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Pachyrhinosaurus is a genus of Centrosaurine Ceratopsian that lived in North America during the Late Cretaceous

History
In 1945-1946 several skulls are discovered from the Horseshoe Canyon Formation. In 1947 Charles Sternberg published a paper on a new Ceratopsian dinosaur representing a new family. In 1950 Sternberg named this species Pachyrhinosaurus canadensis. In 1975 Langston, W. Jr. described more material of P. canadensis from the St. Mary River Formation's Scabby Butte. In the 1980s a young Philip Currie and Darren Tanke were looking through the specimens in the TMP's collection when stumbling on the remains of a Ceratopsian from Pipestone Creek of the Wapiti formation, representing material from, "numerous individuals, with virtually every bone in the skeleton represented." In 2008 Currie, Langston & Tanke formally described the bones as Pachyrhinosaurus lakustai. In the 1990s a multitaxic bonebed in the Prince Creek Formation was discovered. Out of this a multitude of Pachyrhinosaurus skeletons were perserved. In 2012 Fiorillo & Tykoski described these remains as belonging to a new species, Pachyrhinosaurus perotorum. In 2015 Kruk published a thesis on Pachyrhinosaurus ontogeny and histology. In that thesis she named a fourth species of Pachyrhinosaurus, P. "youngi". This specimen was probably collected at the same time as P. lakustai, in the 70s/80s. She says that, "This chapter will be submitted to PLoS ONE for publication." So far an official description hasn't been published, making P. "youngi" a nomen nudum. Finally, TMP 2002.76.1 was discovered in the Iddesleigh of the Dinosaur Park Formation in 1996, and was deposited and cataloged in the TMP in 2002. It was described by ''Ryan et. al''. (2010) as a new Ceratopsian, but wasn't given a name.

Description
Pachyrhinosaurus can be distinguished from Non-Pachyrostran Centrosaurs by the presence of a nasal boss instead of a nasal horn. Further, it can be distinguished from Achelousaurus by the more medially projected, curved P3 epiparietals, more pronounced P1 epiparietals, a shorter, rounder frill, among other features. Further, it can be distinguished from Achelousaurus, the other known Pachyrostran, by, "a relatively broader, longer, and typically thicker boss.", "the nasal rugosity extends posteriorly onto the frontals", and "Supraorbital boss is stouter and less rugose than in Achelousaurus."

P. canadensis is known from the Horsehoe Canyon Formation and the laterally equivalent St. Mary River Formation. It was estimated by to be6 meters long, 2 meters tall, and weigh 3 metric tones. *WIP, add species autapomorphies, ect.*

P. lakustai is known from the Wapiti Formation. It was around 5m long, and weigh 2 metric tons. *WIP, add species autapomorphies, ect.*

P. perotorum is known from the Prince Creek Formation. It was around 5m long, and weigh 2 metric tons *WIP, add species autapomorphies, ect.*

P. "youngi" was found in the Wapiti Formation. Kruk didn't provide estimates on size of P. "youngi", however it can be assumed to be around 5-6m long and weighed 2-3 tons due to it's phylogenetic position. *WIP, add species autapomorphies, ect.*

TMP 2002.76.1 is known from the Dinosaur Park Formation. This specimen is known from a mostly complete skull missing the back of the frill, both radii and ulnae of the front feet, the right humerus, both scapulae, both scapulacoracoids, 3 complete feet, 2 tibiae (one partial one complete), a fibula, around 5 cervical vertebrae, around 14 dorsal vertebrae, and around 11 caudal vertebrae, and around 12 pairs of ribs, as well as a highly incomplete sacrum. If this taxon is Pachyrhinosaurus it would be the oldest species of the genus. Both Kruk (2015) and Tykoski et. al. (2019) recover this taxon as a taxon closer to Pachyrhinosaurus rather than Achelousaurus. However, Savhannah Carpenter's unpublished phylogeny from 2021 as well as ''Ryan et. al. (2010) places TMP 2002.76.1 in a polytomy with Achelousaurus and Pachyrhinosaurus''. The reason why it's position within Pachyrostra hasn't been consistent is due to the preservation of TMP 2002.76.1. TMP 2002.76.1 is missing the top of its frill, which is a vital character that distinguished Ceratopsids in General and Pachyrhinosaurus in particular. Until further expeditions to the Dinosaur Park Formation uncover another specimen of this species, it is unlikely to receive a name (despite several characters that distinguish it from Achelousaurus horneri and the other known Pachyrhinosaurus species) due to the uncertainty of whether it is a new genus, a new species of Achelousaurus, or a new species of Pachyrhinosaurus.

Species

 * P. canadensis (Sternberg, 1950)
 * P. lakustai (Currie, Langston & Tanke, 2008)
 * P. perotorum (Fiorillo & Tykoski, 2012)
 * P. "youngi" (Kruk, 2015, Thesis)
 * P. sp. nov.? TMP 2002.76.1

Phylogeny
This cladogram was recovered by Kruk (2015) in her thesis on P. "youngi".

Horseshoe Canyon Formation/ St. Mary River Formation
*Add ecology*

Wapiti Formation
*Add ecology*

Prince Creek Formation
*Add ecology*

Dinosaur Park Formation
*Add ecology*

Notable Specimens

 * CMN 8867: The holotype of Pachyrhinosaurus canadensis and by proxy Pachyrhinosaurus as a whole.
 * CMN 8866: The paratype of Pachyrhinosaurus canadensis.
 * TMP 1986.55.258: The holotype of Pachyrhinosaurus lakustai.
 * DMNH 21200: The holotype of Pachyrhinosaurus perotorum.
 * DMNH 22558: Paratype of Pachyrhinosaurus perotorum.
 * DMNH 21201: Paratype of Pachyrhinosaurus perotorum.
 * UALVP 53300: Holotype of P. "youngi".
 * TMP 2002.76.1: A new species that probably belongs to Pachyrhinosaurus, the oldest Pachyrhinosaurus known.