| Monoclonius Temporal range: Late Cretaceous,
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| Nasal horn base and lectotype frill of M. crassus | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Neornithischia |
| Clade: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Centrosaurinae |
| Genus: | †Monoclonius Cope, 1876 |
| Type species | |
| †Monoclonius crassus Cope, 1876
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Monoclonius (meaning "single sprout") is an extinct genus of herbivorous ceratopsian dinosaur found in the Late Cretaceous layers of the Judith River Formation in Montana, United States, and the uppermost rock layers of the Dinosaur Park Formation in Alberta, Canada dated to between 75 and 74.6 million years ago.[1][2]
Monoclonius was first named by Edward Drinker Cope in 1876. Later, much taxonomic confusion was caused by the discovery of Centrosaurus, a very similar genus of ceratopsian that is known from much better remains. Today, typical Monoclonius specimens are usually believed to be juvenile Centrosaurus or subadults, in many cases of other genera such as Centrosaurus. Those specimens that remain under the name Monoclonius are mostly too incomplete or immature to be confidently matched with adult specimens from the same time and place. This is especially true of the type species, Monoclonius crassus. Therefore, Monoclonius is often considered a nomen dubium now, pending further study.[2]
- ^ Eberth, D.A. (2005). "The Geology". In Currie, P.J.; Koppelhus, E.B. (eds.). Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed. Bloomington and Indianapolis: Indiana University Press. pp. 54–82.
- ^ a b Ryan, M.J. (2006). "The status of the problematic taxon Monoclonius (Ornithischia: Ceratopsidae) and the recognition of adult-sized dinosaur taxa". Geological Society of America Abstracts with Programs. 38 (4): 62.