identical to Styracosaurus albertensis, differing from it only in the possession of just a single pair of parietal spikes. The middle layers, below the Bearpaw, contained "Transitional Taxon B" that also had a single spike pair but differed in the form of its nasal horn that curved to the front over the anterior branches of the nasal bones. In the upper strata, below the Bearpaw, "Transitional Taxon C" had been excavated. It too had a spike pair but now the nasal horn was fused with the front branches. The upper surface of the horn was elevated and very rough. The orbital horns showed coarse ridges. Subsequently, "Taxon A" was named Stellasaurus, "Taxon B" became Einiosaurus, while "Taxon C" became Achelousaurus. In 1992, Horner et al. did not name these as species for the explicit reason that the entire evolutionary sequence was seen as representing a grade of transitional ceratopsians between Styracosaurus albertensis, known from the Judith River Formation, and the derived, hornless Pachyrhinosaurus from the Horseshoe Canyon Formation, which had the spike pair and bosses on the nose and above the eyes, as well as additional frill ornamentation. In 1997, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.". Horner thought he had found the mechanism driving this evolution, elaborating on ideas he had developed even before he had investigated Landslide Butte. The animals were living on a narrow strip on the east-coast of Laramidia, bordering the Western Interior Seaway and constrained in the west by the high proto-Rocky Mountains. During the Bearpaw Transgression sea levels were rising, steadily reducing the width of their coastal habitat from about to . This led to stronger selection pressures, the severest for Achelousaurus which lived during the phase that the coastal strip was at its narrowest. The lower number of individuals that the smaller habitat could have sustained constituted a population bottleneck, making rapid evolution possible. Increased sexual selection would have induced changes in the sexual ornamentation such as spikes, horns and bosses. A reduced environmental stress by lower sea levels on the other hand, would be typified by adaptive radiation. That sexual selection had indeed been the main mechanism would be proven by the fact that young individuals of all three populations were very similar: they all had two frill spikes, a small nasal horn pointing to the front, and orbital horns in the form of slightly