An adult male Stegoceras would have fought fiercely with another for control of the herd. Although quite small, Stegoceras was a tough creature. A bipedal herbivore, Stegoceras was a member of an unusual group, the pachycephalosaurs (‘thick-headed reptiles’).
This is the best known of the pachycephalosaurids, with dozens of skull fragments known and also a partial skeleton. The structure of the bone in the head dome was such that the bone fibres aligned to absorb impact from the top. The vertebrae of the neck and back were very strong, lashed together with strong tendons that prevented twisting, and aligned to absorb shocks emanating from the head end. The hips were particularly wide and solid. All this is consistent with the idea that the dome was used as a weapon, like a battering ram.
Name: Stegoceras, meaning ‘roof horn’
Size: 2.5-3m long and 1.5m high
Food: plants and ferns
Lived: 75 million years ago in the Late Cretaceous Period in North America
Pachycephalosaurs had one special feature in common – a thick, rounded skull. On Stegoceras’ head was a semicircle of small bony lumps. These bumps ran above its eyes and around the back of its neck. Experts are not certain what the lumps were for, but they could have been used in flank-butting, a widely evidenced theory. The skull was not very thick when Stegoceras was born, but it became thicker as the dinosaur got older.
Some experts believe that they have discovered examples of both male and female Stegoceras. They have found that some skulls are thicker than others and believe that these thick skulls may have belonged to the males. A male Stegoceras could have a skull up to 6cm thick – which is half as thick as a brick.
It was initially proposed that male Stegoceras (and individuals of other pachycephalosaur species) would ram each other headlong, not unlike contemporary bighorn sheep or musk oxen. It was later suggested that they engaged in flank-butting rather than ramming, a theory for which there is a great deal of evidence. Foremost, the rounded shape of the skull roof would lessen the contacted surface area during headbutting, resulting in glancing blows. Second, pachycephalosaurs would not have been able to align their head, neck and body in a perfect horizontal line (which would be needed to transmit stress) – it was more likely that they carried their neck in an S- or U-shaped curve (this to a lesser extent in Stegoceras, due to their thick neck muscles). Lastly, the relatively large width of most pachycephalosaurs would have served to protect vital organs from harm during flank-butting.
For most of the time, Stegoceras was a peaceful herbivore. It moved through the Late Cretaceous vegetation, pulling leaves and flowers from trees and low-lying plants with its beaked mouth. Its teeth were sharp and serrated like a saw. Stegoceras used them to shred leaves and plants, rather like modern goats.
The dome on the head of Stegoceras is high, but not as high as that of others in the group, and is surrounded by a frill of little horns and knobs. The teeth at the front of the jaw are very widely set and the muzzle is broad compared with other pachycephalosaurids. This may indicate a less selective feeding strategy. The very broad hips suggest that the pachycephalosaurids gave birth to live young (this is not widely accepted).