| Styracosaurus | |
|---|---|
 | |
| Holotype skeleton,Canadian Museum of Nature | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Centrosaurinae |
| Clade: | †Eucentrosaura |
| Tribe: | †Centrosaurini |
| Genus: | †Styracosaurus Lambe, 1913 |
| Type species | |
| †Styracosaurus albertensis Lambe, 1913 | |
| Other species | |
| Synonyms | |
S. albertensis synonymy
S. ovatus synonymy
| |
Styracosaurus (/stɪˌrækəˈsɔːrəs/sti-RAK-ə-SOR-əs; meaning "spiked lizard" from theAncient Greekstyrax/στύραξ "spike at the butt-end of a spear-shaft" andsauros/σαῦρος "lizard")[1] is an extinctgenus ofherbivorousceratopsiandinosaur from the LateCretaceous (Campanianstage) of North America. It had four to six long parietal spikes extending from itsneck frill, a smaller jugal horn on each of its cheeks, and a single horn protruding from its nose, which may have been up to 60 centimeters (2 feet) long and 15 centimeters (6 inches) 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 5–5.5 metres (16–18 ft) and weighing about 1.8–2.7 metric tons (2.0–3.0 short tons). It stood about 1.8 meters (5.9 feet) 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 continuousdental batteries, suggesting that the animal sliced up plants. Like other ceratopsians, this dinosaur may have been aherd animal, travelling in large groups, as suggested bybone beds.
Named byLawrence Lambe in 1913,Styracosaurus is a member of theCentrosaurinae. Onespecies,S. albertensis, is currently assigned toStyracosaurus. Another species,S. ovatus, named in 1930 byCharles Gilmore was reassigned to a new genus,Rubeosaurus, by Andrew McDonald andJack Horner in 2010,[2] but it has been considered either its own genus or a species ofStyracosaurus (or even a specimen ofS. albertensis)[3] again, since 2020.
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The firstfossil remains ofStyracosaurus were collected inAlberta, Canada byC. M. Sternberg (from an area now known asDinosaur Provincial Park, in aformation now called theDinosaur Park Formation) and named byLawrence Lambe in 1913. This quarry was revisited in 1935 by aRoyal Ontario Museum crew who found the missing lower jaws and most of the skeleton. These fossils indicate thatS. albertensis was around 5.5–5.8 metres (18–19 ft) in length and stood about 1.65 metres (5.4 ft) high at the hips.[4] An unusual feature of this first skull is that the smallest frill spike on the left side is partially overlapped at its base by the next spike. It appears that the frill suffered a break at this point in life and was shortened by about 6 centimeters (2.4 inches). The normal shape of this area is unknown because the corresponding area of the right side of the frill was not recovered.[5]
Barnum Brown and crew, working for theAmerican Museum of Natural History in New York, collected a nearly complete articulated skeleton with a partial skull in 1915. These fossils were also found in the Dinosaur Park Formation, nearSteveville, Alberta. Brown andErich Maren Schlaikjer compared the finds, and, though they allowed that both specimens were from the same general locality and geological formation, they considered the specimen sufficiently distinct from theholotype to warrant erecting a new species, and described the fossils asStyracosaurus parksi, named in honor ofWilliam Parks.[6] Among the differences between the specimens cited by Brown and Schlaikjer were acheekbone quite different from that ofS. albertensis, and smallertailvertebrae.S. parksi also had a more robust jaw, a shorterdentary, and the frill differed in shape from that of the type species.[6] However, much of the skull consisted of plaster reconstruction, and the original 1937 paper did not illustrate the actual skull bones.[4] It is now accepted as a specimen ofS. albertensis.[5][7]
In the summer of 2006,Darren Tanke of theRoyal Tyrrell Museum of Palaeontology inDrumheller, Alberta relocated the long lostS. parksi site.[5] Pieces of the skull, evidently abandoned by the 1915 crew, were found in the quarry. These were collected and it is hoped more pieces will be found, perhaps enough to warrant a redescription of the skull and test whetherS. albertensis andS. parksi are the same. The Tyrrell Museum has also collected several partialStyracosaurus skulls.[8] At least one confirmed bone bed (bonebed 42) in Dinosaur Provincial Park has also been explored (other proposedStyracosaurus bone beds instead have fossils from a mix of animals, and nondiagnostic ceratopsian remains). Bonebed 42 is known to contain numerous pieces of skulls such as horncores, jaws and frill pieces.[5]
Several other species which were assigned toStyracosaurus have since been assigned to other genera.S. sphenocerus, described byEdward Drinker Cope in 1890 as a species ofMonoclonius and based on anasal bone with a brokenStyracosaurus-like straight nose horn, was attributed toStyracosaurus in 1915.[9] "S. makeli", mentioned informally by amateurpaleontologists Stephen and Sylvia Czerkas in 1990 in a caption to an illustration, is an early name forEiniosaurus.[10] "S. borealis" is an early informal name forS. parksi.[11]
