A third species, "S. ovatus", from the Two Medicine Formation of Montana, was described by Gilmore in 1930. The fossil material is limited, with the best being a portion of the parietal bone of the frill, but one unusual feature is that the pair of spikes closest to the midline converge towards the midline, rather than away from it as in "S. albertensis". There also may only have been two sets of spikes on each side of the frill, instead of three. The spikes are much shorter than in "S. albertensis", with the longest only long. A 2010 review of styracosaur skull remains by Ryan, Holmes, and Russell found it to be a distinct species, and in 2010 McDonald and Horner placed it in its own genus, "Rubeosaurus". However, it may still belong to "Styracosaurus".