Structural Mechanics of Pachycephalosaur Crania Permitted Head-butting Behavior

Pachycephalosaurian dinosaurs have dorsally thickened crania and uniquely shaped frontoparietal domes in some genera, suggested as evidence for heador flank-butting behavior. Trabeculae thought to have resisted impact compression are present as only one histological zone of some pachycephalosaur domes, and are surrounded superficially or replaced by thick compacta in adults. The capabilities of pachycephalosaurian crania for head-butting are testable by finite element analysis (FEA). FEA of 2and 3-D dorsal skull shapes of adult Homalocephale and Pachycephalosaurus reveal that the domes could withstand considerable impact force at certain closing speeds, and that stress and strain would dissipate efficiently throughout the dorsal portion of the skull before reaching the brain. Greater vaulting of the dome permitted higher impact forces. An analysis restricted to the frontoparietal dome of a subadult pachycephalosaurine, with material properties corresponding to histological zones, shows higher compressive strain (not less) in the trabecular region. The trabecular zone, if present, would not have rigidly resisted compression but rather have allowed slight elastic compression and rebound. Modeled keratinous coverings of varying depth indicate reductions in force and energy transmission to underlying bone. FEA therefore leaves open the possibility of head-butting in both flatand dome-headed pachycephalosaurs, especially at low collision speeds.