Arrhinoceratops: a rare horned dinosaur from Alberta

Arrhinoceratops was a large, plant-eating chasmosaurine ceratopsid from the early Maastrichtian of Alberta, Canada. It is known for long brow horns, a smaller nasal horn and a broad frill with paired openings. Skulls from different growth stages reveal how the horns, frill and proportions of the face changed as the animal matured.

The name means “no-nosed-horn face”, but the animal did have a nasal horn. The mistake reflects an early interpretation of the bones around the nose, and the name remains under zoological naming rules. No complete skeleton is known. Estimates of body length and many details of its outline therefore depend on comparison with better-known ceratopsids.

Arrhinoceratops is included in the dinosaur catalogue as a distinct Late Cretaceous horned dinosaur. Its best evidence is the skull, so claims about body mass, behaviour and exact appearance require particular care.

Quick facts

Name and discovery

The genus name can be translated as “face without a nasal horn”. It is an unfortunate name: a small nasal horn was present, although it was much shorter than the long horns above the eyes. The error arose from the original interpretation of the nasal bones. The species name brachyops means “short-faced” or “short-headed”.

The holotype, ROM 796, was found in 1923 by a University of Toronto expedition on the Red Deer River, across from the Neill ranch in Alberta. It is a large, nearly complete skull, but pressure from surrounding rock crushed and distorted it, and the lower jaws are missing. Canadian palaeontologist William Arthur Parks described the fossil in 1925 and named Arrhinoceratops brachyops.

Parks thought the skull lacked a separate nasal horn. Later study showed that a horn core is present as an extension of the nasal bones, as in other ceratopsids. The name was retained under zoological nomenclature even though its literal meaning reflects a mistaken anatomical interpretation.

For many years the genus was known almost entirely from the holotype. A second specimen, ROM 1439, expanded the record. It includes a relatively complete skull with lower jaws, a fused complex of front neck vertebrae and part of the left forelimb. A partial juvenile skull, CMN 8882, was later referred to the species. Together these specimens reveal anatomy and some changes associated with growth.

Classification and the disputed second name

Arrhinoceratops was an ornithischian dinosaur within Ceratopsia and the family Ceratopsidae. Its subfamily placement in Chasmosaurinae is well supported. Many chasmosaurines have relatively long brow horns and an elongated frill, while numerous centrosaurines have a more prominent nasal ornament and shorter frill. These are broad tendencies rather than rules that identify a genus from one feature.

Its more precise position among chasmosaurines has varied among analyses. Some place it near Anchiceratops, Torosaurus or the lineage containing Triceratops. Anchiceratops lived in the same members of the Horseshoe Canyon Formation at roughly the same time, but a close position on an evolutionary tree does not mean that one was the direct ancestor of the other. Incomplete material and conflicting characters make the branching order less stable below the subfamily level.

It is therefore not established as a direct ancestor of Triceratops or another late chasmosaurine. A phylogenetic neighbour is not necessarily a direct parent in a lineage. The secure conclusion is that Arrhinoceratops was a distinct North American chasmosaurine of the early Maastrichtian.

Only one species is generally accepted in the genus: A. brachyops. It is based on ROM 796, with several additional specimens from the same formation referred to it. Differences between adult and juvenile skulls are interpreted as growth-related rather than requiring another species name.

The name Arrhinoceratops? utahensis was proposed in 1946 for fragmentary ceratopsid remains from the North Horn Formation in Utah. The question mark reflected doubt about assigning it to this genus. The material was later moved to Torosaurus and is generally called Torosaurus utahensis. Its exact position has also been discussed, but it is not treated as a second species of Arrhinoceratops. Lists that include two species under this genus usually repeat an outdated combination.

What fossils have been found?

The skull material is the main source of information. The holotype ROM 796 retains much of the skull, including the horns and frill, but has no lower jaws and is noticeably distorted. Its skull is about 1.4 metres long. That measurement does not mean every adult had a head exactly that size.

ROM 1439 is especially valuable because the skull is accompanied by lower jaws and a small set of bones from behind the head. These include the synsacervical complex, where front neck vertebrae are fused, a scapula, humerus, radius and some hand bones. This material supports the robust quadrupedal build expected for a large ceratopsid, but it does not provide a complete set of body proportions.

