as Gorgosaurus lancensis and subsequently referred to its own genus, Nanotyrannus (Bakker et al. 1988) . Because of some similarities with adult Tyrannosaurus rex , these authors and others (Russell 1970a; Carpenter 1992b) voiced suspicion that this skull might represent a juvenile of that larger sympatric species. However, based on the different dental formula, more ziphodont teeth, and apparent cranial suture fusions, this taxon was retained in some recent reviews (Molnar et al. 1990; Holtz 1994). Most of the skull surface, however, demonstrates the presence of striated cortical bone (Carr 1999), as is found in other juvenile ornithodirans (Bennett 1993; Sampson 1993).

Furthermore, the fusion of various cranial bones has been disputed (Carr 1999). 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 (but see Currie 2003a, 2003b).Furthermore, the skull of “ Nanotyrannus ” demonstrates several T. rex autapomorphies (Carr 1999; Holtz 2001b; Brochu 2002; Carr and Williamson, in press). “ Nanotyrannus ” represents either a juvenile Tyrannosaurus rex or the juvenile of another tyrannosaurine sympatric with T. rex , closer to T. rex than to all other tyrannosaurids but not yet recognized from adult material. The discovery of either an adult Nanotyrannus that can be clearly distinguished from Tyrannosaurus rex or a juvenile T. rex of the same skull length and/or ontogenetic stage as, but morphologically distinct from, Nanotyrannus would resolve this taxonomic situation.