Daspletosaurus (Russell, 1970)

Currie, Philip J., 2003, Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, Palaeontologica Polonica 48 (2), pp. 191-226 : 221

Daspletosaurus .

— Daspletosaurus torosus Russell, 1970 is known from the holotype skull and skeleton collected from the Oldman Formation of Dinosaur Park. Another specimen (TMP 2001.36.1) has been collected from the same formation some 250kilometers southeast of Dinosaur Park on the Milk River.

The type specimen of Daspletosaurus torosus was collected from the Oldman Formation, which is the lower of the two terrestrial formations exposed in Dinosaur Provincial Park ( Eberth et al. 2001). Additional skulls with skeletons of Daspletosaurus ( Table 1 ) been recovered from the younger Dinosaur Park Formation in southern Alberta, and a skull (MOR 590) has been found in Montana. A specimen (NMC 11315) from the Horseshoe Canyon formation that was tentatively assigned to this genus ( Russell 1970) is now known to be Albertosaurus . Based on cranial morphology (Currie and Bakker in preparation), the Dinosaur Park Formation specimens seem to represent a distinct species from Daspletosaurus torosus . Furthermore, MOR 590is distinct from both in having a relatively tall, triangular lacrimal horn and seems to represent a distinct species ( Horner et al. 1992). Finally, the specimen described by Lehman and Carpenter (1990) as “ Aublysodon ” may represent another undescribed species of Daspletosaurus based on differences in cranial morphology ( Carr and Williamson 2000). The fact that three or more distinct forms of Daspletosaurus can be recognized justifies the separation of Daspletosaurus from Tyrannosaurus at the generic level. Although there has been a tendency to consider Daspletosaurus as a sister taxon of Tarbosaurus + Tyrannosaurus , there has been no justification to assume this is true without a full phylogenetic analysis of these animals. The analysis of Currie et al. (2003) suggests that Tarbosaurus is the sister taxon of Daspletosaurus + Tyrannosaurus , although the relationship is assumed on the basis of relatively few characters. Furthermore, the geographic and stratigraphic occurrences of these animals suggest the most parsimonious a priori interpretation is that Daspletosaurus and Tyrannosaurus are probably more closely related to each other than either is to Tarbosaurus .

Daspletosaurus shares many derived characters with other tyrannosaurines. It can be distinguished most easily from mature specimens of Tarbosaurus and Tyrannosaurus by its lack of a suborbital process on the postorbitals ( Currie et al. 2003). Contrary to published reports ( Russell 1970, Holtz 2001), the premaxilla and nasal contact each other beneath the external naris. Holtz (2001) also characterized Daspletosaurus as having intergrowth between the premaxillae. However, coossification of the premaxillae in Daspletosaurus is limited to only a single specimen (NMC 8506) of more than ten known individuals. The postorbital region of the skull is laterally expanded in Nanotyrannus and Tyrannosaurus , but like Tarbosaurus , Daspletosaurus is intermediate in the degree of expansion between these genera and albertosaurines.