Stegoceras

Source: Wikipedia, the free encyclopedia.

Stegoceras
Temporal range:
Ma
Two reconstructed S. validum skeletons based on specimen UALVP 2,
Royal Tyrrell Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Ornithischia
Clade: Pachycephalosauria
Family: Pachycephalosauridae
Genus: Stegoceras
Lambe, 1902
Type species
Stegoceras validum
Lambe, 1902
Species
  • S. validum Lambe, 1902
  • ?S. novomexicanum Jasinski & Sullivan, 2011
Synonyms
List
  • Stegoceras validus Lambe, 1902
  • Troodon validus Gilmore, 1924
  • Stegoceras browni Wall & Galton, 1979
  • Ornatotholus browni Galton & Sues, 1983
  • ?Gravitholus albertae Wall & Galton, 1979
  • ?Hanssuesia sternbergi Brown & Schlaikjer, 1943

Stegoceras is a

junior synonyms. Currently only S. validum and S. novomexicanum, named in 2011 from fossils found in New Mexico
, remain. The validity of the latter species has also been debated.

Stegoceras was a small,

tubercles (or round "outgrowths") and nodes (or "knobs"), many in rows, and the largest formed small horns on the shelf. The teeth were small and serrated. The skull is thought to have been flat in juvenile animals and to have grown into a dome with age. It had a rigid vertebral column
, and a stiffened tail. The pelvic region was broad, perhaps due to an extended gut.

Originally known only from skull domes, Stegoceras was one of the first known pachycephalosaurs, and the incompleteness of these initial remains led to many theories about the affinities of this group. A complete Stegoceras skull with associated parts of the skeleton was described in 1924, which shed more light on these animals. Pachycephalosaurs are today grouped with the horned

species recognition. S. validum is known from the Dinosaur Park Formation and the Oldman Formation, whereas S. novomexicanum is from the Fruitland and Kirtland Formation
.

History of discovery

lectotype
of S. validum, shown from the right and underside

The first known remains of Stegoceras were collected by Canadian

As no similar remains had been found in the area before, Lambe was unsure of what kind of dinosaur they were, and whether they represented one species or more; he suggested the domes were "prenasals" situated before the

stegosaurs (plated dinosaurs) as contenders.[5][6] Hatcher doubted whether the Stegoceras specimens belonged to the same species and whether they were dinosaurs at all, and suggested the domes consisted of the frontal, occipital, and parietal bones of the skull.[2] In 1918, Lambe referred another dome (CMN 138) to S. validus, and named a new species, S. brevis, based on specimen CMN 1423 (which he originally included in S. validus). By this time, he considered these animals as members of Stegosauria (then composed of both families of armoured dinosaurs, Stegosauridae and Ankylosauridae), in a new family he called Psalisauridae (named for the vaulted or dome-shaped skull roof).[7]

senior synonym
of Stegoceras

In 1924, the American palaeontologist

nodosaur, the skeleton to an ornithopod, and the supposed gastralia (belly ribs) to a fish. This claim was rebutted by Gilmore and Loris S. Russell in the 1930s.[10]

Gilmore's classification was supported by the American palaeontologists

Pachycephalosauridae after the second genus, as he found that name (meaning "thick head lizard") more descriptive. He also considered T. sternbergi and T. edmontonensis members of Stegoceras, found S. brevis valid, and named a new species, S. lambei, based on a specimen formerly referred to S. validus.[3][11] The split from Troodon was supported by Russell in 1948, who described a theropod dentary with teeth almost identical to those of T. formosus.[12]

pachycephalosaur
skull (left), and 1924 illustrations of same skull with labelled bones (right)

