The cranial roof ornamentation and the degree of neurocentral closure in the holotype of the pachy cephalosaurian dinosaur Homalocephale calathocercos is described in detail for the first time in order to assess its ontogenetic status and taxonomic validity. The parietosquamosal ornamentation consists of five primary nodes along the posterior margin of the skull roof on each side of the midline. The medial-most node is prominently enlarged relative to the others in the series and is bisected by the parietal-squamosal suture. Beneath the primary node row, a second enlarged node occurs immediately medial to lateroventral corner node on each side of the parietosquamosal bar. The degree of suture closure in the vertebral column suggests that the holotype specimen of H. calathocercos is immature, rather than adult. The presence of cranial doming at maturity is therefore unknown in H. calathocercos, and hypotheses that it represents a primitive flat-headed stage in pachycephalosaur evolution or is pedomorphic in the development of its skull roof cannot be supported or refuted at this time. Despite the probable immaturity of the holotype, H. calathocercos has a unique pattern of parietosquamosal ornamentation that is distinct from all other pachycephalosaurs, including Prenocephale prenes from the same host formation. Morphological differences in the cranial ornamentation, jaws, and dentition between the similarly sized holotype skulls of H. calathocercos and P. prenes provide weak support for recent suggestions that H. calathocercos is conspecific with the latter taxon. This study tentatively reaffirms the presence of two pachycephalosaur species in the Nemegt Formation
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Vol. 31 • No. 1