Period: Late Cretaceous
Order, Suborder, Family: Ornithischia, Marginocephalia, Ceratopsidae
Location: North America (Canada, United States)
Length: 20 feet (6 meters)
Monoclonius was discovered by Edward Drinker Cope in 1876 along the Missouri River in Montana. The specimen Cope found was very fragmentary and is one of the first ceratopsids found (no one knew what a ceratopsid looked like in 1876 since no skull or complete skeleton had been found). Because the earliest specimens of Monoclonius were incomplete, it has often been confused with its close relative Centrosaurus. But recent studies have shown they were different dinosaurs.
Charles Sternberg found the first and only complete skull of Monoclonius in Alberta in 1937. At least six other species of Monoclonius have been named, but they were based on incomplete specimens.
Monoclonius, which means "single horn," was a moderate-size centrosaurine ceratopsid without brow horns but with a well-developed and sometimes curved nasal horn. The neck frill was short and round, with scalloped edges and moderately large fenestrae (openings). The frill was thin along the outer rim and did not have the bony projections that were on the frill of Centrosaurus. Monoclonius was closely related to Styracosaurus, Brachyceratops, and Pachyrhinosaurus.