Immature Daspletosaurus sp. specimens from the Dinosaur Park Formation provide insight into ontogenetically invariant tyrannosaurid cranial morphology

Understanding the extent to which tyrannosaurid cranial morphology changed through ontogeny is essential to correctly refer tyrannosaurid specimens at varying life stages to taxa. However, taxonomically diagnostic immature tyrannosaurid bones remain exceedingly elusive. Over the last century the University of Alberta has collected three isolated immature cranial bones referable to Daspletosaurus from Dinosaur Provincial Park, Alberta of Treaty 7 Territory. These bones were all collected from the rocks of the Dinosaur Park Formation and include a premaxilla (UALVP 48717), jugal (UALVP 61561), and lacrimal (UALVP 47955). To assess the taxonomic utility of these bones, they were compared to tyrannosaurid material from the Dinosaur Park and Oldman Formations of Alberta. This provided an opportunity to examine size-independent discrete characters in tyrannosaurid crania. The results of this study suggest that many premaxillary, jugal, and lacrimal discrete characteristics observed in Daspletosaurus are constrained throughout ontogeny. A single ontogenetically invariant character, the dorsal flange of the palatal process, was identified for Daspletosaurus premaxillae. Ontogenetically invariant characters of the jugal include, but are not limited to, a medial lacrimal slot, and the lateral fossa on the postorbital process. Invariant lacrimal characters include an anteroposteriorly short pneumatic recess opening and a subdivided nasal articulation fossa on the medial surface of the bone. The identification of these characters allows specimens of the tyrannosaurine Daspletosaurus and the contemporaneous albertosaurine Gorgosaurus to be confidently identified regardless of ontogenetic stage. These characters were applied to a controversial tyrannosaurid specimen from the Dinosaur Park Formation (TMP 1994.143.0001) resulting in referral to G. libratus, rather than Daspletosaurus sp.