Stegoceras
Encyclopedia
Stegoceras is a genus
of plant-eating pachycephalosaurid
dinosaur
that lived in what is now North America
during the Late Cretaceous
period.
. The teeth are small and curved with serrated edges. The head was supported by an "S"- or "U"-shaped neck. When a partial skeleton of Stegoceras was first discovered, it was thought to have gastralia
, or belly ribs, not typically found in other ornithischia
n dinosaurs. They were subsequently found to be ossified tendon
s. The legs were more than three times the length of the arms.
in 1902
and the type species
is Stegoceras validum. All specimens of S. validum (about 40) were recovered from the Belly River Group
of Alberta, Canada
, with the majority of specimens from the Dinosaur Park Formation
(late Campanian
, 76.5–75 ma) in the Dinosaur Provincial Park
, while the rest of the specimens were recovered from the Oldman Formation
(middle Campanian, 77.5-76.5 ma). It has served as a model for other pachycephalosaurs, due to the completeness of the excavated remains. From the 1920s to 1945, it was believed to be related to, or even synonymous with, Troodon
, due to the similar form of the teeth. This proposal was, however, dispelled after better remains were found. Many species of pachycephalosaurs were described initially as Stegoceras species. S. lambei, S. sternbergi and S. breve, for example, were assigned to new genera, Colepiocephale
, Hanssuesia
and the nomen nudum
Foraminacephale (or Prenocephale
/Sphaerotholus
in previous studies).
Recently, a new species of Stegoceras from San Juan Basin
, New Mexico
have been named. Its specimens were referred to S. validum by Sullivan and Lucas, 2006. Stegoceras novomexicanum was first named by Steven E. Jasinski and Robert M. Sullivan in 2011
on the basis of two partial pachycephalosaurid skull
s, from the upper Fruitland
and lower Kirtland Formation
s. It is known from the holotype
NMMNH
P-33898, nearly complete frontoparietal which was collected from the Fossil Forest Member of the upper Fruitland Formation
(late Campanian
, about 75 ma) and from the paratype
s SMP
VP-2555, partial left frontal and anterior-most portions of left and right frontals (also from the Fossil Forest Member) and SMP VP-2790, incomplete parietal from the Hunter Wash Member of the lower Kirtland Formation
(late Campanian
, about 74 ma). A new phylogenetic analysis
performed by Mahito Watabe, Khishigjaw Tsogtbaatar and Robert M. Sullivan in 2011 fails to recover the monophyly
of the genus Stegoceras.
5450, an incipiently domed frontoparietal, was the first known North America
n flat-headed pachycephalosaurid specimen. It was the only known North American pachycephalosaur specimen with a flat skull roof until the discovery of Dracorex
in South Dakota
. Brown and Schlaikjer, 1943 and Galton, 1971 suggested that there was sexual variation in the degree of doming and hypothesized AMNH 5450 represented a female morph of S. validum. Wall and Galton, 1979 transferred AMNH 5450 to a new species, S. browni and later, Galton and Sues, 1983 erected a new genus for S. browni, Ornatotholus. AMNH 5450 was collected from the Dinosaur Park Formation
of southern Alberta. Goodwin et al., 1998 argued that O. browni might represent a juvenile ontogenetic stage of Stegoceras and Williamson and Carr, 2002 suggested that Ornatotholus shares many similarities with Stegoceras and may represent a juvenile form, making it a nomen dubium
. In June 2011, an article was published in the journal PLoS ONE
by Ryan Schott and others reanalyzing the cranial dome ontogeny of several specimens assigned to Stegoceras. Their results indicated that Ornatotholus is a juvenile Stegoceras. To date, the flat-headed Ornatotholus browni (holotype AMNH
5450) is widely considered a juvenile Stegoceras validum (by Sullivan 2003, Sullivan and Lucas 2006, Longrich et al. 2010, Jasinski and Sullivan 2011 and Schott et al. 2011).
