Pachyrhinosaurus

Pachyrhinosaurus (meaning "thick-nosed lizard") is an extinct genus of centrosaurine 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 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 only from the lower Horseshoe Canyon Formation, about 71.5-71 Ma ago.[1] Fossils of the youngest species, P. perotorum, have been recovered from the Prince Creek Formation of Alaska, and date to 70-69 million years ago.[2] The presence of three known species makes this genus the most speciose among the centrosaurines.

A Pachyrhinosaurus was selected as the mascot for the 2010 Arctic Winter Games held in Grande Prairie, Alberta.

Description
The largest Pachyrhinosaurus species were 8 metres (26 ft) long.[4] 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.

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.[5][6]

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.

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 1974, 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.

P. perotorum holotype 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.

In 2013, Fiorillo et al. described a new specimen, an incomplete nasal bone attributable to Pachyrhinosaurus perotorum which was collected from the Kikak-Tegoseak Quarry in northern Alaska. This bone, designated DMNH 21460 belongs to an immature individual. This discovery expands the known age profile of this dinosaur genus from this particular site. The specimen has nasal ornamentation that is dorsally enlarged, representing an intermediate stage of growth. Of note, the authors pointed out that the posterior part of the nasal shows evidence for "a degree of integument complexity not previously recognized in other species" of Pachyrhinosaurus. It was determined that the dorsal surface of the nasal boss bore a thick, cornified pad and sheath.

Classification
The cladogram below shows the phylogenetic position of all currently known Pachyrhinosaurus species following Anthony R. Fiorillo and Ronald S. Tykoski, 2011 (only members of the Pachyrhinosaurini are shown below).