Pachyrhinosaurus (from Ancient Greek Παχυ (pachy), thick; ρινό (rinó), nose; and σαυρος (sauros), lizard)[1] is an extinct genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous period of North America.

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 metre, with a total of 3,500 bones and 14 skulls.

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.

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.

[7][8] 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 on the Colville River in Alaska.

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.

[3] P. perotorum was thought to have two unique, flattened horns which projected forward and down from the top edge of the frill, but 2019 it was shown that these had been inaccurately reconstructed and it instead had the same backward pointed as its sister species.

[3] The cladogram below shows the phylogenetic position of all currently known Pachyrhinosaurus species following Chiba et al. (2017):[14] Diabloceratops eatoni Machairoceratops cronusi Avaceratops lammersi (ANSP 15800)

The development of characteristics useful in sexual selection, including competition between males, such as pronounced nasal bosses, occurred at approximately nine years of age.

[16] The development of characteristics useful in sexual selection, including competition between males, such as pronounced nasal bosses, occurred when the dinosaur was roughly 73% the size of a full-grown adult.

The region where dinosaurs lived was bounded by mountains to the west, and included ancient channels, small freshwater ponds, streams, and floodplains.

Pachyrhinosaurus shared its paleoenvironment with other dinosaurs, such as the ceratopsians Anchiceratops and Montanoceratops cerorhynchus, the armored nodosaur Edmontonia longiceps, the duckbilled hadrosaur Edmontosaurus regalis, the theropods Saurornitholestes and Troodon, possibly the ornithopod Thescelosaurus, and the tyrannosaurid Albertosaurus, which was likely the apex predator in its ecosystem.

[18] Vertebrates present in the St. Mary River Formation at the time of Pachyrhinosaurus included the actinopterygian fishes Amia fragosa, Lepisosteus, Belonostomus, Paralbula casei, and Platacodon nanus, the mosasaur Plioplatecarpus, the turtle Boremys and the diapsid reptile Champsosaurus.

A fair number of mammals lived in this region, which included Turgidodon russelli, Cimolestes, Didelphodon, Leptalestes, Cimolodon nitidus, and Paracimexomys propriscus.

[19][20] Flora of the region include the aquatic angiosperm Trapago angulata, the amphibious heterosporous fern Hydropteris pinnata, rhizomes, and taxodiaceous conifers.

[23] 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.

[23] P. canadensis coexisted with ankylosaurids Anodontosaurus lambei and Edmontonia longiceps, the maniraptorans Atrociraptor marshalli, Epichirostenotes curriei, the troodontid Albertavenator curriei, the alvarezsaurid theropod Albertonykus borealis, the ornithomimids Dromiceiomimus brevitertius, Ornithomimus edmontonicus, and an unnamed species of Struthiomimus, the bone-head pachycephalosaurids Stegoceras, and Sphaerotholus edmontonensis, the ornithopod Parksosaurus warreni, the hadrosaurids Edmontosaurus regalis, Hypacrosaurus altispinus, and Saurolophus osborni, the ceratopsians Anchiceratops ornatus, Arrhinoceratops brachyops, Eotriceratops xerinsularis, Montanoceratops cerorhynchus, and the tyrannosaurid Albertosaurus sarcophagus, which was the apex predator of this paleoenvironment.

Reptiles such as turtles and crocodilians are rare in the Horseshoe Canyon Formation, and this was thought to reflect the relatively cool climate which prevailed at the time.

A study by Quinney et al. (2013), however, showed that the decline in turtle diversity, which was previously attributed to climate, coincided instead with changes in soil drainage conditions, and was limited by aridity, landscape instability, and migratory barriers.