Chasmosaurus: the long-frilled ceratopsid after its 2026 revision

Chasmosaurus belli was a plant-eating horned dinosaur from the Campanian Age of the Late Cretaceous, about 76–75.5 million years ago. It lived in what is now southern Alberta, Canada. This quadrupedal ceratopsid was roughly 4.5–5 metres long, with a beak, tooth batteries, modest facial horns and a very long frill pierced by two large openings.

Chasmosaurus is known from several skulls, partial and nearly complete skeletons, skin impressions and a rare juvenile. The fossils help researchers study ceratopsid growth, but they also show how easily age or individual variation can be mistaken for species differences. A 2026 revision moved the former Chasmosaurus russelli into the separate genus Cryptarcus; the only securely named species retained in Chasmosaurus is C. belli.

Quick facts

Name and discovery

The name Chasmosaurus combines Greek chasma, “opening” or “fissure”, with sauros, “lizard”. It refers to the two large parietal openings in the frill. These are normal anatomical openings, not breaks in the fossil. In life, soft tissues covered them, so they did not appear as empty windows.

Lawrence Lambe collected the first remains of the animal near the Red Deer River in the late 1890s. In 1902 he described a partial parietal bone, CMN 491, as Monoclonius belli, naming the species for Robert Bell. Although fragmentary, the type preserves a diagnostic part of the frill.

In 1913, Charles Sternberg and members of his family found more complete skulls and skeletons. The material showed that the dinosaur differed from Monoclonius. Lambe proposed the name Protorosaurus in 1914, but that name was already used for a Permian reptile. He replaced it with Chasmosaurus. One early skeleton, CMN 2245, preserved both substantial parts of the skeleton and patches of skin.

Classification and changing species assignments

Chasmosaurus is an ornithischian, a ceratopsian and a member of Ceratopsidae. Within the family it belongs to Chasmosaurinae, a branch in which long frills are common. Centrosaurines more often had shorter frills, though the distinction has exceptions. The broader Ceratopsian overview places these branches among the wider diversity of horned dinosaurs.

Chasmosaurus is usually recovered among early branches of Chasmosaurinae. It lived before Triceratops and belongs to the same subfamily, but it is not established as Triceratops’s direct ancestor. Its precise position changes among phylogenetic analyses. Researchers code deformed skulls, immature individuals and disputed marginal bones differently, which can alter the resulting family tree. The dinosaur catalogue places Chasmosaurus among other named dinosaur profiles.

The most cautious current arrangement recognises one secure species, Chasmosaurus belli. Its rear parietal bar is usually nearly straight or has a shallow central notch. Even the specimens assigned to C. belli remain under discussion: some skulls traditionally placed in the species combine features that bring them closer to Vagaceratops.

Chasmosaurus russelli, described by Charles Sternberg in 1940 from skull CMNFV 8800, was long treated as the second species. Re-preparation and redescription of the holotype identified features of the premaxilla and frill ossifications that differ from typical Chasmosaurus. In 2026, researchers established the genus Cryptarcus for it. Other fossils formerly assigned to C. russelli did not automatically become Cryptarcus: some are left as Chasmosaurus sp., while others need new study.

Names such as C. kaiseni, C. canadensis and C. brevirostris are based on skulls that lack a securely known rear frill margin. Long brow horns or a short muzzle alone are not enough to establish a species, so these fossils are generally treated as indeterminate Chasmosaurus or Chasmosaurus sp. The Texas species once called C. mariscalensis is now Agujaceratops, and C. irvinensis was moved to Vagaceratops. The status of Mojoceratops perifania remains disputed. The occasionally repeated name Chasmosaurus priscus was never properly published and is not a valid species name.

Fossils, skin and growth

The known material ranges from the partial frill of the type to nearly complete skulls, associated skeletons and individuals of different ages. In NHMUK R4948 the skull bones are disarticulated while the postcranial skeleton is comparatively undistorted. That specimen allowed detailed study of the braincase, vertebrae and limbs.

Skin impressions are associated with CMN 2245 on the right side and around the pelvis. They show small, non-overlapping scales among larger polygonal scales surrounded by smaller ones in rosette-like patterns. These impressions document the skin on those regions. They do not reveal the animal’s full-body colour.

