When the public imagines a “one-horned dinosaur,” they are typically thinking of the Ceratopsidae family, a group of large, plant-eating dinosaurs from the Late Cretaceous period. The specific group characterized by a single, prominent horn growing from the nose is the Centrosaurine subfamily. This feature immediately distinguishes them from relatives like Triceratops, which had three horns, with two situated over the eyes. Centrosaurines focused their elaborate cranial ornamentation on the central part of the face.
Identifying the Primary Single-Horned Dinosaurs
The Centrosaurinae subfamily contains the most representative examples of single-horned dinosaurs. Centrosaurus is the type genus for the subfamily. Its name, meaning “pointed lizard,” refers to the small hornlets lining its frill, rather than its large nasal horn.
Monoclonius is another historical example, whose name translates to “single horn.” However, its taxonomic standing is complex; many specimens are now considered juvenile forms of other Centrosaurine genera, including Centrosaurus. Today, Monoclonius is often classified as a nomen dubium because its original fossil material is too incomplete to be definitively assigned to a unique species.
Other Centrosaurines, such as Styracosaurus, are also characterized by a large nasal horn, though their frill is decorated with numerous long spikes, giving them a distinct profile. These creatures are contrasted with the Chasmosaurinae subfamily, which includes Triceratops and Chasmosaurus. Chasmosaurines typically featured a reduced nasal horn and two long horns situated over the eyes.
Distinguishing Anatomical Features of the Nasal Horn
The defining feature of the Centrosaurines is the prominent nasal horncore, a bony outgrowth of the nasal bones that sits high on the snout. The size and curvature of this horn vary significantly among species, with some curving forwards, others backward, and some being almost straight.
In life, the bony core was covered by a thick sheath of keratin, the same fibrous protein found in human fingernails and rhino horns. This keratinous covering would have extended the horn’s length far beyond the fossilized bone, potentially adding 15 to 25 percent to its total size and sharpening its point. The entire horn structure was anchored to the massive skull, which was supported by a fusion of the first several neck vertebrae, known as a syncervical, providing a stable base for potential impact.
The nasal horn is intimately linked with the parietosquamosal frill, the bony shield projecting from the back of the skull. Centrosaurines generally possessed a shorter, more rectangular frill compared to the long, triangular frills of Chasmosaurines. This frill often featured elaborate spikes, hooks, or bosses around its edge, collectively known as epiparietals and episquamosals, which served as species-specific display structures.
Evolutionary Theories for the Horn’s Function
The consensus suggests that the Centrosaurine nasal horn and frill were primarily adaptations for social signaling rather than defense against large predators. One prominent theory is that the unique morphology facilitated species recognition.
In the diverse ecosystems of the Late Cretaceous, where multiple species of horned dinosaurs coexisted, the distinct ornamentation allowed individuals to quickly identify members of their own group, a requirement for social herding. Furthermore, the structures are strongly linked to sexual selection, likely serving as visual displays to attract mates. This hypothesis is supported by evidence that the most elaborate ornamentation did not fully develop until the animals reached sexual maturity.
The nasal horn was also likely a tool for intraspecific combat. Evidence of puncture wounds and healed injuries found on the fossilized skulls of Centrosaurus and other Ceratopsids suggests they engaged in ritualized pushing or wrestling matches, similar to modern horned mammals. The horn would have been used to strike the flanks of rivals, while the frill may have protected the neck muscles and eyes during contests for dominance or mating rights.
Time Period and Geographical Range
Single-horned Centrosaurine dinosaurs thrived during the Late Cretaceous period, specifically the Campanian epoch, roughly 82 to 74 million years ago. Their existence predates the appearance of Triceratops, which belonged to the later Maastrichtian epoch.
Fossil remains are almost exclusively found in western North America, a region that was once the island continent of Laramidia. The richest deposits are concentrated in the northern part of this landmass, particularly in the Canadian provinces of Alberta and Saskatchewan. Bonebeds containing thousands of individuals of Centrosaurus have been discovered in the Dinosaur Park Formation, providing insight into their social behavior and population structure.
These Centrosaurines inhabited a lush, subtropical environment characterized by floodplains and coastal regions adjacent to the Western Interior Seaway, a large inland sea that bisected North America. While most single-horned Ceratopsids are North American, the discovery of a relative like Sinoceratops in China suggests that some Centrosaurines managed to migrate into Asia.