In 1953,

holotype specimen (specimen AMNH 5450 in the American Museum of Natural History) in Alberta.[15] In 1983, Galton and Hans-Dieter Sues moved S. browni to its own genus, Ornatotholus (ornatus is Latin for "adorned" and tholus for "dome"), and considered it the first known American member of a group of "flat-headed" pachycephalosaurs, previously known from Asia.[16] In a 1987 review of the pachycephalosaurs, Sues and Galton emended the specific name validus to validum, which has subsequently been used in the scientific literature. These authors synonymized S. brevis, S. sternbergi, and S. lambei with S. validum, found that S. bexelli differed from Stegoceras in several features, and considered it an indeterminate pachycephalosaur.[3][4] In 1998, Goodwin and colleagues considered Ornatotholus a juvenile S. validum, therefore a junior synonym.[17]

21st century developments

CT images
of juvenile, flat-headed AMNH 5450 (formerly considered the distinct genus "Ornatotholus"), with sections on the right

In 2000, Robert M. Sullivan referred S. edmontonensis and S. brevis to the genus Prenocephale (until then only known from the Mongolian species P. prenes), and found it more likely that S. bexelli belonged to Prenocephale than to Stegoceras, but considered it a nomen dubium (dubious name, without distinguishing characters) due to its incompleteness, and noted its holotype specimen appeared to be lost.[18] In 2003, Thomas E. Williamson and Thomas Carr considered Ornatotholus a nomen dubium, or perhaps a juvenile Stegoceras.[19] In a 2003 revision of Stegoceras, Sullivan agreed that Ornatotholus was a junior synonym of Stegoceras, moved S. lambei to the new genus Colepiocephale, and S. sternbergi to Hanssuesia. He stated that the genus Stegoceras had become a wastebasket taxon for small to medium-sized North American pachycephalosaurs until that point. By this time, dozens of specimens had been referred to S. validum, including many domes too incomplete to be identified as Stegoceras with certainty. UALVP 2 is still the most complete specimen of Stegoceras, upon which most scientific understanding of the genus is based.[4] S. brevis was moved to the new genus Foraminacephale in 2016 by Ryan K. Schott Schott and David C. Evans,[20] and S. bexelli to Sinocephale in 2021 by Evans and colleagues.[21] In 2023, Aaron D. Dyer and colleagues analysed sutures and individual elements in the skulls of the pachycephalosaurs Gravitholus and Hanssuesia, and found no significant distinction between them and Stegoceras validum. They considered both as junior synonyms, with Gravitholus representing the end-stage in the growth of Stegoceras.[22]

In 2002, Williamson and Carr described a dome (specimen NMMNH P-33983 in the New Mexico Museum of Natural History and Science) from the San Juan Basin, New Mexico, which they considered a juvenile pachycephalosaur of uncertain species (though perhaps Sphaerotholus goodwini). In 2006, Sullivan and Spencer G. Lucas considered it a juvenile S. validum, which would expand the range of the species considerably.[23][24] In 2011, Steven E. Jasinski and Sullivan considered the specimen an adult, and made it the holotype of the new species Stegoceras novomexicanum, with two other specimens (SMP VP-2555 and SMP VP-2790) as paratypes.[25] A 2011 phylogenetic analysis by Watabe and colleagues did not place the two Stegoceras species close to each other.[26]

In 2016, Williamson and

stratigraphic interval and geographic range.[28] Dyer and colleagues found that the S. novomexicanum holotype could be an immature Sphaerotholus, and therefore a junior synonym in 2023.[22]

Description

Size of S. validum compared to a human

Stegoceras is one of the most completely known North American pachycephalosaurs, and one of the few known from

postcranial remains; S. validum specimen UALVP 2 is the most complete Stegoceras individual known to date. Its length is estimated to have been about 2 to 2.5 metres (6.6 to 8.2 ft), comparable to the size of a goat.[29][30][31] The weight has been estimated to be about 10 to 40 kilograms (22 to 88 lb).[32] Stegoceras was small to medium in size compared to other pachycephalosaurs.[3] S. novomexicanum appears to have been smaller than S. validum, but it is disputed whether the known specimens (incomplete skulls) are adults or juveniles.[25][27]

Skull and dentition

CT images of skull UALVP 2 in multiple views, showing relative surface densities of bone (blue: low, yellow: high)

