Daspletosaurus sp.

Miyashita, Tetsuto, Tanke, Darren H. & Currie, Philip J., 2009, Variation in premaxillary tooth count and a developmental abnormality in a tyrannosaurid dinosaur, Acta Palaeontologica Polonica 54 (4), pp. 635-643 : 636-638

cf. Daspletosaurus sp.

Fig. 1 .

Material.—TMP 2007.20.124, a partial left premaxilla.

Description.—TMP 2007.20.124 is a left tyrannosaurid premaxilla, based on the angle between the alveolar margin and the interpremaxillary suture, which is more than 50 ° ( Fig. 1 ). The angle is 60 °, and is comparable to that of immature tyrannosaurines (63 ° in TMP 1994.143.1 Daspletosaurus sp. ), but larger than that of albertosaurines (35 ° and 47 ° in TMP 1991.36.500 and TMP 1999.33.1, Gorgosaurus libratus ). Because of abrasion, it lacks the supranarial and subnarial processes, and most of the medial surface, including the interdental plates, is missing. The arc length along the complete alveolar margin is 20% larger than that of a juvenile Daspletosaurus sp. (TMP 1994.143.1) and about as large as that of an adult Gorgosaurus libratus (TMP 2005.9.9) ( Table 2; ACL). When in its life position, the premaxilla is relatively short anteroposteriorly and wide lateromedially, just as in those of similar−sized tyrannosaurids. The narial fossa is partially preserved. The interpremaxillary suture is a flat surface that extends along the medial edge of the anterior wall of the first alveolus. The interdental plates are missing, but two ridges clearly mark the boundaries between the three teeth. The partially preserved, smooth surface along the posterior edge of the bone is part of the maxillary suture. The texture is unlike the pitted, spongy alveolar surface. The three alveoli are variable in size ( Table 2). The first alveolus is the largest, and the next two alveoli are successively smaller. The three alveoli gradually taper dorsally, following the external bone profile. There is no indication of a space for an extra alveolus anywhere along the alveolar margin.

Stratigraphic and geographic range.—Dinosaur Park Formation, Belly River Group (Campanian, Cretaceous); 5 m above the boundary with the underlying Oldman Formation. Denhart Coulee, southern Alberta, Canada .

Figures

Fig. 1. A–D. A left partial tyrannosaurid premaxilla with three alveoli, from the Dinosaur Park Formation (Campanian), Alberta, Canada; TMP 2007.124.20; in lateral (A, B) and posteromedial (C, D) views; illustrations (A, C) and photographs (B, D). E. Stratigraphic column showing succession of tyrannosaurids during the Upper Cretaceous of Alberta, Canada. Shaded units indicate occurrence of tyrannosaurids identifiable down to generic level.