Pachyrhinosaurus (meaning in Greek "thick-nosed lizard", from (), thick; (), nose; and (), lizard) is an extinct genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous period of North America. The first examples were discovered by Charles M. Sternberg in Alberta, Canada, in 1946, and named in 1950. Over a dozen partial skulls and a large assortment of other fossils from various species have been found in Alberta and Alaska. A great number were not available for study until the 1980s, resulting in a relatively recent increase of interest in Pachyrhinosaurus. Three species have been identified. P. lakustai, from the Wapiti Formation, the bonebed horizon of which is roughly equivalent age to the upper Bearpaw and lower Horseshoe Canyon Formations, is known to have existed from about 73.5–72.5 million years ago. P. canadensis is younger, known from the lower Horseshoe Canyon Formation, about 71.5–71 Ma ago and the St. Mary River Formation. Fossils of the youngest species, P. perotorum, have been recovered from the Prince Creek Formation of Alaska, and date to 70–69 Ma ago. The presence of three known species makes this genus the most speciose among the centrosaurines. Discovery and species Pachyrhinosaurus canadensis, was described in 1950 by Charles Mortram Sternberg based on the holotype incomplete skull NMC 8867, and the paratype incomplete skull NMC 8866, which included the anterior part of the skull but was lacking the right lower mandible, and the "beak". These skulls were collected in 1945 and 1946 from the sandy clay of the Horseshoe Canyon Formation in Alberta, Canada. In the years to come, additional material would be recovered at the Scabby Butte locality of the St. Mary River Formation near Lethbridge, Alberta, from terrestrial sediments considered to be between 74 and 66 million years old. These were among the first dinosaur sites found in the province, in the 1880s. The significance of these discoveries was not understood until shortly after World War II when preliminary excavations were conducted. Another Pachyrhinosaurus skull was taken out of the Scabby Butte locality in 1955, and then in 1957 Wann Langston Jr. and a small crew excavated additional pachyrhinosaur remains. The University of Calgary has plans to reopen this important site some day as a field school for university-level paleontology students. Several specimens, NMC 21863, NMC 21864, NMC 10669 assigned in 1975 by W. Langston, Jr. to Pachyrhinosaurus were also recovered at the Scabby Butte locality.