My attempt at another ceratopsian; this time, Monoclonius crassus - but I realised after reading up on this genus that my drawing is probably more like Centrosaurus. Damn! I really wanted to draw Monoclonius but I guess the picture I found on the Wikipedia that was labeled "Monoclonius" is probably a Centrosaurus...
According to Sampson et al. (1997), "Monoclonius specimens are generally defined on the presence of a thin, scalloped parietal and on the absence of hooks, spikes and horns seen on the posterior transverse ramus of other genera". The only complete skull specimen of Monoclonius, a specimen previously attributed to M. lowei, has a short nasal horn core and a pair of low rounded supraorbital horns (Sampson et al. 1997).
The absence of elaborate cranial ornamentations in Monoclonius and its occurrences in slightly older strata can be indicative of a primitive condition in Monoclonius. On the other hand, these features are also commonly associated with juvenile and subadult centrosaurine specimens, indicating a possible paedomorphosis in Monoclonius. However, it seems more likely that Monoclonius is based on subadult specimens rather than being adults with primitive/juvenile characteristics. Sampson et al. (1997) regard Monoclonius as a numen dubium because 1, diagnostic characters of Monoclonius are present in subadults of other centrosaurines; 2, suture closure and bone surface texture supports subadult status of Monoclonius specimens; and 3, Monoclonius-type cranial elements have been found in bonebeds of other centrosaurines spanning a large time period.
The third point is the most convincing argument against the validity of Monoclonius. Monoclonius-type parietals have been found in association with Centrosaurus, Einiosaurus, and Pachyrhinosaurus bonebeds in Montana and Alberta. These are indistinguishable to isolated parietals usually assigned to Monoclonius. Perhaps, Monoclonius lived alongside other centrosaurines in several different locations over a large time period. Sampson et al. (1997) however think this unlikely and proposes that Monoclonius is likely made up of subadult specimens of Styracosaurus, Centrosaurus, Einiosaurus, and/or Achelousaurus and that all centrosaurines go through a 'Monoclonius stage' through ontogeny (Sampson et al. 1997).
On the other hand, Dodson (1990) argued that the type specimen of Monoclonius is diagnostic and that this genus is valid based on biometric results. Dodson et al. (2004 in The Dinosauria, 2nd edition) also seem to follow this argument as they treat Monoclonius as a valid monotypic genus with the complete skull, M. lowei, included in the type species. Though not discussed in detail, Dodson et al. (2004) citing Tumarkin and Dodson (1998) mention the possibility of paedomorphosis in Monoclonius based on the large adult-sized 'M. lowei' - i.e. retaining juvenile characteristics while attaining adult size.
As much as I love the name Monoclonius, it seems more study is in need in order to either invalidate or validate the genus. As Dodson et al. (2004) mention, aging techniques may perhaps provide more evidences. Bone histology comes to mind...
Dodson, P. 1990. On the status of the ceratopsids Monoclonius and Centrosaurus. In: Carpenter, K. and Currie, P. J. (eds.). Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press, Cambridge. Pp. 231-243.
Dodson, P., Forster, C. A., and Sampson, S. D. 2004. Ceratopsidae. In: Weishampel, D. B., Dodson, P., and Osmóslka, H. (eds.). The Dinosauria (Second Edition). California University Press, London. Pp. 494-513.
Sampson, S. D., Ryan, M. J., and Tanke, D. H. 1997. Craniofacial ontogeny in centrosaurine dinosaurs (Ornithischia: Ceratopsidae): taxonomic and behavioral implications. Zoological Journal of the Linnean Society, 121: 293–337.
Tumarkin, A. R. and Dodson, P. 1998. A heterochronic analysis of enigmatic ceratopsids. Journal of Vertebrate Paleontology, 18(suppl.): 83A.