Kinetic limitations of intracranial joints in Brachylophosaurus canadensis and Edmontosaurus regalis (Dinosauria: Hadrosauridae), and their implications for the chewing mechanics of hadrosaurids.

The highly specialized tooth morphology and arrangement of the dental battery of hadrosaurids has led to much speculation surrounding the chewing mechanics of this successful group of herbivorous dinosaurs. Pleurokinesis, a long established hypothesis explaining the ornithopod chewing mechanism, proposes a transverse power stroke in hadrosaurids that was accommodated by vertical adduction of the mandible, lateral rotation of the maxilla at the maxilla-premaxilla joint, lateral rotation of the jugal-maxilla complex at its contact with the lacrimal, and posterolateral rotation of the quadrate at its contact with the squamosal. A secondary series of movements were also thought to have occurred as a consequence of these primary movements. In this article, the intracranial joint morphology is described for both Brachylophosaurus canadensis and Edmontosaurus regalis and their permissive kinematics are established. Based on this evidence, the movements associated with pleurokinesis are not accommodated in these hadrosaurine dinosaurs. Rather, the movements that seem most likely to have produced the observed dental wear patterns are those associated with the mandible about the jaw joint. The structure of this joint appears well-suited to have accommodated some translation as well as rotation of the mandible about the quadrate condyle. Three-dimensional modeling of the alternate mandibular movements reveals that not all the combined labiolingual width of the lingual and buccal facets of the tooth row was involved in the power stroke. Rather, limits on the degree of mandibular long axis rotation suggest that only the lingual facet and the more medial portion of the buccal facet were utilized.