The skull of Stegoceras was roughly triangular in shape when viewed from the side, with a relatively short snout. The

tubercles (or round outgrowths), and a row of nodes (knobs) extended along its upper edges, ending in a pointed tubercle (or small horn) on each side at the back of the skull. An inner row of smaller tubercles ran parallel with the larger one. Except for the upper surface of the dome, much of the skull was ornamented with nodes, many arranged in rows.[3]

The large orbit was shaped like an imperfect ellipse (with the longest axis from front to back), and faced to the side and slightly forward. The

palpebral bones were fused and formed a thick ridge above the orbit. The relatively large jugal bone formed the lower margin of the orbit, extending far forwards and down towards the jaw joint. It was ornamented with ridges and nodes in a radiating arrangement.[3]

The nasal openings were large and faced frontwards. The

predentary bone is indicated by facets at the front of the lower jaw.[3] Like other pachycephalosaurs, it would have had a small beak.[33]

ornithischian
dinosaurs

Stegoceras had teeth that were

tooth crowns bore enamel, and both sides were divided vertically by a ridge. Each edge had about seven or eight denticles, with the front edge usually having the most.[3]

The skull of Stegoceras can be distinguished from those of other pachycephalosaurs by features such as its pronounced parietosquamosal shelf (though this became smaller with age), the "incipient" doming of its frontopariental (though the doming increased with age), its inflated nasal bones, its ornamentation of tubercles on the sides and back of the squamosal bones, rows of up to six tubercles on the upper side of each squamosal, and up to two nodes on the backwards projection of the parietal. It is also distinct in its lack of nasal ornamentation, and in having a reduced diastema.[4][34] The skull of S. novomexicanum can be distinguished from that of S. validum in features such as the backwards extension of the parietal bone being more reduced and triangular, having larger supratemporal fenestrae (though this may be due to the possible juvenile status of the specimens), and having roughly parallel suture contacts between the squamosal and parietal. It also appears to have had a smaller frontal boss than S. validum,[25][27] and seems to have been more gracile overall.[28]

Postcranial skeleton

Bones of UALVP 2 including those of the limbs, pelvis, vertebrae, ribs, and ossified tail tendons

The

teleost fish; the feature is unique to pachycephalosaurs among tetrapod (four-limbed) animals, and may have functioned in stiffening the tail.[9]

The

metatarsus of the foot appears to have been narrow, and the single known ungual (claw bone) of a toe was slender and slightly curved.[3] Though the limbs of Stegoceras are not completely known, they were most likely like other pachycephalosaurs in having five-fingered hands and four toes.[33]

Classification

Reconstructed skeleton showing ossified tail tendons in place, National Museum of Natural History

During the 1970s, more pachycephalosaur genera were described from Asian fossils, which provided more information about the group. In 1974, Maryańska and Osmólska concluded that pachycephalosaurs are distinct enough to warrant their own

suborder within Ornithischia, Pachycephalosauria. In 1978, the Chinese palaeontologist Dong Zhiming split Pachycephalosauria into two families; the dome-headed Pachycephalosauridae (including Stegoceras) and the flat-headed Homalocephalidae (originally spelled Homalocephaleridae).[36] Wall and Galton did not find suborder status for the pachycephalosaurs justified in 1979.[15] By the 1980s, the affinities of the pachycephalosaurs within Ornithischia were unresolved. The main competing views were that the group was closest to either ornithopods or ceratopsians, the latter view due to similarities between the skeleton of Stegoceras and the "primitive" ceratopsian Protoceratops. In 1986, American palaeontologist Paul Sereno supported the relationship between pachycephalosaurs and ceratopsians, and united them in the group Marginocephalia, based on similar cranial features, such as the "shelf"-structure above the occiput. He conceded that the evidence for this grouping was not overwhelming, but the validity of the group was supported by Sues and Galton in 1987.[3]

By the early 21st century, few pachycephalosaur genera were known from postcranial remains, and many taxa were only known from domes, which made classification within the group difficult. Pachycephalosaurs are thus mainly defined by cranial features, such as the flat to domed frontoparietal, the broad and flattened bar along the postorbital and squamosal bones, and the squamosal bones being deep plates on the occiput.

