The ceratopsid nasal horn is comprised of towering, fused nasal bones. In chasmosaurines, a small epinasal bone sits atop the nasals until it is incorporated into the bony structure of the horn. The epinasal is most obvious in certain juvenile and subadult Triceratops specimens. Unlike the postorbitals, nasal horns may not be primitive for Ceratopsidae— Zuniceratops lacks any kind of nasal horn, and the plasticity of the nasal structure in both ceratopsid subgroups suggests that they formed independently in both groups. We’ll talk about chasmosaurine nasal horns first, as they are the most standardized and vanilla.
Chasmosaurine nasal horns are never all that tall, and while they appear early in life, the horn itself does not become truly noticeable until late in life. Certainly the suggestion of a horn is there in subadult animals—in fact, it forms a noticeable “step up” from the rostrum to the forehead (see growth series below). However, aside from the small epinasal, a true horn does not develop until the chasmosaurine reaches maturity. In almost all genera, the horn is angled anteriodorally, forming a “ski slope” toward the rostrum. Once chasmosaurines reach maturity, the nasal horn continues to thicken and elongate—in old adult Triceratops specimens, the horn points subhorizontally. Some chasmosaurines, such as Torosaurus, place less emphasis on the nasal horn, seemingly content with developing a narrow, almost rounded horn. Agujaceratops follows a similar pattern. Even as its postorbital horns rise ever higher, the nasal horn remains short and rounded, though definitely more distinct than in Torosaurus.
Nedoceratops (= Diceratops, Diceratus ) apparently has no nasal horn, but a close inspection of the type (and only) skull reveals that in fact it has a very small one. Lacking an epinasal, the stepped structure of the horn may suggest that the individual is simply a subadult who has not developed its nasal horn yet. On the other hand, it may also be related to Torosaurus (as a recent phylogeny suggests), which itself has an abbreviated nasal horn. Much has been made of Arrhinoceratops ’ alleged lack of nasal horn (thus its name), but rest assured that a nasal horn is present, just partially broken off. In fact, its nasal horn may have been quite impressive.
Centrosaurine nasal horns are divided basically into two types: horns and bosses. A small banana-shaped boss is present on Albertaceratops, and two veeery small horns are present on an undescribed taxon from Utah (seems they could easily be sections of a broken boss). Horns are generally more prevalent than bosses among centrosaurines, though they come in some weird shapes. Centrosaurus (= Monoclonius ) has a large, singular horn which curves every which way in different individuals. As you can imagine, early American fossil hunters gave different genus/species names based on different horn shapes. The result is that Centrosaurus has perhaps the most complex taxonomic history of all ceratopsids (except maybe Triceratops ). Most skulls assigned to Monoclonius have been sunk into Centrosaurus, or have been unassignable to specific genera given what we now know of ceratopsid ontogeny. Styracosaurus albertensis was once restored with a towering nasal horn, but a reappraisal of the genus by Michael Ryan showed that this is wrong— Styracosaurus actually had a mid-sized nasal horn that may not have been all that pointy.
A sampling of ceratopsid nasal horns. From top to bottom: Triceratops prorsus (YPM 1922); Torosaurus latus (MOR 981); Pachyrhinosaurus lakustai (TMP 1989.55.1234); and "Monoclonius flexus" (AMNH 5239).
Midway between horn and boss is Einiosaurus, a mid-sized taxon from the Two Medicine Formation whose horn goes through quite a transformation as it ages. In young animals, the horn is short and slightly recurved. Once adulthood is reached, though, the horn quickly dominates the face as it grows up and then dips forward. It ends up looking something like a hook. The horn is laterally narrow, but its anterior-posterior breadth is impressive, and the horn narrows to a blunt point. Further toward the bossy end of things is Achelousaurus, from the same formation, but quite distinct (for an interesting interpretation of their differences, see Sampson 1995). Achelousaurus is a basal member of the Pachyrhinosaurini. As it reaches adulthood, all horny growths on the nasal and postorbital are reabsorbed, leaving large, rough-textured bosses in their place. The living bony tissue was probably fairly brittle toward the surface, and it would have been protected by a large keratin or horny covering. I like the suggestion that pachyrhinosaurine bosses were covered with oxen-like horn—the texturing is consistent.
Horn growth in chasmosaurines, as exemplified by Triceratops. Note the "stepped-up" look of the rostrum in everyone except MOR 004.
How did ceratopsids use their nasal horns? In addition to figuring out that Triceratops used its postorbital horns for intraspecific combat, Andrew Farke also discovered that Centrosaurus must not have fought the same way, as the frills he studied carried virtually no scars attributable to intraspecific battle. This is not so surprising—without large postorbital horns, it’s difficult to see who centrosaurines head-to-head in quite the same manner, but it would appear that even the elongate nasal horn did not inflict injury on conspecifics. Absent any pathological evidence, it becomes very difficult to figure out what the nasal horns were used for. In chasmosaurines, they may have simply served as age indicators—Horner & Goodwin demonstrated that nasal horn morphology changes significantly along with postorbital morphology during growth in Triceratops. Like the angle and curve of the postorbitals, chasmosaurines may have looked to the development of the nasal horn to assess age and fitness amongst themselves. But what of centrosaurines, who exhibit a far greater range in nasal horn morphology?
Overall, nasal horn morphology is more variable than postorbital horn morphology and may hold more value phylogenetically.