THE STATUS OF THE PROBLEMATIC TAXON MONOCLONIUS (ORNITHISCHIA:CERATOPSIDAE) AND THE RECOGNITION OF ADULT-SIZED DINOSAUR TAXA

The late Campanian centrosaurine ceratopsid Monoclonius crassus, including the junior synonym M. lowei, was declared a nomen dubium by Sampson et al. in 1997, based on the fact that all referred material exhibits juvenile- or long-grain-bone texture indicative of juvenile and subadult age, respectively, and lacks diagnostic adult cranial ornamentation that could be used to assign any of the specimens to a recognized taxon. In 1998 Tumarkin and Dodson used a heterochronic argument to support the validity of the taxon M. lowei based, in part, on the large size of the holotype (and only) skull (CMN 8790) and suggested that its subadult characters are really adult paedomorphic characters. This position was accepted by Dodson et al. in 2004 who included M. crassus in their list of recognized ceratopsid taxa.

Pertinent to the status of Monoclonius is the identification of a previously unrecognized broken base of a spike-like parietal process at locus P3 on the left side of the parietal of CMN 8790. This indicates that, despite its large size, M. lowei was in the process of developing its diagnostic parietal ornamentation at the time of its death, and was therefore, like all other Monoclonius specimens, of subadult age and not a fully mature (but paedomorphic) adult. Additionally, Pachyrhinosaurus from Horseshoe Canyon Formation of southern Alberta, recently recognized to be of late Campanian age, is more massive than any other centrosaur and would have had a longer basal skull length than CMN 8790.

CMN 8790 is now known to have been recovered from the uppermost portion of the Dinosaur Park Formation in the southernmost Alberta. The only centrosaur known from this portion of the formation in Canada is Styracosaurus, but both Achelousaurus and Einiosaurus occur at approximately the same stratigraphic level in the lithologically equivalent Two Medicine Formation of Montana. This suggests that M. lowei could potentially be referable to one of these spike-bearing centrosaurs. As the final morphology of the cranial ornamentation of M. lowei is unknowable at the present time, we advocate the return of M. crassus to the status of nomen dubium but recognize that further work is warranted.