Pachyrhinosaurus (from Ancient Greek ' (), thick; ' (), nose; and (), lizard) is a genus of centrosaurine ceratopsid dinosaur 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 Pachyrhinosaurus.
Three species have been identified. P. lakustai, from the Wapiti Formation, the bonebed horizon of which is roughly equivalent in 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 73–71 Ma ago and the St. Mary River Formation. Fossils of the northernmost species, P. perotorum, have been recovered from the Prince Creek Formation of Alaska, and date to 73 Ma ago. The presence of three known species makes this genus the most speciose among the centrosaurines.
Discovery and species
Pachyrhinosaurus canadensis
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.
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.
Pachyrhinosaurus lakustai
thumb|Skulls of the three species compared: P. perotorum (with inaccurate [[Wikt:epiparietal|epiparietal placement), P. canadensis, and P. lakustai]]Another Pachyrhinosaurus bonebed, on the Wapiti River south of Wembley in northwestern Alberta, was worked briefly by staff of the Royal Tyrrell Museum in the late 1980s but is now worked annually each summer (since 2015) by the Philip J. Currie Dinosaur Museum in Wembley, 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 metre, with a total of 3,500 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.
In 2024, excavation at Pipestone Creek by the Philip J. Currie Dinosaur Museum unearthed one of the largest known P. lakustai skulls. The specimen came to be known as "Big Sam" (PCB.2024.666). The 138 cm long skull has a nasal boss that is 43 cm in diameter. The 1-3 spikes that normally ran down the middle of the frill in P. lakustai was strongly reduced in this specimen. As of October 2025, the specimen is still being prepared.
Pachyrhinosaurus perotorum
In 2012, Fiorillo & Tykoski described Pachyrhinosaurus perotorum. This Alaskan species was named after the Texas oil billionaire and benefactor, Ross Perot.
Description
thumb|left|Size comparison of P. canadensis
Size estimates for the largest Pachyrhinosaurus species, P. canadensis indicate lengths of and a weight of . The other species, P. lakustai and P. perotorum, have been estimated by Greg Paul at in length and in weight. 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 horns as its sister species.
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. lakustai bore another comb-like horn arising from the middle of the frill behind the eyes.
Paleobiology
In the Prince Creek Formation, P. perotorum engaged in niche partitioning with Edmontosaurus; the latter favoured lowland deltas while the former preferred more upland environments.
Growth rates
thumb|right|Skull differences between different growth stages of P. perotorum
During the first few years of development, P. perotorum show extremely rapid growth. When the animals were one year old, they had already reached 28% of their adult body size. By age two, they were almost half the size of a mature adult. However, the rate of growth slows considerably after that, and maximum size is not fully attained until about age twenty. The development of characteristics useful in sexual selection, including competition between males, such as pronounced nasal bosses, occurred at approximately nine years of age. This presumably corresponds to the age of sexual maturity.
Unlike other Pachyrhinosaurus species, P. perotorum shows highly conspicuous growth banding in the bones, indicating retarded growth during the winter. This is perhaps not surprising, considering that P. perotorum experienced much harsher winters than southerly species within the genus.
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 age of sexual maturity is unknown. Due to the lack of conspicuous growth banding, more detailed analyses of P. lakustai growth rates cannot be performed.
A skull of P. lakustai was found with severe lesions on the right side of the skull. The largest of these lesions was 195 mm in diameter, and harboured an additional large lesion inside it. This resulted in a massive hole on the right side of the face, so deep that bones of the left side of the skull are visible within the hole. This hole also partially destroyed the right eye socket, which probably would've rendered the animal blind on that side. This affliction did not kill the animal; this individual died as part of a mass death assemblage alongside many of its herd mates. This specimen came to be known as "Harvey" (TMP 1989.055.1234), after the DC Comics character Harvey Dent.
Paleoecology
St. Mary River Formation
Habitat
thumb|right|Restoration of a confrontation between P. canadensis and [[Albertosaurus]]
The St. Mary River Formation has not undergone a definitive radiometric dating, however, the available stratigraphic correlation has shown that this formation was deposited between 74 and 66 million years ago, during the Campanian and the late Maastrichtian, during the final regression of the mid-continental Bearpaw Seaway. It ranges from as far south as Glacier County, Montana to as far north as the Little Bow River in Alberta. The St. Mary River Formation is part of the Western Canadian Sedimentary Basin in southwestern Alberta, which extends from the Rocky Mountains in the west to the Canadian Shield in the east. It is laterally equivalent to the Horseshoe Canyon Formation. The region where dinosaurs lived was bounded by mountains to the west, and included ancient channels, small freshwater ponds, streams, and floodplains.
Paleofauna
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. Flora of the region include the aquatic angiosperm Trapago angulata, the amphibious heterosporous fern Hydropteris pinnata, rhizomes, and taxodiaceous conifers.
Horseshoe Canyon Formation
Habitat
The Horseshoe Canyon Formation has been radiometrically dated as being between 74 and 67 million years old. It was deposited during the gradual withdrawal of the Western Interior Seaway, during the Campanian and Maastrichtian stage of the Late Cretaceous period. The Horseshoe Canyon Formation is a terrestrial unit which is part of the Edmonton Group that includes the Battle Formation and the Whitemud Member, both in Edmonton. The valley where dinosaurs lived included ancient meandering estuary channels, straight channels, peat swamps, river deltas, floodplains, shorelines and wetlands. Due to the changing sea levels, many different environments are represented in the Horseshoe Canyon Formation, including offshore and near-shore marine habitats and coastal habitats like lagoons, and tidal flats. The area was wet and warm with a temperate to subtropical climate. Just prior to the Campanian–Maastrichtian boundary, the mean annual temperature and precipitation in this region dropped rapidly. The dinosaurs from this formation form part of the Edmontonian land vertebrate age, and are distinct from those in the formations above and below.
Modern life at high elevations in lower latitudes resembles life at low elevation in higher latitudes.
During the Edmontonian, in North America's northern biome, there is a general trend of reduced centrosaurine diversity, with only Pachyrhinosaurus surviving. The saltwater plesiosaur Leurospondylus was present and freshwater environments were populated by turtles, Champsosaurus, and crocodilians like Leidyosuchus and Stangerochampsa. Evidence has shown that multituberculates and the early marsupial Didelphodon coyi were present. Vertebrate trace fossils from this region included the tracks of theropods, ceratopsians and ornithopods, which provide evidence that these animals were also present. Non-vertebrates in this ecosystem included both marine and terrestrial invertebrates.
See also
- Timeline of ceratopsian research