A growth series of Pachycephalosaurus wyomingensis skulls and cranial elements from oldest to youngest.

Skull length increases through ontogeny. The squamosal horns and nodal ornaments reach maximum expression in ��Stygimoloch�� with the concurrent inflation of a relatively high and narrow frontoparietal dome. Ontogenetically, the dome continues to expand and incorporate the lateral cranial elements in adults. The squamosal horns and nodes erode and form clusters of robust, blunt ornaments along the dorsolateral margins of the squamosals. The adult condition is best exemplified by AMNH 1696. A series of adult Pachycephalosaurus skulls confirm the cranial morphology marked by a relatively smooth, massive frontoparietal dome, lack of a parietosquamosal shelf with clusters of conical, blunt squamosal nodes that are variable in number, sometimes asymetrical but morphologically consistent between adults. See Figures 3,6 and 7.