Styracosaurus ( ; meaning "spiked lizard" from the Ancient Greek / "spike at the butt-end of a spear-shaft" and / "lizard") is an extinct genus of herbivorous dinosaur from the Late Cretaceous (Campanian stage) of North America. It had four to six long parietal spikes extending from its neck frill, a smaller jugal horn on each of its cheeks, and a single horn protruding from its nose, which may have been up to long and wide. The function or functions of the horns and frills have been debated for many years.
Styracosaurus was a relatively large dinosaur, reaching lengths of and weighing about . It stood about tall. Styracosaurus possessed four short legs and a bulky body. Its tail was rather short. The skull had a beak and shearing cheek teeth arranged in continuous dental batteries, suggesting that the animal sliced up plants. Like other ceratopsians, this dinosaur may have been a herd animal, travelling in large groups, as suggested by bone beds.
Named by Lawrence Lambe in 1913, Styracosaurus is a member of the Centrosaurinae. One species, S. albertensis, is currently assigned to Styracosaurus. Another species, S. ovatus, named in 1930 by Charles Gilmore was reassigned to a new genus, Rubeosaurus, by Andrew McDonald and Jack Horner in 2010, but it has been considered either its own genus or a species of Styracosaurus (or even a specimen of S. albertensis) Among the differences between the specimens cited by Brown and Schlaikjer were a cheekbone quite different from that of S. albertensis, and smaller tail vertebrae. S. parksi also had a more robust jaw, a shorter dentary, and the frill differed in shape from that of the type species. "S. makeli", mentioned informally by amateur paleontologists Stephen and Sylvia Czerkas in 1990 in a caption to an illustration, is an early name for Einiosaurus. "S. borealis" is an early informal name for S. parksi.
Styracosaurus ovatus
thumb|left|Holotype frill of S. ovatus, which was previously in the genus Rubeosaurus
A species, Styracosaurus ovatus, from the Two Medicine Formation of Montana, was described by Gilmore in 1930, named for a partial parietal under the accession number USNM 11869. Unlike S. albertensis, the longest parietal spikes converge towards their tips, instead of projecting parallel behind the frill. There also may only have been two sets of spikes on each side of the frill, instead of three. As estimated from the preserved material, the spikes are much shorter than in S. albertensis, with the longest only long. An additional specimen from the Two Medicine Formation was referred to Styracosaurus ovatus in 2010 by Andrew McDonald and John Horner, having been found earlier in 1986 but not described until that year. Known from a premaxilla, the nasal bones and their horncore, a postorbital bone and a parietal, the specimen Museum of the Rockies 492 was considered to share the medially-converging parietal spikes with the only other specimen of S. ovatus, the holotype. Following this additional material, the species was added to a phylogenetic analysis where it was found to group not with Styracosaurus albertensis, but in a clade including Pachyrhinosaurus, Einiosaurus and Achelousaurus, and therefore McDonald and Horner gave the species the new genus name Rubeosaurus. Another specimen, the partial immature skull USNM 14768, which was earlier referred to the undiagnostic genus Brachyceratops, was also referred to Rubeosaurus ovatus by McDonald and colleagues in 2011. While the medial spikes of USNM 14768 were too incomplete to show if it shared the convergence seen in other R. ovatus specimens, it was considered to be the same species as it was also found in the older deposits of the Two Medicine Formation, and had a unique combination of parietal features only shared completely with the other specimens of the species.
thumb|[[Paleoart|Life restoration of the "S. ovatus" individual]]
Though it was originally found to nest closer to Einiosaurus and later centrosaurines by McDonald and colleagues in both 2010 and 2011, revisions of phylogenetic analyses in 2013 by Scott Sampson and colleagues, and further expansions and modifications of the same dataset, instead placed Rubeosaurus ovatus as the sister taxon of Styracosaurus albertensis, as had been originally considered when the species was first named, though the two species were not moved into the same genus as originally named. A review of the variability within known Styracosaurus specimens by Robert Holmes and colleagues in 2020 found that USNM 11869, the type specimen of Rubeosaurus ovatus, fell within the variation seen in other specimens from the older deposits of the Dinosaur Park Formation S. albertensis is known from. While no phylogenetic analysis was conducted, previous results of updated analyses showed that Rubeosaurus ovatus and Styracosaurus albertensis were not distantly related, so the justification for naming the genus Rubeosaurus was not present, and the variability in Styracosaurus albertensis specimens also did not support the distinction of Styracosaurus ovatus, with Holmes et al. considering the latter a junior synonym of the former.
Description
thumb|left|Size compared to a human
Individuals of the genus Styracosaurus were approximately long as adults and weighed about . The skull was massive, with a large nostril, a tall straight nose horn, and a parietal squamosal frill (a neck frill) crowned with at least four large spikes. Each of the four longest frill spines was comparable in length to the nose horn, at long. The nasal horn was estimated by Lambe at long in the type specimen,
Classification
Styracosaurus is a member of the Centrosaurinae. Other members of the clade include Centrosaurus (from which the group takes its name), Pachyrhinosaurus, Avaceratops, Achelousaurus, and Monoclonius, However, most other researchers have not accepted Monoclonius nasicornis as a female Styracosaurus, instead regarding it as a synonym of Centrosaurus apertus. While sexual dimorphism has been proposed for an earlier ceratopsian, Protoceratops, there is no firm evidence for sexual dimorphism in any ceratopsid.