dimorphic trait. In 2006, Sullivan argued against the idea that the extent of doming was useful in determining taxonomic affinities between pachycephalosaurs.[31] In 2003, Sullivan found Stegoceras itself to be more basal (or "primitive") than the "fully-domed" members of the subfamily Pachycephalosaurinae, elaborating on conclusions reached by Sereno in 1986.[4]

A 2013

phylogenetic analysis by Evans and colleagues found that some flat-headed pachycephalosaur genera were more closely related to "fully" domed taxa than to the "incompletely" domed Stegoceras, which suggests they represent juveniles of domed taxa, and that flat heads do not indicate taxonomic affinities.[38] The cladogram below shows the placement of Stegoceras within Pachycephalosauridae according to Schott and colleagues, 2016:[20]

Partial skull of a juvenile, flat-headed S. validum, UCMZ(VP) 2008.001
Pachycephalosauria

Wannanosaurus yansiensis

Pachycephalosauridae

Hanssuesia sternbergi

Colepiocephale lambei

Stegoceras validum

Stegoceras novomexicanum

Pachycephalosaurinae

Goyocephale lattimorei

Homalocephale calathocercos

Tylocephale gilmorei

Foraminacephale brevis

Amtocephale gobienses

Prenocephale prenes

Acrotholus audeti

Pachycephalosaurus wyomingensis

Alaskacephale gangloffi

Dracorex hogwartsia

Stygimoloch spinifer

Sphaerotholus goodwini

Sphaerotholus buchholtzae

The

taphonomic bias; smaller animals are less likely to be preserved through fossilisation. More delicate bones are also less likely to be preserved, which is why pachycephalosaurs are mainly known from their robust skulls.[4][38]

Palaeobiology

Feeding mechanics

Mandible and teeth of UALVP 2

It is uncertain what pachycephalosaurs ate; having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that their sharp, serrated teeth were ideally suited for a mixed diet of leaves, seeds, fruit and insects.

replacement teeth, and tooth replacement happened in backwards progression in sequential threes. The occipital region of Stegoceras was well-demarcated for muscle-attachment and it is believed that the jaw movement of Stegoceras and other pachycephalosaurs was mostly limited to up-and-down motions with only a slight capability for jaw rotation. This is based on the structure of the jaw and dental microwear and wear facets of the teeth indicate that the bite-force was used more for shearing than for crushing.[3][40]

In 2021, the Canadian palaeontologist Michael N. Hudgins and colleagues examined the teeth of Stegoceras and

ecomorphospaces (though Stegoceras and Thescelosaurus themselves were not contemporaries).[41]

Nasal passages

In 1989, Emily B. Griffin found that Stegoceras and other pachycephalosaurs had a good sense of smell (olfaction), based on the study of cranial

cartilaginous and therefore not preserved through mineralization.[43]

Ontogenetic changes

Growth series of S. validum skulls, according to Schott et al., 2011

Several explanations have historically been proposed for the variation seen in the skulls of Stegoceras and other pachycephalosaurs. Brown and Schlaikjer suggested that there was

histological study of an S. validum skull that showed the dome consisted of vascular, fast-growing bone, consistent with an increase in doming through age. These authors found that the supposedly distinct features of Ornatotholus could easily be the results of ontogeny.[34][17]

CT images of TMP 84.5.1, a juvenile or subadult S. validum, with sections

In 2003, Williamson and Carr published a hypothetical growth series of S. validum, showing Ornatotholus as the juvenile stage. They suggested that juveniles were characterized by a flat, thickened frontoparietal roof, with larger supratemporal fenestrae, and studded with closely spaced tubercles and nodes. The parietosquamosal shelf was not reduced in size, and the frontoparietal suture was open. Sub-adults had mound-like domes, with the back part of the parietal and skull-roof being flat. The supratemporal fenestrae showed asymmetry in size, and the closure of the frontoparietal suture was variable. The nodes were stretched or almost obliterated as the dome expanded during growth, with a tesserated surface remaining. The pattern was often obliterated at the highest point (apex) of the dome, the area where maximum expansion occurred. The tubercles on the skull were stretched in different directions, and those at the margin of the parietosquamosal shelf may have been hypertrophied (enlarged) tubercles. The back and sides of sub-adult and adult skulls were ornamented by less modified tubercles. Before being incorporated into the enlarging dome, the skull bones expanded, resulting in junctions between these bones. The adult dome was broad and convex, and incorporated most of the shelf, which was reduced in size and overhung the occiput as a thick "lip". The supratempooral fenestrae were closed, but the suture between the frontoparietal and connected skull bones was not always closed in adults and subadults.[19]

