| Summary: | A new specimen attributable to an immature individual of Pachyrhinosaurus perotorum (Dinosauria, Ceratopsidae) from the Kikak-Tegoseak Quarry in northern Alaska preserves a mix of features that provides refinement to the sequence of ontogenetic stages and transformations inferred for the development of the nasal boss in Pachyrhinosaurus. The new specimen consists of an incomplete nasal that includes the posterior part of the nasal horn, the dorsal surface between the horn and the left-side contacts for the prefrontal and frontal, and some of the left side of the rostrum posteroventral to the nasal horn. The combination of morphologies in the new specimen suggests either an additional stage of development should be recognized in the ontogeny of the nasal boss of Pachyrhinosaurus, or that the ontogenetic pathway of nasal boss development in P. perotorum was notably different from that of P. lakustai. Additionally, the presence of a distinct basal sulcus and the lateral palisade texture on the nasal horn of the specimen described here indicate that a thick, cornified horn sheath was present well before the formation of a dorsal cornified pad. A separate rugose patch on the nasal well posterior to the nasal horn is evidence for a cornified integumentary structure, most likely a thick cornified pad, on the posterior part of the nasal separate from the nasal horn prior to the onset of nasal boss formation in P. perotorum.
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