or musk oxen. This theory has been challenged. Paleontologists such as Carpetner (1997) have contended that the rounded dome of Stegoceras would have given a very small area for potential impact. Thus head-butting animals will have made glancing blow, unless the impact was prefectly centered. Another problem noted is that Stegoceras and other dome-headed pachycephalosaurs would not have been able to align their head, neck, and body in a perfect horizontal line (which would be needed to transmit stress) -- it was more likely that they carried their neck in an "S"- or "U"-shaped curve (Stegoceras seemed to carry their spine in a less extreme curve, due to their thick neck muscles). Carpenter suggests flank-butting as an alternative and contends that the thickness of the dome would have used for increasing the powerful behind a blow to the sides. It is possible that Stegoceras and similar pachycephalosaurs would have delivered the blows with a movement of the neck from the side and a rotaton of the head. The dorsolateral area of the dome has the greatest surface area and may have been the point of impact. This would insure that the force of the impact would not cause serious injury to the opponent. The bone rim above the orbit may have protected the aggressor's eye when making a blow. Also, the relatively large width of Stegoceras may have served to protect vital organs from harm during flank-butting. Snively and Theodor (2011) conducted CT scans of the skulls of Stegoceras validum, Prenocephale prenes and several head-striking artiodactyls. They found that the skull of Stegoceras was well-equipped for head-butting. It appears that Stegoceras had extra layer of dense bone in the middle of its dome, which may have provided extra protection for the brain.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of plant-eating pachycephalosaurid
Pachycephalosauria
Pachycephalosauria is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls...
dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
that lived in what is now North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
during the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period.
Description
Stegoceras had an estimated length of up to 2 metres (6.6 ft) and weighed 120-150 lbs. It had a relatively large brain that was encased in a dome of 3 inches (7.6 cm) of thick bone divided into two parts. The dome had a fairly smooth surface, but was irregularly pitted by foramina which gives an entrance to channels within the bone. Stegoceras (at least S. validum) can be disingushed by having a prominent parietosquamosal shelp with open supratemporal fosse, incipient doming of the frontopariental and minute node in clusters on postorbital and squamosals. Stegoceras had rounded eye sockets that faced forward, which suggests had good vision and was capable of binocular visionBinocular vision
Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...
. The teeth are small and curved with serrated edges. The head was supported by an "S"- or "U"-shaped neck. When a partial skeleton of Stegoceras was first discovered, it was thought to have gastralia
Gastralium
Gastralia are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae...
, or belly ribs, not typically found in other ornithischia
Ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...
n dinosaurs. They were subsequently found to be ossified tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
s. The legs were more than three times the length of the arms.
Discovery and classification
Stegoceras was first named by Lawrence LambeLawrence Lambe
Lawrence Morris Lambe was a Canadian geologist and palaeontologist from the Geological Survey of Canada .His published work, describing the diverse and plentiful dinosaur discoveries from the fossil beds in Alberta, did much to bring dinosaurs into the public eye and helped usher in the Golden...
in 1902
1902 in paleontology
-Crurotarsi:-Newly named dinosaurs:Data courtesy of George Olshevsky's dinosaur genera list and Dr. Jeremy Montague's dinosaur genus database.-Prolacertiformes:...
and the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is Stegoceras validum. All specimens of S. validum (about 40) were recovered from the Belly River Group
Belly River Group
The Belly River Formation is a stratigraphical unit of Late Cretaceous age in the Western Canadian Sedimentary Basin.It takes the name from the Belly River, a tributary of the Oldman River in southern Alberta, and was first described in outcrop on the banks of the Oldman River and Bow River by...
of Alberta, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, with the majority of specimens from the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
(late Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
, 76.5–75 ma) in the Dinosaur Provincial Park
Dinosaur Provincial Park
Dinosaur Provincial Park is a UNESCO World Heritage Site located about two and a half hours drive southeast of Calgary, Alberta, Canada or , about a half hour drive, northeast of Brooks....
, while the rest of the specimens were recovered from the Oldman Formation
Oldman Formation
The Oldman Formation is the middle member of the Judith River Group, a major geologic unit in southern Alberta. The formation is widely recognized as bearing a great number of well preserved dinosaur skeletons, as well as other fossils.-Age:...
(middle Campanian, 77.5-76.5 ma). It has served as a model for other pachycephalosaurs, due to the completeness of the excavated remains. From the 1920s to 1945, it was believed to be related to, or even synonymous with, Troodon
Troodon
Troodon is a genus of relatively small, bird-like dinosaur from the Late Cretaceous period . Discovered in 1855, it was among the first dinosaurs found in North America...