In 2011, Schott and colleagues made a more comprehensive analysis of cranial dome ontogeny in S. validum. The study found that the parietosquamosal shelf conserved the arrangement of ornamentation throughout growth, and that vascularity of the frontoparietal domes decreased with size. It also found that dome shape and size was strongly correlated with growth, and that growth was

allometric (in contrast to isometric) from flat to domed, supporting Ornatotholus as a juvenile Stegoceras. They also hypothesized that this model of dome growth, with dramatic changes from juvenile to adult, was the common developmental trajectory of pachycephalosaurs. These researchers noted that though Williamson and Carr's observation that the supratemporal fenestrae closed with age was generally correct, there was still a high degree of individual variation in the size of these fenestrae, regardless of the size of the frontoparietal, and this feature may therefore have been independent of ontogeny.[34]

A 2012 study by Schott and Evans found that the number and shape of the individual nodes on the squamosal shelf of the examined S. validum skulls varied considerably, and that this variability does not seem to correlate with ontogenic changes, but was due to individual variation. These researchers found no correlation between the width of supratemporal fenestrae and the size of the squamosal.[45]

Dome function

The function of pachycephalosaur domes has been debated, and Stegoceras has been used as a model for experimentation in various studies. The dome has mainly been interpreted as a weapon used in

species recognition.[46][47]

Combat

Apex of S. validum and Prenocephale skulls (left, arrows), with possible combat orientations (right), according to Peterson et al., 2013

The hypothesis that the domed skulls of Stegoceras and other pachycephalosaurs were used for butting heads was first suggested by American palaeontologist

plexi-glass models. The impact would be absorbed through the neck and body, and neck ligaments and muscles would prevent injuries by glancing blows (as in modern bighorn sheep). Sues also suggested that the animals could have butted each other's flanks.[35][48][49]

In 1997, the American palaeontologist Kenneth Carpenter pointed out that the dorsal vertebrae from the back of the pachycephalosaur Homalocephale show that the back curved downwards just before the neck (which was not preserved), and unless the neck curved upwards, the head would point to the ground. He therefore inferred that the necks of Stegoceras and other pachycephalosaurs were held in a curved posture (as is the norm in dinosaurs), and that they would therefore not have been able to align their head, neck, and body horizontally straight, which would be needed to transmit stress. Their necks would have to be held below the level of the back, which would have risked damaging the spinal cord on impact. Modern bighorn sheep and bison overcome this problem by having strong ligaments from the neck to the tall neural spines over the shoulders (which absorb the force of impact), but such features are not known in pachycephalosaurs. These animals also absorb the force of impact through sinus chambers at the base of their horns, and their foreheads and horns form a broad contact surface, unlike the narrow surface of pachycephalosaur domes. Because the dome of Stegoceras was rounded, it would have given a very small area for potential impact, and the domes would have glanced off each other (unless the impact was perfectly centred). Combating pachycephalosaurs would have had difficulty seeing each other while their heads were lowered, due to the bony ridges above the eyes.[35]

CT images showing the internal density of bone in UALVP 2 (blue: low, red: high)
CT cross-sections of the skulls of UALVP 2 (left) and a white-bellied duiker (right)