A jointed juvenile about 1.5 metres long preserves the skull, backbone and rear of the skeleton. Its frill was short and narrow, and the horn cores were only developing. It provides evidence of growth, but it does not by itself prove parental care. A single young animal is not evidence of a nest, family group or feeding behaviour.

The fossil record is unusually useful compared with that of many dinosaurs, but historical identifications are not all equally secure. In ceratopsids, species traits often occur on the back edge of the frill. If that region is missing, distorted or restored, even a good skeleton may only be assigned to Chasmosaurus sp.

Size and anatomy

Adult Chasmosaurus probably reached about 4.5–5 metres in length. Mass is commonly estimated at around 1.5–2.5 tonnes. The range reflects how complete each skeleton is and how researchers reconstruct soft tissues; neither number is a direct measurement from a complete living animal.

The skull was low and elongated, with a narrow muzzle and a keratinous beak supported by the rostral bone. Behind the beak, rows of replacement teeth formed dental batteries. Their stacked cutting surfaces sliced plant fibres as worn teeth were replaced. This was effective food processing, but it was not the side-to-side chewing seen in mammals.

The nasal horn was usually small. Above the eyes were horn cores that varied markedly among specimens and changed during growth. They supported keratinous sheaths, so the outline of the living horns is unknown and could have extended beyond the preserved bone. The horn cores may also have remodelled as the animal matured.

The main feature was a long frill formed by the parietal and squamosal bones. It widened toward the rear and contained two large openings that reduced its weight. Small ossifications lined parts of the margin. Their number and position can help distinguish taxa, but pieces may be lost or displaced. In C. belli, the rear edge was straight or only shallowly notched.

The body was deep, the neck strong and the limbs robust. Chasmosaurus walked on four legs. Its forelimbs were not fully splayed sideways as in some old restorations, yet they were not perfectly vertical pillars under the body either. The tail was relatively short compared with the heavy trunk.

Habitat, feeding and behaviour

Chasmosaurus lived on a low plain west of the Western Interior Seaway during the Cretaceous Period. Rivers crossed floodplains, abandoned channels, wet areas and wooded patches. Deposits of the Dinosaur Park Formation record the gradual approach of a marine shoreline. Vegetation included conifers, ferns, horsetails and flowering plants.

The beak could crop or clip shoots, and the dental batteries cut plant matter. Skull shape and jaw position are consistent with feeding at low to intermediate heights. The exact diet is unknown: no stomach contents or droppings securely tied to Chasmosaurus have been found. Differences among the skulls of large herbivores may indicate partitioning of food resources, but they do not reveal a precise menu.

The horns and frill may have served several roles. Researchers have considered display, recognition of other members of the species, defence and physical contests. Large openings and age-related changes in ornament are compatible with a conspicuous signalling role, but do not rule out protection. No direct observation of Chasmosaurus behaviour survives in the fossil record, so one function cannot be presented as proven.

No mass deposit of a single Chasmosaurus species is known, and ceratopsid trackways from the formation cannot be assigned to this genus. Herds, migrations and social hierarchy therefore remain unconfirmed. Nests, eggs, sounds, speed and parental care are also unknown.

Outdated ideas and what remains uncertain

An outdated scheme treated C. belli and C. russelli as two secure species. Since 2026, the holotype of the latter belongs to Cryptarcus russelli. That does not mean every old specimen labelled C. russelli can now be called Cryptarcus; most still need review.

Long and short brow horns were once explained as male and female forms. Reliable sexing criteria are not available for these skeletons, and horn proportions varied with age and between individuals. A long-horned “male” and short-horned “female” should not be treated as established anatomy.

The frill is sometimes drawn either as a solid shield of armour or as a bare structure with open windows. Both pictures miss part of the evidence. Bone may have offered some protection, but the thin margins around the openings occupied much of its area. Soft tissue covered the openings in life; its colour and any bright pattern have not been preserved.

The juvenile skeleton is also not proof of parental care. It shows how proportions changed during growth. Without a nest or an associated group of adults and young, that behavioural conclusion is unsupported. Chasmosaurus is a well-represented but taxonomically complex ceratopsid, and revisions of museum fossils continue to change how its species are recognised.