Morphology and histology of new cranial specimens of Pachycephalosauridae (Dinosauria: Ornithischia) from the Nemegt Formation, Mongolia

Previously, only three pachycephalosaur specimens have been reported from the Nemegt Formation, two of which are holotypes (Homalocephale calathocercos and Prenocephale prenes) and all of which were collected from the classic Nemegt locality. Here, we describe several new cranial specimens from this formation, but which originate from Bügiin Tsav, Guriliin Tsav, and Tsagaan Khushuu localities. The specimens consist of an incomplete frontoparietal dome that is slightly smaller than the holotype of P. prenes, and two specimens that are larger than the holotype -a second incomplete frontoparietal and a complete squamosal with ornamentation. All specimens are referable to P. prenes or cf. Prenocephale. Cranial histology is investigated in an Asian pachycephalosaur for the first time on the basis of these new specimens. The specimens increase the ontogenetic representation for Prenocephale, and show that the squamosal ornamentation pattern is consistent across the known size range. These specimens expand the geographic range of pachycephalosaurs in the Nemegt Formation, and provide new ontogenetic data for P. prenes that reinforces the taxonomic distinction between it and Homalocephale.