A species,Styracosaurus ovatus, from theTwo Medicine Formation ofMontana, was described byGilmore in 1930, named for a partial parietal under the accession number USNM 11869. UnlikeS. 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 inS. albertensis, with the longest only 295 millimeters (11.6 inches) long.[12] An additional specimen from the Two Medicine Formation was referred toStyracosaurus ovatus in2010 by Andrew McDonald andJohn Horner, having been found earlier in 1986 but not described until that year. Known from apremaxilla, thenasal bones and their horncore, apostorbital bone and a parietal, the specimenMuseum of the Rockies 492 was considered to share the medially-converging parietal spikes with the only other specimen ofS. ovatus, the holotype. Following this additional material, the species was added to a phylogenetic analysis where it was found to group not withStyracosaurus albertensis, but in a clade includingPachyrhinosaurus,Einiosaurus andAchelousaurus, and therefore McDonald and Horner gave the species the new genus nameRubeosaurus.[13] Another specimen, the partial immature skull USNM 14768, which was earlier referred to the undiagnostic genusBrachyceratops, was also referred toRubeosaurus ovatus by McDonald and colleagues in2011. While the medial spikes of USNM 14768 were too incomplete to show if it shared the convergence seen in otherR. 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.[14]
Though it was originally found to nest closer toEiniosaurus and later centrosaurines by McDonald and colleagues in both 2010 and 2011, revisions of phylogenetic analyses in2013 by Scott Sampson and colleagues, and further expansions and modifications of the same dataset, instead placedRubeosaurus ovatus as the sister taxon ofStyracosaurus 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 knownStyracosaurus specimens by Robert Holmes and colleagues in2020 found that USNM 11869, the type specimen ofRubeosaurus ovatus, fell within the variation seen in other specimens from the older deposits of the Dinosaur Park FormationS. albertensis is known from. While no phylogenetic analysis was conducted, previous results of updated analyses showed thatRubeosaurus ovatus andStyracosaurus albertensis were not distantly related, so the justification for naming the genusRubeosaurus was not present, and the variability inStyracosaurus albertensis specimens also did not support the distinction ofStyracosaurus ovatus, with Holmeset al. considering the latter ajunior synonym of the former.[3] The conclusion of Holmes and colleagues was supported by a later 2020 study authored by Caleb Brown, Holmes, andPhilip J. Currie, who described a new juvenileStyracosaurus specimen and determined that there were several specimens that are otherwise consistent withS. albertensis have been found with inward angled midline frill spikes, though not the same degree asS. ovatus. Though they considered thatS. ovatus represented an extreme end of theS. albertensis variation not only in morphology but also as it was stratigraphically younger, they cautioned that at the least the current diagnosis ofS. ovatus was inadequate.[15]
Later in 2020, the supposed specimen MOR 492 was redescribed by John Wilson and colleagues, who reinterpreted its anatomy in a way that contrasted McDonald and Horner who referred it toStyracosaurus ovatus. While Wilsonet al. agreed that the close relationship betweenS. albertensis andS. ovatus meant that the genus nameRubeosaurus should be abandoned, they cautioned against synonymization. MOR 492 was moved into its own taxon,Stellasaurus ancellae, which nested alongsideEiniosaurus,Achelousaurus andPachyrhinosaurus in a similar result to McDonald and Horner when the specimen was included as part of theS. ovatus hypodigm. Wilson and colleagues also suggested that the new taxon may have been ancestral to the later forms it was found related to, suggesting that gradual evolution throughanagenesis could be the reason for the intermediate morphologies of many specimens and species found in the Two Medicine Formation, possibly also includingS. ovatus. As the holotype ofStyracosaurus ovatus was found in deposits much younger than the remainder ofStyracosaurus specimens, and was considered to have the most extreme morphology while still falling within plausible variation as Holmeset al. had concluded, Wilson and colleagues advised thatS. ovatus was retained as a separate, probably directly descended fromS. albertensis, species ofStyracosaurus. The immature specimen USNM 14768, referred toS. ovatus by McDonaldet al. in 2011, was considered too immature to be diagnostic, and thusS. ovatus was limited to its holotype USNM 11869.[16]
Individuals of the genusStyracosaurus were approximately 5–5.5 metres (16–18 ft) long as adults and weighed about 1.8–2.7 metric tons (2.0–3.0 short tons).[17][18] The skull was massive, with a largenostril, a tall straight nose horn, and a parietal squamosal frill (aneck frill) crowned with at least four large spikes. Each of the four longest frill spines was comparable in length to the nose horn, at 50 to 55 centimeters (20 to 22 inches) long.[4] The nasal horn was estimated by Lambe at 57 centimeters (22 inches) long in thetype specimen,[19] but the tip had not been preserved. Based on other nasal horn cores fromStyracosaurus andCentrosaurus, this horn may have come to a more rounded point at around half of that length.[5]
Aside from the large nasal horn and four long frill spikes, the cranial ornamentation was variable. Some individuals had small hook-like projections and knobs at theposterior margin of the frill, similar to but smaller than those inCentrosaurus. Others had less prominent tabs. Some, like the type individual, had a third pair of long frill spikes. Others had much smaller projections, and small points are found on the side margins of some but not all specimens. Modest pyramid-shaped brow horns were present in subadults, but were replaced by pits in adults.[5] Like most ceratopsids,Styracosaurus had largefenestrae (skull openings) in its frill. The front of the mouth had a toothless beak.