, due to the similar form of the teeth. This proposal was, however, dispelled after better remains were found. Many species of pachycephalosaurs were described initially as Stegoceras species. S. lambei, S. sternbergi and S. breve, for example, were assigned to new genera, Colepiocephale
Colepiocephale
Colepiocephale is a genus of pachycephalosaurid dinosaur from Late Cretaceous deposits of Alberta, Canada. It was collected from the Foremost Formation . The type species, C. lambei, was originally described by Sternberg , and later renamed by Sullivan in 2003. C...
, Hanssuesia
Hanssuesia
Hanssuesia is a genus of pachycephalosaurid dinosaur from the late Cretaceous period. It lived in what is now Alberta and Montana.Hanssuesia based on material originally named Troodon sternbergi by Barnum Brown and Erich Maren Schlaikjer in 1943. Later, it was transferred to Stegoceras as S....
and the nomen nudum
Nomen nudum
The phrase nomen nudum is a Latin term, meaning "naked name", used in taxonomy...
Foraminacephale (or Prenocephale
Prenocephale
Prenocephale was a small pachycephalosaurid dinosaur genus from the Late Cretaceous and was similar in many ways to its close relative, Homalocephale, which may simply represent Prenocephale juveniles. Adult Prenocephale probably weighed around and measured around long...
/Sphaerotholus
Sphaerotholus
Sphaerotholus is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States. To date, two species have been described: the type species, S. goodwini, from the Den-na-zin Member of the Kirtland Formation of San Juan County, New Mexico, and a second species, S...
in previous studies).
Recently, a new species of Stegoceras from San Juan Basin
San Juan Basin
The San Juan Basin is a geologic structural basin in the Four Corners region of the Southwestern United States; its main portion covers around , encompassing much of northwestern New Mexico, southwest Colorado, and parts of Arizona and Utah....
, New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
have been named. Its specimens were referred to S. validum by Sullivan and Lucas, 2006. Stegoceras novomexicanum was first named by Steven E. Jasinski and Robert M. Sullivan in 2011
2011 in paleontology
-Lobopods:-Arachnids:-Insects:-Newly named jawless vertebrates:-Newly named acanthodian:-Newly named cartilaginous fishes:-Newly named bony fishes:-Newly named lepospondyls:-Newly named temnospondyls:-Newly named lissamphibians:...
on the basis of two partial pachycephalosaurid skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
s, from the upper Fruitland
Fruitland Formation
The Fruitland Formation is a sedimentary geological formation containing layers of sandstone, shale, and coal. It was laid down in marshy delta conditions, with poor drainage and frequent flooding, under a warm, humid and seasonal climate...
and lower Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...
s. It is known from the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
NMMNH
New Mexico Museum of Natural History and Science
The New Mexico Museum of Natural History and Science is a natural history and science museum in Albuquerque, New Mexico near Old Town Albuquerque. The Museum was founded in 1986....
P-33898, nearly complete frontoparietal which was collected from the Fossil Forest Member of the upper Fruitland Formation
Fruitland Formation
The Fruitland Formation is a sedimentary geological formation containing layers of sandstone, shale, and coal. It was laid down in marshy delta conditions, with poor drainage and frequent flooding, under a warm, humid and seasonal climate...
(late Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
, about 75 ma) and from the paratype
Paratype
Paratype is a technical term used in the scientific naming of species and other taxa of organisms. The exact meaning of the term paratype when it is used in zoology is not the same as the meaning when it is used in botany...
s SMP
State Museum of Pennsylvania
The State Museum of Pennsylvania is a non-profit museum at 300 North Street in downtown Harrisburg, run by the Commonwealth of Pennsylvania through the Pennsylvania Historical and Museum Commission to preserve and interpret the region's history and culture. It is a part of the Pennsylvania State...
VP-2555, partial left frontal and anterior-most portions of left and right frontals (also from the Fossil Forest Member) and SMP VP-2790, incomplete parietal from the Hunter Wash Member of the lower Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...
(late Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
, about 74 ma). A new phylogenetic analysis
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
performed by Mahito Watabe, Khishigjaw Tsogtbaatar and Robert M. Sullivan in 2011 fails to recover the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of the genus Stegoceras.