CMN 8882 belonged to a juvenile. Compared with adults, its brow horns were shorter and differently inclined; the frill-margin bones were taller before they became lower and fused; and the face was proportionally shorter. These are real growth-related differences visible in the bones, although a sample of only a few skulls cannot represent every stage of development.

A separate nasal horn, CMN 56500, has a cross-section useful for recognising the characteristic form of this structure. Along with the skulls, it disproves the literal reading that the animal had no nose horn.

No complete vertebral column, pelvis, hind limbs or tail securely belonging to Arrhinoceratops has been found. There are no known skin impressions, eggs, nests, trackways or digestive contents. Reconstructions of these missing details rely on other ceratopsids and should be understood as comparative.

Size and anatomy

Body length is usually estimated at about 4.5–6 metres. The broad range reflects the absence of a complete skeleton, not demonstrated extreme variation among individuals. The trunk is scaled from the skull and the few limb bones using proportions of other chasmosaurines. No species-specific volumetric model provides a reliable mass, so a precise number of tonnes would suggest evidence that does not exist.

The skull carried two long brow horns and a much smaller nasal horn. In adults the brow horn cores were elongated and pointed mainly forward. The juvenile had shorter cores at a different angle. The brow horns were comparable in scale to those of Triceratops, but the frill and proportions of the face differed. Keratin sheaths probably extended the visible horns in life, but their precise shape and thickness cannot be established from bone alone.

The frill was broad, roughly rectangular in outline, and contained a paired opening in the parietal bones. The openings reduced weight, while the remaining bone still provided a large area for soft-tissue attachment. Separate ossifications ornamented the edge. Their shape and degree of fusion changed as the animal grew, so a juvenile frill should not be compared with an adult without accounting for age.

A beak at the front of the mouth cropped plants. Behind it, dental batteries with continually replaced teeth processed tough vegetation. The lower jaw had a pronounced bony ridge along its side, and the surface of the predentary bone was steeply angled. Such features help distinguish the genus from related chasmosaurines.

The fused front neck vertebrae strengthened the neck and helped support a heavy head. The known forelimb was robust, but it cannot establish running speed or every detail of gait. Arrhinoceratops was a quadruped; images of it standing on its hind legs are artistic hypotheses rather than evidence-based posture.

Habitat, diet and possible behaviour

The Horseshoe Canyon Formation preserves river channels, floodplains, wetlands and coastal lowlands along the eastern side of Laramidia. Conditions were seasonal, and the shoreline of the ancient Western Interior Seaway shifted over time. Arrhinoceratops lived among varied plants and other dinosaurs during the Late Cretaceous Period.

Herbivory is supported by the beak and dental batteries. The animal could crop low and medium-height vegetation, then process it with successive rows of replacement teeth. Its exact menu is unknown. No stomach contents or plant remains directly associated with a feeding individual establish a preference for particular ferns, conifers or flowering plants.

The function of the horns and frill remains debated. They may have contributed to display, individual recognition, competition or defence. Changes in horn shape during growth are consistent with these structures becoming important as the animal matured, but do not prove a specific ritual or fighting style. There is no large sample of injuries that unambiguously records combat between Arrhinoceratops.

There is no direct evidence that this genus lived in herds. Its bones do not form a known mass accumulation that can confidently be interpreted as a single group. Nests and juveniles associated with adults are also unknown. Migration, parental care and complex social organisation remain hypotheses, not established facts.

Skin and colour have not been preserved. Illustrations often show scales by comparison with other ceratopsids, but that is a reconstruction. The same distinction applies to facial soft tissues, horn sheaths, posture in a particular scene and the vegetation surrounding the animal.

A distinctive skull, an incomplete animal

Arrhinoceratops was a large chasmosaurine of the early Maastrichtian in Alberta and is represented by one well-supported species, A. brachyops. Several skulls helped correct the supposed absence of a nasal horn and document how the brow horns, frill and face changed with age. The missing postcranial skeleton limits the precision of body-size estimates and reconstructions of its anatomy and behaviour.