The bulky body ofStyracosaurus resembled that of arhinoceros. It had powerful shoulders which may have been useful in intraspecies combat.Styracosaurus had a relatively short tail. Each toe bore a hooflikeungual which was sheathed in horn.[17]
Various limb positions have been proposed forStyracosaurus and ceratopsids in general, including forelegs which were held underneath the body, or, alternatively, held in a sprawling position. The most recent work has put forward an intermediate crouched position as most likely.[20]
Styracosaurus is a member of theCentrosaurinae. Other members of theclade includeCentrosaurus (from which the group takes its name),[21][22]Pachyrhinosaurus,[21][23]Avaceratops,[21]Einiosaurus,[23][24]Albertaceratops,[24]Achelousaurus,[23]Brachyceratops,[7] andMonoclonius,[21] although these last two aredubious. Because of the variation between species and even individual specimens of centrosaurines, there has been much debate over which genera and species are valid, particularly whetherCentrosaurus and/orMonoclonius are valid genera, undiagnosable, or possibly members of the opposite sex. In 1996,Peter Dodson found enough variation betweenCentrosaurus,Styracosaurus, andMonoclonius to warrant separate genera, and thatStyracosaurus resembledCentrosaurus more closely than either resembledMonoclonius. Dodson also believed one species ofMonoclonius,M. nasicornis, may actually have been a femaleStyracosaurus.[25] However, most other researchers have not acceptedMonoclonius nasicornis as a femaleStyracosaurus, instead regarding it as a synonym ofCentrosaurus apertus.[5][26] Whilesexual dimorphism has been proposed for an earlier ceratopsian,Protoceratops,[27] there is no firm evidence for sexual dimorphism in any ceratopsid.[28][29][30]
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Thecladogram depicted below represents a phylogenetic analysis by Chibaet al. (2017):[31]
| Centrosaurinae |
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The evolutionary origins ofStyracosaurus were not understood for many years because fossil evidence for early ceratopsians was sparse. The discovery ofProtoceratops, in 1922, shed light on early ceratopsid relationships,[32] but several decades passed before additional finds filled in more of the blanks. Fresh discoveries in the late 1990s and 2000s, includingZuniceratops, the earliest known ceratopsian with brow horns, andYinlong, the first-knownJurassic ceratopsian, indicate what the ancestors ofStyracosaurus 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.[7]
Goodwin and colleagues proposed in 1992 thatStyracosaurus was part of the lineage leading toEiniosaurus,Achelousaurus andPachyrhinosaurus. This was based on a series of fossil skulls from theTwo Medicine Formation of Montana.[33] The position ofStyracosaurus in this lineage is now equivocal, as the remains that were thought to representStyracosaurus have been transferred to the genusRubeosaurus.[13]
Styracosaurus is known from a higher position in the formation (relating specifically to its own genus) than the closely relatedCentrosaurus, suggesting thatStyracosaurus displacedCentrosaurus as the environment changed over time and/or dimension.[26] It has been suggested thatStyracosaurus albertensis is a direct descendant ofCentrosaurus (C. apertus orC. nasicornis), and that it in turn evolved directly into the slightly later speciesRubeosaurus ovatus. Subtle changes can be traced in the arrangement of the horns through this lineage, leading fromRubeosaurus toEiniosaurus, toAchelousaurus andPachyrhinosaurus. However, the lineage may not be a simple, straight line, as a pachyrhinosaur-like species has been reported from the same time and place asStyracosaurus albertensis.[2]
In 2020, during the description ofStellasaurus, Wilsonet al. foundStyracosaurus (includingS. ovatus) to be the earliest member of a single evolutionary lineage that eventually developed intoStellasaurus,Achelousaurus, andPachyrhinosaurus.[14]
Styracosaurus and other horned dinosaurs are often depicted in popular culture asherd animals. A bonebed composed ofStyracosaurus remains is known from theDinosaur Park Formation of Alberta, about halfway up the formation. This bonebed is associated with different types ofriver deposits.[8][34] The mass deaths 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.[35]
PaleontologistsGregory Paul and Per Christiansen proposed that large ceratopsians such asStyracosaurus were able to run faster than anelephant, based on possible ceratopsiantrackways which did not exhibit signs of sprawling forelimbs.[36]