Because of the problems he found with the head-butting hypothesis, Carpenter instead suggested the domes were adaptations for flank-butting (as seen in some large African mammals); he imagined that two animals would stand parallel, facing each other or the same direction, and direct blows to the side of the opponent. The relatively large body width of pachycephalosaurs may consequently have served to protect vital organs from harm during flank-butting. It is possible that Stegoceras and similar pachycephalosaurs would have delivered the blows with a movement of the neck from the side and a rotation of the head. The upper sides of the dome have the greatest surface area, and may have been the point of impact. The thickness of the dome would have increased the power behind a blow to the sides, and this would ensure that the opponent felt the force of the impact, without being seriously injured. The bone rim above the orbit may have protected the aggressor's eye when making a blow. Carpenter suggested that the pachycephalosaurs would have first engaged in

threat display by bobbing and presenting their heads to show the size of their domes (intimidation), and thereafter delivered blows to each other, until one opponent signalled submission.[35]

In 2008, Eric Snively and Adam Cox tested the performance of 2D and 3D pachycephalosaur skulls through

pneumatic sinuses that are found below the point of impact in the skulls of head-striking artiodactyls, it instead had vascular struts which could have similarly acted as braces, as well as conduits to feed the development of a keratin covering.[51]

Two S. validum domes with lesions (arrows, left) shown from above, and distribution of observed pachycephalosaurid skull lesions shown on a S. validum skull model (right)

In 2012, Caleb M. Brown and Anthony P. Russell suggested that the stiffened tails were probably not used as defence against flank-butting, but may have enabled the animals to take a tripodal stance during intra-specific combat, with the tail as support. Brown and Russell found that the tail could thereby help in resisting compressive, tensile, and torsional loading when the animal delivered or received blows with the dome.[9] A 2013 study by Joseph E. Peterson and colleagues identified lesions in skulls of Stegoceras and other pachycephalosaurs, which were interpreted as infections caused by trauma. Lesions were found on 22% of sampled pachycephalosaur skulls (a frequency consistent across genera), but were absent from flat-headed specimens (which have been interpreted as juveniles or females), which is consistent with use in intra-specific combat (for territory or mates). The distribution of lesions in these animals tended to concentrate at the top of the dome, which supports head-butting behaviour. Flank-butting would probably result in fewer injuries, which would instead be concentrated on the sides of the dome. These observations were made while comparing the lesions with those on the skulls and flanks of modern sheep skeletons. The researchers noted that modern head-butting animals use their weapons for both combat and display, and that pachycephalosaurs could therefore also have used their domes for both. Displaying a weapon and willingness to use it can be enough to settle disputes in some animals.[46]

Pelvic, hind limb, pectoral, and brachial musculature reconstructions of S. validum

Bryan R. S. Moore and colleagues examined and reconstructed the limb musculature of Stegoceras in 3D in 2022, using the very complete UALVP 2 specimen as basis. They found that the musculature of the forelimbs was conservative, particularly compared to those of early bipedal

positive allometry, and may have been used in combat, these researchers suggested it may have been the case for the hindlimb muscles as well, if they were used to propel the body forwards during head-butting. They cautioned that while UALVP 2 is very complete for a pachycephalosaur, their study was limited by it missing large portions of its vertebral column and outer limb elements.[52]

Other suggested functions

Restoration of a pair with different coloured domes

In 1987, J. Keith Rigby and colleagues suggested that pachycephalosaur domes were

bovids) and that use in sexual display was only secondary. They further speculated that the external covering of the domes was brightly coloured in life, or may have changed colour seasonally.[47]

In 2011, American palaeontologists

John R. Horner proposed that "bizarre structures" in dinosaurs in general (including domes, frills, horns, and crests) were primarily used for species recognition, and dismissed other explanations as unsupported by evidence. Among other studies, these authors cited Goodwin et al.'s 2004 paper on pachycephalosaur domes as support of this idea, and they pointed out that such structures did not appear to be sexually dimorphic.[53] In a response to Padian and Horner the same year, Rob J. Knell and Scott D. Sampson argued that species recognition was not unlikely as a secondary function for "bizarre structures" in dinosaurs, but that sexual selection (used in display or combat to compete for mates) was a more likely explanation, due to the high cost of developing them, and because such structures appear to be highly variable within species.[54] In 2013, the British palaeontologists David E. Hone and Darren Naish criticized the "species recognition hypothesis", and argued that no extant animals use such structures primarily for species recognition, and that Padian and Horner had ignored the possibility of mutual sexual selection (where both sexes are ornamented).[55]

