Colepiocephale lambei from the Foremost Formation of Alberta is a problematic pachycephalosaurid that has recently been hypothesized as (1) synonymous with Stegoceras validum, (2) a distinct species within the Stegoceras clade, and (3) an unusual taxon more derived than Stegoceras. Conflict among these hypotheses arises from different interpretations of the parietosquamosal region of the dome. Re-evaluation of the morphology of C. lambei based on all known material from Alberta indicates that the posteromedial extension of the parietal is broken off at its base in every specimen, and what was interpreted as the posterior margin of the dome actually is the anterior margins of large supratemporal fenestrae. The squamosals are not preserved in any specimen, but the form of the parietal suggests that they were present in the typical pachycephalosaurid configuration. Colepiocephale lambei is considered to be a distinct taxon based on several autapomorphies and stratigraphic segregation from S. validum. Phylogenetic analyses that include new anatomical data confirm that C. lambei occupies a basal position within domed pachycephalosaurids, but its precise systematic position with respect to S. validum remains equivocal. This general interpretation is more consistent with its stratigraphic position as one of the oldest known members of Pachycephalosauridae. Additionally, this study results in the recognition of the first material referable to this taxon from the Judith River Formation of Montana, which results in a geographic range extension for this taxon and extends its stratigraphic range into the equivalent of the lower Oldman Formation in Alberta.
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