| Pachyrhinosaurus Temporal range: 73.5–69 Ma |
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| P. canadensis skull, Royal Tyrrell Museum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | †Ornithischia |
| Suborder: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Centrosaurinae |
| Tribe: | †Pachyrhinosaurini |
| Node: | †Pachyrostra |
| Genus: | †Pachyrhinosaurus Sternberg, 1950 |
| Type species | |
| Pachyrhinosaurus canadensis Sternberg, 1950 |
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| Species | |
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P. canadensis Sternberg, 1950 |
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Pachyrhinosaurus (meaning "thick-nosed lizard") is a genus of ceratopsid dinosaurs 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 the Pachyrhinosaurus. Three species have been identified. P. lakustai, from 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 only from the lower Horseshoe Canyon Formation, about 71.5-71 Ma ago.[1] Fossils of the youngest species, unofficially named P. "perotorum", have been recovered from the Prince Creek Formation of Alaska, and date to 70-69 million years ago.[2]
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The largest Pachyrhinosaurus species were 8 metres (26 ft) long.[3] It weighed about four tons. They were herbivorous and possessed strong cheek teeth to help them chew tough, fibrous plants.
Instead of horns, their skulls bore massive, flattened bosses; a large boss over the nose and a smaller one over the eyes. A prominent pair of horns grew from the frill and extended upwards. The skull also bore several smaller horns or ornaments that varied between individuals and between species. In P. canadensis and P. perotorum, the bosses over the nose and eyes nearly grew together, and were separated only by a narrow groove. In P. lakustai, the two bosses were separated by a wide gap. In P. canadensis and P. lakustai, the frill bore two additional small, curved, backward-pointed horns. These were not present in P. perotorum, and in fact some specimens of P. lakustai also lack them, which may indicate that the presence of these horns varied by age or sex.[2]
Various ornaments of the nasal boss have also been used to distinguish between different species of Pachyrhinosaurus. Both P. lakustai and P. perotorum bore a jagged, comb-like extension at the tip of the boss which was missing in P. canadensis. P. perotorum was unique in having a narrow dome in the middle of the back portion of the nasal boss, and P. lakustai had a pommel-like structure projecting from the front of the boss (the boss of P. canadensis was mainly flat on top and rounded). P. perotorum bore two unique, flattened horns which projected forward and down from the top edge of the frill, and P. lakustai bore another comb-like horn arising from the middle of the frill behind the eyes.[2]
The type species, Pachyrhinosaurus canadensis, was described in 1950 by Charles Mortram Sternberg. This specimen and others came from the Scabby Butte locality near Lethbridge, Alberta and was among the first dinosaur sites found in the province (in the 1880s), but its significance was not understood until shortly after World War II when some preliminary excavations were conducted. Two skulls were collected by Charles M. Sternberg from the Scabby Butte site and another location on the Little Bow River nearby. 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.
Another Pachyrhinosaurus bonebed, on the Wapiti River south of Beaverlodge in northwestern Alberta, was worked briefly by staff of the Royal Tyrrell Museum in the late 1980s but is now worked annually for a couple weeks each summer (since 2006) by the University of Alberta. Material from this site appears referable to Pachyrhinosaurus canadensis.
In 1972, Grande Prairie, Alberta science teacher Al Lakusta found a large bonebed along Pipestone Creek in Alberta. When the area was finally excavated between 1986 and 1989 by staff and volunteers of the Royal Tyrrell Museum of Palaeontology, paleontologists discovered an amazingly large and dense selection of bones—up to 100 per square meter, with a total of 3500 bones and 14 skulls. This was apparently the site of a mass mortality, perhaps a failed attempt to cross a river during a flood. Found amongst the fossils were the skeletons of four distinct age groups ranging from juveniles to full grown dinosaurs, indicating that the Pachyrhinosaurus cared for their young.
The adult skulls had both convex and concave bosses as well as unicorn-style horns on the parietal bone just behind their eyes. The concave boss types might be related to erosion only and not reflect male/female differences. In 2008, a detailed monograph describing the skull of the Pipestone Creek pachyrhinosaur, and penned by Philip J. Currie, Wann Langston, Jr., and Darren Tanke, classified the specimen as a second species of Pachyrhinosaurus, named P. lakustai after its discoverer.[4]
Modern life at high elevations in lower latitudes resembles life at low elevation in higher latitudes.[5] There may be parallels to this phenomenon in Cretaceous ecosystems, for instance, Pachyrhinosaurus species are found in both Alaska and upland environments in southern Alberta.[5]
During the Edmontonian, in North America's northern biome, there is a general trend of reduced centrosaurine diversity, with only Pachyrhinosaurus surviving.[5] Inland faunas are distinguished by a Saurolophus-Anchiceratops association while more coastal areas were characterized by Pachyrhinosaurus and Edmontosaurus.[5]
A Pachyrhinosaurus was selected as the mascot for the 2010 Arctic Winter Games held in Grande Prairie, Alberta.[6]