Long-horned Ceratopsidae from the Foremost Formation (Campanian) of southern Alberta.

The horned Ceratopsidae represent one of the last radiations of dinosaurs, and despite a decade of intense work greatly adding to our understanding of this diversification, their early evolution is still poorly known. Here, two postorbital horncores from the upper Foremost Formation (Campanian) of Alberta are described, and at ∼78.5 Ma represent some of the geologically oldest ceratopsid material. The larger of these specimens is incorporated into a fused supraorbital complex, and preserves a massive, straight, postorbital horncore that is vertical in lateral view, but canted dorsolaterally in rostral view. Medially, the supracranial sinus is composed of a small, restricted caudal chamber, and a large rostral chamber that forms the cornual diverticulum. This morphology is distinct from that of the long-horned Chasmosaurinae, and similar to, but still different from, those of younger Centrosaurinae taxa. The smaller specimen represents an ontogenetically younger individual, and although showing consistent morphology to the larger specimen, is less taxonomically useful. Although not certain, these postorbital horns may be referable to a long-horned basal (i.e., early-branching, non-pachyrhinosaurini, non-centrosaurini) centrosaurine, potentially the contemporaneous , largely known from the parietosquamosal frill. These specimens indicate the morphology of the supracranial sinus in early, long-horned members of the Ceratopsidae, and add to our understanding of the evolution of the cranial display structures in this iconic dinosaur clade.