A new paper is available online that will shock you:
P. -E. Dieudonné, P. Cruzado-Caballero, P. Godefroit & T. Tortosa (2020) A new phylogeny of cerapodan dinosaurs. Historical Biology, DOI: 10.1080/08912963.2020.1793979
The fossil record of Pachycephalosauria is currently confined to the Late Cretaceous, as the putative Early Cretaceous pachycephalosaurs Stenopelix and Yaverlandia have been removed from the group and re-assigned to Ceratopsia and Maniraptora respectively. Xu et al. (2006) proposed that Heterodontosauridae is closely related to Marginocephalia and coined the name Heterodontosauriformes for the Heterodontosauridae+Marginocephalia clade, but that scheme found virtually no support in subsequent Ornithischia-related cladistic studies. However, the placement of Heterodontosauridae inside Pachycephalosauria could be a possible explanation for the absence of Pachycephalosauria in pre-Cenomanian deposits, because the bauplan of early ceratopsians is not known.
Xu X., X.; Forster, C.A.; Clark, J.M.; Mo J. (2006). A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences 273 (1598): 2135–2140. doi:10.1098/rspb.2006.3566. PMC 1635516. PMID 16901832.