thumb|Ceratopsid skull casts positioned in a [[phylogenetic tree, in the Natural History Museum of Utah, with Styracosaurus at the far left]]
thumb|upright|Skull of the holotype specimen
The cladogram depicted below represents a phylogenetic analysis by Chiba et al. (2017):
Origins and evolution
The evolutionary origins of Styracosaurus were not understood for many years because fossil evidence for early ceratopsians was sparse. The discovery of Protoceratops, in 1922, shed light on early ceratopsid relationships, but several decades passed before additional finds filled in more of the blanks. Fresh discoveries in the late 1990s and 2000s, including Zuniceratops, the earliest known ceratopsian with brow horns, and Yinlong, the first-known Jurassic ceratopsian, indicate what the ancestors of Styracosaurus may have looked like. These new discoveries have been important in illuminating the origins of horned dinosaurs in general, and suggest that the group originated during the Jurassic in Asia, with the appearance of true horned ceratopsians occurring by the beginning of the late Cretaceous in North America. The position of Styracosaurus in this lineage is now equivocal, as the remains that were thought to represent Styracosaurus have been transferred to the genus Rubeosaurus. The mass deaths at this site may have been a result of otherwise non-herding animals congregating around a waterhole in a period of drought, with evidence suggesting the environment may have been seasonal and semi-arid.
Paleontologists Gregory Paul and Per Christiansen proposed that large ceratopsians such as Styracosaurus were able to run faster than an elephant, based on possible ceratopsian trackways which did not exhibit signs of sprawling forelimbs.
Dentition and diet
Styracosaurs were herbivorous dinosaurs; they probably fed mostly on low growth because of the position of the head. They may, however, have been able to knock down taller plants with their horns, beak, and bulk. The jaws were tipped with a deep, narrow beak, believed to have been better at grasping and plucking than biting.
Ceratopsid teeth, including those of Styracosaurus, were arranged in groups called batteries. Older teeth on top were continually replaced by the teeth underneath them. Unlike hadrosaurids, which also had dental batteries, ceratopsid teeth sliced but did not grind. while others have suggested ferns. Dodson has proposed that Late Cretaceous ceratopsians may have knocked down angiosperm trees and then sheared off leaves and twigs.
Horns and frill
thumb|upright|Close-up of the AM5372 skull, [[American Museum of Natural History]]
The large nasal horns and frills of Styracosaurus are among the most distinctive facial adornments of all dinosaurs. Their function has been the subject of debate since the first horned dinosaurs were discovered.
Early in the 20th century, paleontologist R. S. Lull proposed that the frills of ceratopsian dinosaurs acted as anchor points for their jaw muscles. He later noted that for Styracosaurus, the spikes would have given it a formidable appearance. In 1996, Dodson supported the idea of muscle attachments in part and created detailed diagrams of possible muscle attachments in the frills of Styracosaurus and Chasmosaurus, but did not subscribe to the idea that they completely filled in the fenestrae. C. A. Forster, however, found no evidence of large muscle attachments on the frill bones.
It was long believed that ceratopsians like Styracosaurus used their frills and horns in defence against the large predatory dinosaurs of the time. Although pitting, holes, lesions, and other damage on ceratopsid skulls are often attributed to horn damage in combat, a 2006 study found no evidence for horn thrust injuries causing these forms of damage (for example, there is no evidence of infection or healing). Instead, non-pathological bone resorption, or unknown bone diseases, are suggested as causes.
thumb|left|Variation in frill morphology; the top row are subadults, the rest are mature.
However, a newer study compared incidence rates of skull lesions in Triceratops and Centrosaurus and showed that these were consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure, while lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation, or a form of combat focused on the body rather than the head; as Centrosaurus was more closely related to Styracosaurus and both genera had long nasal horns, the results for this genus would be more applicable for Styracosaurus. The researchers also concluded that the damage found on the specimens in the study was often too localized to be caused by bone disease.
The large frill on Styracosaurus and related genera also may have helped to increase body area to regulate body temperature, like the ears of the modern elephant. A similar theory has been proposed regarding the plates of Stegosaurus, although this use alone would not account for the bizarre and extravagant variation seen in different members of the Ceratopsidae. Evidence that visual display was important, either in courtship or in other social behavior, can be seen in the fact that horned dinosaurs differ markedly in their adornments, making each species highly distinctive. Also, modern living creatures with such displays of horns and adornments use them in similar behavior.
The use of the exaggerated structures in dinosaurs as species identification has been questioned, as no such function exists in vast majority of modern species of tetrapods (terrestrial vertebrates).
A skull discovered in 2015 from a Styracosaurus indicates that individual variation was likely commonplace in the genus. The asymmetrical nature of the horns in the specimen has been compared to deer, which often have asymmetrical antlers in various individuals. The study carried out may also indicate that the genus Rubeosaurus may be synonymous with Styracosaurus as a result.
The Dinosaur Park Formation is interpreted as a low-relief setting of rivers and floodplains that became more swampy and influenced by marine conditions over time as the Western Interior Seaway transgressed westward. The climate was warmer than present-day Alberta, without frost, but with wetter and drier seasons. Conifers were apparently the dominant canopy plants, with an understory of ferns, tree ferns, and angiosperms.
In the Two Medicine Formation, dinosaurs that lived alongside Styracosaurus ovatus included the basal ornithopod Orodromeus, hadrosaurids (such as Hypacrosaurus, Maiasaura, and Prosaurolophus), the centrosaurines Brachyceratops and Einiosaurus, the leptoceratopsid Cerasinops, the ankylosaurs Edmontonia and Euoplocephalus, the tyrannosaurid Daspletosaurus (which appears to have been a specialist of preying on ceratopsians), as well as the smaller theropods Bambiraptor, Chirostenotes, Troodon, and Avisaurus.