Albertosaurus libratus

The holotype of A. libratus (CMN 2120)

was first named Gorgosaurus libratus by Lambe (1914). Its slender upper incisiform teeth distinguished it from the robust dentition of Deinodon horridus Leidy, 1856 from the equivalent Judith River Formation of Montana. Later, Matthew and Brown (1922) showed that the type teeth of Deinodon and Gorgosaurus were indistinguishable, and that the two taxa likely were congeneric. However, the authors retained generic distinction in the absence of diagnostic skeletal material for Deinodon. Russell (1970), in agreement with Gilmore (1946), determined that the lectotype of D. horridus, comprising two incomplete incisiform teeth, is indistinguishable from the Dinosaur Park material, and D. horridus is therefore a nomen dubium (for an opposing view, see Sahni, 1972).

Russell (1970) concluded that Gorgosaurus libratus and Albertosaurus sarcophagus Osborn, 1905 were congeneric on the basis of overall similarity, a position that is upheld herein on a phylogenetic systematic basis (see Carr, 1996). Albertosaurus was rediagnosed and distinguished from Daspletosaurus by Russell (1970).

I agree with Russell (1970) that the types and referred material of G. libratus and A. sarcophagus appear to be identical, and are different from specimens referred to Tyrannosaurus bataar Maleev, 1955b and T. rex Osborn, 1905. I also agree with Carpenter (1992) that discrete differences exist between the tyrannosaurid skulls from the Dinosaur Park and Horseshoe Canyon formations.

Ontogenetic changes to discrete morphological characters are based on specimens referred to A. libratus, with the exception of FMNH PR308. Morphologically, this specimen is identical to the morphotype represented by D. torosus. Although the skull is considered representative of A. libratus (Russell, 1970: fig. 1; Paul, 1988:335; Carpenter, 1992: figs. 1, 2E), the specimen is less complete (Fig. 1A, B) than usually shown. Also, FMNH PR308 has been focal in discussions of tyrannosaurid diversity, with specific reference to tooth size and number (Bakker et al., 1988; Paul, 1988). In fact, every upper tooth and all but 13 dentary teeth are restored in plaster (pers. obs.), rendering the material basis of the former aspect moot.

The relationship between size and morphology has not been adequately studied in large theropods, and no attempt has been made to do so herein. This important question would best be answered by a quantitative and comparative study in the realms of biomechanics and functional morphology. The ontogenetic characters in this study were chosen cognizant of this issue. Thus, inclusive features of possible structural importance to the skull were avoided.