Mandibular force profiles and tooth morphology in growth series of Albertosaurus sarcophagus and Gorgosaurus libratus (Tyrannosauridae: Albertosaurinae) provide evidence for an ontogenetic dietary shift in tyrannosaurids1

The albertosaurines Albertosaurus sarcophagus and Gorgosaurus libratus are among the best represented tyrannosaurids, known from nearly complete growth series. These specimens provide an opportunity to study mandibular biomechanical properties and tooth morphology to infer changes in feeding behaviour and bite force through ontogeny in tyrannosaurids. Mandibular force profiles reveal that the symphyseal region of albertosaurines is consistently stronger in bending than the mid-dentary region, indicating that the anterior extremity of the jaws played an important role in prey capture and handling through ontogeny. The symphyseal region was better adapted to withstand torsional stresses than in most non-avian theropods, but not to the extent seen in Tyrannosaurus rex, suggesting that albertosaurine feeding behaviour may have involved less bone crushing or perhaps relatively smaller prey than in Tyrannosaurus rex. The constancy of these biomechanical properties at all known growth stages indicates that although albertosaurines maintained a similar feeding strategy through ontogeny, prey size/type had to change between juvenile and mature individuals. This ontogenetic dietary shift likely happened when individuals reached a mandibular length of ∼58 cm, a size at which teeth shift from ziphodont to incrassate in shape and bite force begins to increase exponentially. The fact that large albertosaurines were capable of generating bite forces equivalent to similar-sized tyrannosaurines suggests that no significant differences in jaw-closing musculature existed between the two clades and that the powerful bite of Tyrannosaurus rex is the result of its large body size rather than of unique adaptations related to a specialized ecology.