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A densely packed bone bed near Grande Prairie, Alberta, has produced abundant remains of a new species of ceratopsid. A minimum number of 27 individuals is represented in the part of the bone bed that has been excavated so far. The new animal— Pachyrhinosaurus lakustai sp. nov.— is closely related to the centrosaurine Pachyrhinosaurus canadensis, which has been recovered from younger beds in southern Alberta. It differs from the geologically younger species in having a relatively shorter nasal boss that is well separated from the supraorbital bosses. Juveniles of the new taxon resemble juveniles of Centrosaurus and other centrosaurines. However, the cranial morphology underwent a remarkable ontogenetic change, in which the nasal and supraorbital horns of the juveniles transformed into a huge nasal boss and smaller supraorbital bosses, and the frill became adorned with spikes and horns on top of and at the back of the parietal. Although there is some indication that the species may have been sexually dimorphic at maturity, it is not possible to separate sexual variation from individual and ontogenetic variation without a much larger sample. It is quite clear that the nasal boss supported some sort of keratinous structure, although it is not possible to determine its shape and function. No cause has been determined for the apparent catastrophic demise of a herd of P. lakustai.

Key words: Dinosauria, Ceratopsia, Pachyrhinosaurus lakustai, Late Cretaceous, Alberta, Canada.

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