Morphological variation in the hadrosauroid dentary

Abstract: The near global success reached by hadrosaurid dinosaurs during the Cretaceous has been attributed to their ability to masticate (chew). This behavior is more commonly recognized as a mammalian adaptation and, as a result, its occurrence in a non-mammalian lineage should be accompanied with several evolutionary modifications associated with food collection and processing. The current study investigates morphological variation in a specific cranial complex, the dentary, a major element of the hadrosauroid lower jaw. 89 dentaries were subjected to morphometric and statistical analyses to investigate the clade’s taxonomic-, ontogenetic-, and individual variation in dentary morphology. Results indicate that food collection and processing became more efficient in saurolophid hadrosaurids through a complex pattern of evolutionary and growth-related changes. The diastema (space separating the beak from the dental battery) grew longer relative to dentary length, specializing food collection anteriorly and food processing posteriorly. The diastema became ventrally directed, hinting at adaptations to low-level grazing, especially in younger individuals. The coronoid process became anteriorly directed, and was relatively more elongate, resulting in increased moment arm length, with muscles being re-directed to pull the jaw more posteriorly, and mechanical advantage increasing. Although all hadrosauroid groups went through relative dental battery elongation during growth, by incorporating more teeth into each row, the dental battery became deeper in saurolophids. Previous research supports the interpretation that this is the result of more tooth rows being stacked vertically, allowing the dental battery to work as a shock-absorber during mastication, and allowed teeth to be replaced without interruption to food consumption. The increased anterior inclination of the coronoid process, and relative elongation of the diastema in saurolophids are herein suggested to have evolved through hypermorphosis, a version of peramorphosis where the growth trajectory in the descendant taxon extends beyond the ancestral state, whereas the relative elongation of the coronoid process, the relative deepening of the dental battery, and the increased ventral deflection of the diastema are the result of a novel juvenile condition.