Cranial anatomy and diagnosis of Stygimoloch spinifer (Ornithischia: Pachycephalosauria) with comments on cranial, display structures in agonistic behavior

A new skull of Stygimoloch spinifer (MPM 8111) from the Upper Cretaceous Hell Creek Formation of North Dakota is the most complete specimen discovered to date. It allows much of the skull and braincase of this unusual pachycephalosaurid to be described for the first time and confirms a suite of diagnostic characters for the species. The skull is long with a vaulted, transversely narrow frontoparietal dome and a robust squamosal forming a prominent posterior shelf The shelf is ornamented by three to four large, low-angle horns and multiple clusters of smaller bony nodes. The orientation of the squamosal is preserved along an unambiguous contact with the frontoparietal suture, allowing definitive determination of the orientation of the squamosal horns. These cranial features indicate a different mode of agonistic behavior than previously suggested for Stegoceras and Pachycephalosaurus. The high, narrow dome of S. spinifer is not suited for head-butting, and the orientation of its squamosal horns and ornamental nodes strongly suggest display functions. Several additional specimens are described and referred to S. spinifer.