In 2012, Schott and Evans suggested that the regularity in squamosal ornamentation throughout the ontogeny of Stegoceras was consistent with species recognition, but the change from flat to domed frontoparietals in late age suggests that the function of this feature changed through ontogeny, and was perhaps sexually selected, possibly for intra-specific combat.[45] Dyer and colleagues found in 2023 that Stegoceras specimens differed in the thickness of the frontonasal boss, and that skulls with the most bone pathologies were those with the tallest bosses, which they considered indication that variation in boss thickness represents intersexual variation.[22]

Palaeoenvironment

The Dinosaur Park Formation in Alberta, Canada, where S. validum was first discovered

S. validum is known from the late Late Cretaceous Belly River Group (the Canadian equivalent to the Judith River Group in the US), and specimens have been recovered from the Dinosaur Park Formation (late Campanian, 76.5 to 75 mya) in Dinosaur Provincial Park (including the lectotype specimen), and the Oldman Formation (middle Campanian, 77.5 to 76.5 mya) of Alberta, Canada. The pachycephalosaurs Hanssuesia (if not a synonym of Stegoceras) and Foraminacephale are also known from both formations.[4][22] S. novomexicanum is known from the Fruitland (late Campanian, about 75 mya) and lower Kirtland Formation (late Campanian, about 74 mya) of New Mexico, and if this species correctly belongs in Stegoceras, the genus would have had a broad geographic distribution.[25] The presence of similar pachycephalosaurs in both the west and north of North America during the latest Cretaceous shows that they were an important part of the dinosaur faunas there.[27]

It has traditionally been suggested that pachycehalosaurs inhabited mountain environments; wear of their skulls was supposedly a result of them having been rolled by water from upland areas, and comparisons with bighorn sheep reinforced the theory. In 2014, Jordan C. Mallon and Evans disputed this idea, as the wear and original locations of the skulls is not consistent with having been transported in such a way, and they instead proposed that North American pachycephalosaurs inhabited

alluvial (associated with water) and coastal plain environments.[56]

The Dinosaur Park Formation is interpreted as a low-relief setting of

dromaeosaurid Saurornitholestes and possibly an albertosaurine tyrannosaur.[60]

