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 Albertosaurus and Gorgosaurus— Albertosaurus libratus is based on a nearly complete skeleton (NMC 2120) described by Lambe (1914a, b, 1917) as Gorgosaurus libratus. The holotype skull is crushed, but most of it is preserved. It hails from the Dinosaur Park Formation and is the best-sampled North American tyrannosaurid, with multiple well-preserved specimens having been collected since Lambe’s time. Albertosaurus sarcophagus Osborn, 1905 is based on a partial skeleton (NMC 5600) from the overlying Edmonton Formation of Alberta ( Osborn, 1905 ). It is not as well preserved as the type of A. libratus. A partial skeleton with no skull from the Edmonton was later described by Parks (1928) as Albertosaurus arctunguis', most authorities now regard A. arctunguis to be a junior synonym of A. sarcophagus (Molnar et al., 1990; Carpenter, 1992). Russell (1970) was the first to synonymize Gorgosaurus with Albertosaurus . I agree with Holtz (2001a) that the character states diagnosing a more inclusive Albertosaurus are plesiomorphic (and thus problematic), and the characters used by Russell (1970) to separate them as species rely on skull and limb proportions that might be subject to preservational or ontogenetic variation. Ongoing work by Philip Currie (P. Currie, pers. comm.) is clarifying the diagnoses of these fossils, but for purposes of this analysis, they will be considered species of Albertosaurus . Carpenter (1992) argued that the promaxillary fenestra lies close to the maxillary fenestra in A. libratus, but not in A. sarcophagus , and that the promaxillary fenestra of A. libratus is invisible in lateral view. In all mature A. libratus and A. sarcophagus maxillae available for this study, the morphology of the maxillary and promaxillary fenestrae was identical. The reconstruction of A. libratus figured by Carpenter (1992:fig. 2E) looks similar to that published by Russell (1970 :fig. 1), but in Russell’s reconstruction, the lateral morphological details are based on FMNH PR308. In that specimen, the promaxillary fenestra is not preserved on either side. In fact, based on the number of dentary and maxillary alveoli, this specimen may pertain to Daspletosaurus , not Albertosaurus ( Carr, 1999 ; Holtz, 2001a)—and the promaxillary fenestra is not visible in lateral view in Daspletosaurus , as implied by Carpenter (1992) for A. libratus.