Paleobiology
Stegoceras validum likely had a broad geographic distribution, due to its occurrence in Alberta and New Mexico.Dimorphism
One specimen, AMNHAmerican Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
5450, an incipiently domed frontoparietal, was the first known North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n flat-headed pachycephalosaurid specimen. It was the only known North American pachycephalosaur specimen with a flat skull roof until the discovery of Dracorex
Dracorex
Dracorex is a dinosaur genus of the family Pachycephalosauridae, from the Late Cretaceous of North America.The type species is Dracorex hogwartsia, meaning "dragon king of Hogwarts". It is known from one nearly complete skull , as well as four cervical vertebrae including the atlas, third, ninth...
in South Dakota
South Dakota
South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux American Indian tribes. Once a part of Dakota Territory, South Dakota became a state on November 2, 1889. The state has an area of and an estimated population of just over...
. Brown and Schlaikjer, 1943 and Galton, 1971 suggested that there was sexual variation in the degree of doming and hypothesized AMNH 5450 represented a female morph of S. validum. Wall and Galton, 1979 transferred AMNH 5450 to a new species, S. browni and later, Galton and Sues, 1983 erected a new genus for S. browni, Ornatotholus. AMNH 5450 was collected from the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
of southern Alberta. Goodwin et al., 1998 argued that O. browni might represent a juvenile ontogenetic stage of Stegoceras and Williamson and Carr, 2002 suggested that Ornatotholus shares many similarities with Stegoceras and may represent a juvenile form, making it a nomen dubium
Nomen dubium
In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
. In June 2011, an article was published in the journal PLoS ONE
PLoS ONE
PLoS ONE is an open access peer-reviewed scientific journal published by the Public Library of Science since 2006. It covers primary research from any discipline within science and medicine. All submissions go through an internal and external pre-publication peer review but are not excluded on the...
by Ryan Schott and others reanalyzing the cranial dome ontogeny of several specimens assigned to Stegoceras. Their results indicated that Ornatotholus is a juvenile Stegoceras. To date, the flat-headed Ornatotholus browni (holotype AMNH
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
5450) is widely considered a juvenile Stegoceras validum (by Sullivan 2003, Sullivan and Lucas 2006, Longrich et al. 2010, Jasinski and Sullivan 2011 and Schott et al. 2011).
Agonistic behavior
It was initially proposed that male Stegoceras (and individuals of other pachycephalosaurid species) would ram each other headlong, not unlike contemporary bighorn sheepBighorn Sheep
The bighorn sheep is a species of sheep in North America named for its large horns. These horns can weigh up to , while the sheep themselves weigh up to . Recent genetic testing indicates that there are three distinct subspecies of Ovis canadensis, one of which is endangered: Ovis canadensis sierrae...
or musk oxen. This theory has been challenged. Paleontologists such as Carpetner (1997) have contended that the rounded dome of Stegoceras would have given a very small area for potential impact. Thus head-butting animals will have made glancing blow, unless the impact was prefectly centered. Another problem noted is that Stegoceras and other dome-headed pachycephalosaurs would not have been able to align their head, neck, and body in a perfect horizontal line (which would be needed to transmit stress) -- it was more likely that they carried their neck in an "S"- or "U"-shaped curve (Stegoceras seemed to carry their spine in a less extreme curve, due to their thick neck muscles). Carpenter suggests flank-butting as an alternative and contends that the thickness of the dome would have used for increasing the powerful behind a blow to the sides. It is possible that Stegoceras and similar pachycephalosaurs would have delivered the blows with a movement of the neck from the side and a rotaton of the head. The dorsolateral area of the dome has the greatest surface area and may have been the point of impact. This would insure that the force of the impact would not cause serious injury to the opponent. The bone rim above the orbit may have protected the aggressor's eye when making a blow. Also, the relatively large width of Stegoceras may have served to protect vital organs from harm during flank-butting. Snively and Theodor (2011) conducted CT scans of the skulls of Stegoceras validum, Prenocephale prenes and several head-striking artiodactyls. They found that the skull of Stegoceras was well-equipped for head-butting. It appears that Stegoceras had extra layer of dense bone in the middle of its dome, which may have provided extra protection for the brain.