See also

References

  1. ^ a b Lambe, L. M. (1902). "New genera and species from the Belly River Series (mid-Cretaceous)". Geological Survey of Canada, Contributions to Canadian Palaeontology. 3: 68.
  2. ^ .
  3. ^ a b c d e f g h i j k l Sues, H. D. & Galton, P. M. (1987). "Anatomy and classification of the North American Pachycephalosauria (Dinosauria: Ornithischia)". Palaeontographica Abteilung A. 198: 1–40.
  4. ^
    S2CID 85894105
    .
  5. ^ Nopcsa, F. (1903). "Über Stegoceras und Stereocephalus". Centralblatt für Mineralogie, Geologie und Paläontologie. 1903: 266–267.
  6. PMID 17746863
    .
  7. ^ Lambe, L. M. (1918). "The Cretaceous genus Stegoceras typifying a new family referred provisionally to the Stegosauria". Transactions of the Royal Society of Canada. 12 (4): 23–36.
  8. ^ Gilmore, C. W. (1924). "On Troodon validus, an orthopodous dinosaur from the Belly River Cretaceous of Alberta, Canada". Department of Geology, University of Alberta Bulletin. 1: 1–43.
  9. ^
    PMID 22272307
    .
  10. ^ .
  11. .
  12. .
  13. ^ Bohlin, B., 1953. Fossil reptiles from Mongolia and Kansu. Reports from the Scientific Expedition to the North-western Provinces of China under Leadership of Dr. Sven Hedin. VI. Vertebrate Palaeontology 6. The Sino-Swedish Expedition Publication 37:1–113
  14. ^ Kuhn, O., 1964, Fossilium Catalogus I: Animalia Pars 105. Ornithischia (Supplementum I), IJsel Pers, Deventer, 80 pp
  15. ^ .
  16. .
  17. ^ .
  18. ^ Sullivan, Robert M. (2000). "Prenocephale edmontonensis (Brown and Schlaikjer) new comb. and P. brevis (Lambe) new comb. (Dinosauria: Ornithischia: Pachycephalosauria) from the Upper Cretaceous of North America". New Mexico Museum of Natural History and Science Bulletin. 17: 177–90.
  19. ^
    S2CID 86112901
    .
  20. ^ .
  21. .
  22. ^ .
  23. ^ Sullivan, R. M.; Lucas, S. G. (2006). "The pachycephalosaurid dinosaur Stegoceras validum from the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico". New Mexico Museum of Natural History and Science Bulletin. 35: 329–330.
  24. ^ Williamson, T. E.; Carr, T. D. (2002). "A juvenile pachycephalosaur (Dinosauria: Pachycephalosauridae) from the Fruitland Formation". New Mexico: New Mexico Geology. 24: 67–68.
  25. ^ a b c d Jasinski, S. E.; Sullivan, R. M. (2011). "Re-evaluation of pachycephalosaurids from the Fruitland-Kirtland transition (Kirtlandian, late Campanian), San Juan Basin, New Mexico, with a description of a new species of Stegoceras and a reassessment of Texascephale langstoni" (PDF). Fossil Record 3. New Mexico Museum of Natural History and Science, Bulletin. 53: 202–215.
  26. ^ Watabe, M.; Tsogtbaatar, K.; Sullivan, R. M. (2011). "A new pachycephalosaurid from the Baynshire Formation (Cenomanian-late Santonian), Gobi Desert, Mongolia" (PDF). Fossil Record 3. New Mexico Museum of Natural History and Science, Bulletin. 53: 489–497.
  27. ^ .
  28. ^ a b Jasinski, S. E.; Sullivan, R. M. (2016). "The validity of the Late Cretaceous pachycephalosaurid Stegoceras novomexicanum (Dinosauria: Pachycephalosauridae)". In Sullivan, Robert M.; Lucas, Spencer G. (eds.). Fossil Record 5: Bulletin 74. New Mexico Museum of Natural History and Science. pp. 107–116.
  29. .
  30. .
  31. ^ .
  32. .
  33. ^ .
  34. ^ .
  35. ^ a b c d Carpenter, Kenneth (1 December 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia, Dinosauria); a new look at head-butting behavior". Rocky Mountain Geology. 32 (1): 19–25.
  36. ^ Perle, A.; Osmólska, H. (1982). "Goyocephale lattimorei gen. et sp. n., a new flat-headed pachycephalosaur (Ornlthlschia, Dinosauria) from the Upper Cretaceous of Mongolia – Acta Palaeontologica Polonica". Acta Palaeontologica Polonica. 27: 115–127.
  37. ^ Sereno, P. C., 2000. The fossil record, systematics and evolution of pachycephalosaurs and ceratopsians from Asia. 480–516 in Benton, M.J., M.A. Shishkin, D.M. Unwin & E.N. Kurochkin (eds.), The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press, Cambridge.
  38. ^
    PMID 23652016
    .
  39. .
  40. .
  41. .
  42. .
  43. .
  44. .
  45. ^ .
  46. ^ .
  47. ^ .
  48. .
  49. ^ Sues, H. D. (1978). "Functional morphology of the dome in pachycephalosaurid dinosaurs". Neues Jahrbuch für Geologie und Paläontologie - Monatshefte. 8: 459–472.
  50. ^ Snively, E.; Cox, A. (2008). "Structural Mechanics of Pachycephalosaur Crania Permitted Head-butting Behavior". Palaeontologia Electronica. 11: 1–17.
  51. PMID 21738658
    .
  52. .
  53. .
  54. .
  55. .
  56. .
  57. .
  58. .
  59. .
  60. .

External links