The fossil record reveals a stunning variety of physical traits, including the horns and crests that adorned the heads of many prehistoric creatures. Among the most distinctive are dinosaurs that sported a single, prominent horn directly on the snout. While most famous horned dinosaurs were plant-eaters, the unique distinction of having a horn on its nose belongs to a large, meat-eating predator from the Jurassic period.
Identifying the Dinosaur: Ceratosaurus
The dinosaur specifically identified by its nasal horn is Ceratosaurus, a name that translates appropriately to “horned lizard” in Greek. This large predatory theropod lived during the Late Jurassic period, approximately 153 to 148 million years ago, making it a contemporary of other giants like Allosaurus and Stegosaurus. Fossils of Ceratosaurus have been primarily recovered from the Morrison Formation in the western United States.
The most distinguishing feature of Ceratosaurus is the prominent, blade-like projection located on the midline of its snout, formed by the fusion of its nasal bones. This nasal horn was relatively short in comparison to the massive horns of later dinosaurs, with the bony core of the type specimen measuring about 5.1 inches (13 cm) long and 2.8 inches (7 cm) tall. In the living animal, this bony core would have been covered by a keratinous sheath, giving it a more elongated and pointed appearance.
Unlike most other large theropods, Ceratosaurus also featured a pair of smaller, bony ridges over its eyes, composed of the lacrimal bones, in addition to its nasal horn. It was a bipedal carnivore with powerful hindlimbs and proportionally short forelimbs. The combination of its deep jaws, long blade-like teeth, and three cranial horns made it a formidable, mid-sized predator in its ecosystem.
Nasal Horns in Ceratopsians
While Ceratosaurus is the specific answer for a theropod with a nose horn, the most recognized group for these features is the Ceratopsidae, or Ceratopsians (“horned faces”). These were large, quadrupedal herbivores that lived much later, during the Late Cretaceous period (82 to 66 million years ago). Ceratopsians, including species like Triceratops, Centrosaurus, and Styracosaurus, were characterized by beaks, shearing teeth, and elaborate cranial ornamentation, including horns and a large bony frill.
The nasal horn in Ceratopsians, which develops from the nasal bones, varied dramatically in size and shape among different species. For instance, the Centrosaurine group, which includes Centrosaurus and Styracosaurus, were known for having a prominent, often long and curved, horn on their nose. In Centrosaurus, the nasal horn was long and well-developed, while the brow horns above the eyes were small or sometimes absent.
Conversely, in the Chasmosaurine group, which includes Triceratops, the nasal horn was typically reduced in size, sometimes appearing as a low boss or bump. The focus of their ornamentation shifted to the two massive horns situated over the eyes, which could reach lengths of over three feet. This diverse array of horn arrangements suggests that the features served different, highly specialized functions within the various Ceratopsian lineages.
Purpose and Function of the Nasal Horn
The function of cranial ornamentation like the nasal horn has long been debated among paleontologists, with evidence suggesting multiple uses rather than a single purpose. The consensus is that these structures were primarily involved in social signaling and display. The unique shapes and sizes of horns and frills likely allowed dinosaurs to recognize members of their own species, aiding in successful mating and group cohesion.
The horns also played a role in sexual selection, serving as a display to attract mates or establish dominance within a group. For the Ceratosaurus, the relatively thin, blade-like structure of its nasal horn suggests it was better suited for visual display than for intense, forceful combat. Its likely use was to signal maturity or health to rivals and potential mates.
For the robust Ceratopsian horns, the function was likely multi-faceted, including both display and physical interaction. While defense against predators like Tyrannosaurus is plausible, the more frequently cited function is intra-species combat, such as head-butting, to assert territorial or mating rights. Abnormalities found in some Ceratopsian frills suggest these animals engaged in physical contests, using their horns to lock and wrestle with rivals.