Gorgosaurus libratus, Lambe, 1914

Holtz, TR jr., 2004, Tyrannosauroidea, The Dinosauria, University of California Press, pp. 111-136 : 3-1

Gorgosaurus libratus

is known from more complete material than any other North American species of tyrannosaurid (Lambe 1914), including most of the ontogenetic series (Carr 1999; Currie 2003a). To date, diagnostic material has been confirmed from only the upper Campanian Dinosaur Park Formation of Alberta; additional specimens, and especially isolated postcrania from other contemporaneous formations referred to this taxon (e.g., the Two Medicine and Judith River formations of Montana and the Fruitland and Lower Kirtland formations of New Mexico), have yet to be demonstrated as actually representing G. libratus . In fact, Carr and Williamson (2000) failed to discover any remains of albertosaurine material from the numerous tyrannosaurid bones from the Late Cretaceous of New Mexico.

The improper assignment to this species of a skull referable to Daspletosaurus torosus (Carr 1999) has unfortunately formed the basis of the main reconstruction of the dinosaur in recent decades (Russell 1970a). Gorgosaurus libratus can be diagnosed by the following autapomorphies: the first maxillary tooth incisiform; a palatine with a slotlike articular surface for the maxilla; and the articular surface of the maxilla not reaching the dorsal margin of the maxillary ramus of the palatine (Carr et al., in press). The orbit of Gorgosaurus is more circular than those of other tyrannosaurids, even in adults; this circularity is achieved by the cranial extension of the ventral end of the ventral ramus of the postorbital to nearly contact the descending ramus of the lacrimal. Gorgosaurus is known to have femoral lengths of 105 cm and skull lengths of almost 1 m; adult individuals range up to approximately 9 m in length.