Cranial anatomy and stratigraphy of a new specimen of Daspletosaurus from the Judith River Formation of Central Montana

The tyrannosaurine Daspletosaurus contains three recognized species from the Campanian of Montana and Alberta: Daspletosaurus torosus, Daspletosaurus wilsoni, and Daspletosaurus horneri. The recently named D. wilsoni has been proposed to represent a transitional anagenetic form between D. torosus and D. horneri, a hypothesis contingent on both the stratigraphic succession of these three taxa and the presence of an intermediate morphology in D. wilsoni. Adequate testing of this hypothesis is hampered by limited knowledge of the morphological variation and stratigraphic ranges of both D. wilsoni and D. torosus. We introduce a new, ontogenetically mature specimen of Daspletosaurus from the upper Campanian Coal Ridge Member of the Judith River Formation of central Montana that is well constrained to ~76.3–75.8 Ma. This specimen has a combination of features not yet reported in Daspletosaurus, increasing the known range of morphological disparity within this genus. The cranial morphology and stratigraphic position of this specimen precludes its referral to D. horneri. Although stratigraphically equivalent to D. wilsoni, this specimen lacks one of the three characters purported to distinguish that taxon from D. torosus (dorsal quadrate process of quadratojugal broadly visible laterally). We propose that this character is intraspecifically variable within Daspletosaurus and therefore not diagnostic, thus weakening the case that D. wilsoni is distinct from D. torosus. Additional specimens with stratigraphic controls are necessary to determine if D. wilsoni is a valid taxon.