BOSS VERSUS HORN: PACHYRHINOSAURUS

The skulls of many dinosaurs had bumps, crests or horns for display, scaring rivals and impressing potential mates.

Instead of the sharp nose horn of many of their large plant-eating cousins, Pachyrhinosuaurs had a big, bony lump on their snout.

Pachyrhinosaurus were ceratopsian dinosaurs that lived in North America during the Late Cretaceous. 

Unlike their Triceratops cousins, Pachyrhinosaurus can be distinguished by their large lumps or bosses over their nose rather than horns.

The first fossil specimens were found in western Canada in Alberta by Charles M. Sternberg in 1946 and named in 1950. Charles Mortram Sternberg (1885–1981) was an American-Canadian fossil collector and palaeontologist, and son of the famous Charles Hazelius Sternberg (1850-1943) who worked with Edward Drinker Cope and Othniel C. Marsh, and for the British Museum, the San Diego Natural History Museum and other museums. 

Late in his career, Charles M. collected and described Pachyrhinosaurus, Brachylophosaurus, Parksosaurus and Edmontonia. A contemporary author wrote, "No published study of Canadian dinosaurs is possible today without citing one or another of Sternberg's papers. 

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.