Styracosaurs wereherbivorous dinosaurs; they probably fed mostly on low growth because of the position of the head. They may, however, have been able to knock down tallerplants with their horns,beak, and bulk.[7][37] Thejaws were tipped with a deep, narrow beak, believed to have been better at grasping and plucking than biting.[38]
Ceratopsid teeth, including those ofStyracosaurus, were arranged in groups called batteries. Older teeth on top were continually replaced by the teeth underneath them. Unlikehadrosaurids, which also had dental batteries, ceratopsid teeth sliced but did not grind.[7] Some scientists have suggested that ceratopsids likeStyracosaurus atepalms andcycads,[39] while others have suggestedferns.[40] Dodson has proposed that Late Cretaceous ceratopsians may have knocked downangiosperm trees and then sheared off leaves and twigs.[41]
The large nasal horns and frills ofStyracosaurus 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, paleontologistR. S. Lull proposed that the frills of ceratopsian dinosaurs acted as anchor points for their jaw muscles.[42] He later noted that forStyracosaurus, the spikes would have given it a formidable appearance.[43] In 1996, Dodson supported the idea of muscle attachments in part and created detailed diagrams of possible muscle attachments in the frills ofStyracosaurus andChasmosaurus, but did not subscribe to the idea that they completely filled in the fenestrae.[44] C. A. Forster, however, found no evidence of large muscle attachments on the frill bones.[28]
It was long believed that ceratopsians likeStyracosaurus 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 boneresorption, or unknown bone diseases, are suggested as causes.[45]
However, a newer study compared incidence rates of skull lesions inTriceratops andCentrosaurus and showed that these were consistent withTriceratops using its horns in combat and the frill being adapted as a protective structure, while lower pathology rates inCentrosaurus may indicate visual rather than physical use of cranial ornamentation, or a form of combat focused on the body rather than the head;[46] asCentrosaurus was more closely related toStyracosaurus and both genera had long nasal horns, the results for this genus would be more applicable forStyracosaurus. The researchers also concluded that the damage found on the specimens in the study was often too localized to be caused by bone disease.[47]
The large frill onStyracosaurus and related genera also may have helped to increase body area toregulate body temperature,[48] like the ears of the modernelephant. A similar theory has been proposed regarding the plates ofStegosaurus,[49] although this use alone would not account for the bizarre and extravagant variation seen in different members of theCeratopsidae.[7] This observation is highly suggestive of what is now believed to be the primary function, display.
The theory of frill use in sexual display was first proposed in 1961 by Davitashvili. This theory has gained increasing acceptance.[28][50] 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.[51]
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).[52]
A skull discovered in 2015 from aStyracosaurus 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 genusRubeosaurus may be synonymous withStyracosaurus as a result.[3]
Styracosaurus is known from the Dinosaur Park Formation, and was a member of a diverse and well-documentedfauna of prehistoric animals that included horned relatives such asCentrosaurus andChasmosaurus, duckbills such asProsaurolophus,Lambeosaurus,Gryposaurus,Corythosaurus, andParasaurolophus,ornithomimidsStruthiomimus,tyrannosauridsGorgosaurus, andDaspletosaurus, andarmoredEdmontonia andEuoplocephalus.[53]
The Dinosaur Park Formation is interpreted as a low-relief setting ofrivers andfloodplains that became moreswampy and influenced bymarine conditions over time as theWestern Interior Seawaytransgressed westward.[54] Theclimate was warmer than present-day Alberta, withoutfrost, but with wetter and drier seasons.Conifers were apparently the dominantcanopy plants, with anunderstory offerns,tree ferns, andangiosperms.[55]
In the Two Medicine Formation, dinosaurs that lived alongsideStyracosaurus ovatus included the basalornithopodOrodromeus,hadrosaurids (such asHypacrosaurus,Maiasaura, andProsaurolophus), thecentrosaurinesBrachyceratops andEiniosaurus, theleptoceratopsidCerasinops, theankylosaursEdmontonia andEuoplocephalus, thetyrannosauridDaspletosaurus (which appears to have been a specialist of preying on ceratopsians), as well as the smallertheropodsBambiraptor,Chirostenotes,Troodon, andAvisaurus.
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