Daspletosaurus torosus (Russell, 1970)

Currie, P. J., 1987, Theropods of the Judith River Formation of dinosaur Provincial Park, Alberta, Canada, Fourth Symposium on Mesozoic Terrestrial Ecosystems, Drumheller, Alberta: Tyrrell Museum of Paleontology, pp. 51-60 : 54

Tyrannosaurid frontals are very distinctive in their relationships with the bones they contact. Two or three prongs of the nasal overlap the rostral end of the frontal. The prefrontal is always present, although in some species it may be fused to either the nasal or lacrimal, and in others it is covered by the lacrimal. The prefrontal sits in a pit on the doraolateral surface of the frontal. Primitively ( Figs. 1f, g ), the pit is overlapped posteriorly by the dorsal surface of the frontal, so that the caudal end of the prefrontal actually inserts into a socket in the frontal, and is completely surrounded by that bone. ln more progressive forms (Molnar 1980; Bakker, Williams and Currie, in preparation), the socket is lost, and one suspects that this is the case in Daspletosaurus torosus ( Fig. 1h ). The orbital rim of the frontal is very short, and in some species is reduced to a slot passing between the lacrimal and postorbital ( Fig. 1g ). The frontal-postorbital suture is expanded in all tyrannosaurids ( Figs. 1f, g, h ). At the rostral end of the contact, the frontal develops a pronounced buttress with a posterolaterally oriented suture ( Fig. 1f ), behind which the frontal is broadly overlapped by the postorbital. The relationship between the frontal and postorbital changes both ontogenetically and in more progressive tyrannosaurids however, and the contact becomes more elongate rostrocaudally and more vertical. A rostrodorsal process of the parietals separates the frontals posteriorly, and the sagittal crest extends onto the frontals. Tyrannosaurid frontals are essentially flat dorsally between the orbits. The temporal musculature extends far onto the dorssl surface of the frontal, its anterior boundary being a low sinuous ridge between the sagittal crest and the postorbital buttress.

Figures

FIG. 1 Theropod frontals from Dinosaur Provincial Park. (a) Troodon formosus. TMP 86.49. 10; (b) Dromicelomimus samueli, TMP 85.56.62; (c) cf. Erilcosaurus sp., NMC 12355; (d) Saurornitholestes langstoni, TMP 86. 77. 57; (e) Dromaeosaurus albertensis, NMC 12349, AMNH 5356; (f) cf. Aublysodon sp., TMP 80.16.485; (g) cf. Gorgosaurus libratus, TMP 80.16.924; (h) Daspletosaurus torosus (after Russell. 1970). (1) Caudal limit of dorsalateral exposure of the lacrimal; (2) caudal limit of orbital rim; (3) caudal limit of postorbital suture; (4) lacrimal slot; (5) Postorbital buttress. Scale= 1cm.