''Pachycephalosaurus'' (; meaning "thick-headed lizard", from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''pachys-/'' "thick", ''kephale/'' "head" and ''sauros/'' "lizard") is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
pachycephalosaurid
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s. The
type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
, ''P. wyomingensis'', is the only known
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, but some researchers argue that there might be a second species, ''P. spinifer''. It lived during the Late
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
Period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
(
Maastrichtian
The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...
stage
Stage or stages may refer to:
Acting
* Stage (theatre), a space for the performance of theatrical productions
* Theatre, a branch of the performing arts, often referred to as "the stage"
* ''The Stage'', a weekly British theatre newspaper
* Sta ...
) of what is now North America. Remains have been excavated in
Montana
Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
,
South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota people, Lakota and Dakota peo ...
,
Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
, and
Alberta
Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
. It was a
herbivorous
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
creature which is primarily known from a single skull and a few extremely thick
skull roof
The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.
In comparati ...
s, at 22 centimetres (9 inches) thick. More complete fossils have been found in recent years. ''Pachycephalosaurus'' was among the last non-avian dinosaurs before the
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
. The genus ''Tylosteus'' has been
synonymized with ''Pachycephalosaurus'', as have the genera ''Stygimoloch'' and ''Dracorex'' in recent studies.
Like other pachycephalosaurids, ''Pachycephalosaurus'' was a
biped
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...
al
herbivore
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
with an extremely thick
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...
roof. It possessed long hindlimbs and small forelimbs. ''Pachycephalosaurus'' is the largest-known pachycephalosaur. The thick skull domes of ''Pachycephalosaurus'' and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. This hypothesis has been disputed in recent years.
History of discovery
![Pachycephalosaur Skull (AMNH 1696)](https://upload.wikimedia.org/wikipedia/commons/4/40/Pachycephalosaur_Skull_%28AMNH_1696%29.jpg)
Remains attributable to ''Pachycephalosaurus'' may have been found as early as the 1850s. As determined by
Donald Baird
Donald is a masculine given name derived from the Gaelic name ''Dòmhnall''.. This comes from the Proto-Celtic *''Dumno-ualos'' ("world-ruler" or "world-wielder"). The final -''d'' in ''Donald'' is partly derived from a misinterpretation of the ...
, in 1859 or 1860
Ferdinand Vandeveer Hayden
Ferdinand Vandeveer Hayden (September 7, 1829 – December 22, 1887) was an American geologist noted for his pioneering surveying expeditions of the Rocky Mountains in the late 19th century. He was also a physician who served with the Union Ar ...
, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the
Missouri River, from what is now known to be the
Lance Formation
The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69 - 66 Ma) rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the late ...
in southeastern Montana.
This specimen, now
ANSP 8568, was described by
Joseph Leidy
Joseph Mellick Leidy (September 9, 1823 – April 30, 1891) was an American paleontologist, parasitologist and anatomist.
Leidy was professor of anatomy at the University of Pennsylvania, later was a professor of natural history at Swarthmore ...
in 1872 as belonging to the dermal
armor
Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or fr ...
of a reptile or an
armadillo
Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...
-like animal.
It became known as ''Tylosteus''. Its actual nature was not found until Baird studied it again over a century later and identified it as a
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal The quadratojugal is a skull bone present in many vertebrates, including ...
(bone from the back of the skull) of ''Pachycephalosaurus'', including a set of bony knobs corresponding to those found on other specimens of ''Pachycephalosaurus''.
[ Because the name ''Tylosteus'' predates ''Pachycephalosaurus'', according to the ]International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the ...
''Tylosteus'' would normally be preferred. In 1985, Baird successfully petitioned to have ''Pachycephalosaurus'' used instead of ''Tylosteus'' because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithostrati ...
information.[ICZN Opinion 1371,]
''Pachycephalosaurus'' Brown & Schlaikjer, 1943 and ''Troodon wyomingensis'' Gilmore, 1931 (Reptilia, Dinosauria): Conserved.
''Bulletin of Zoological Nomenclature'', 43 (1): April 1986. This may not be the end of the story; Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of ''Dracorex'' than that of ''Pachycephalosaurus''. The issue is of uncertain importance, though, if ''Dracorex'' actually represents a juvenile ''Pachycephalosaurus'', as has been recently proposed.
In 1890 during the Bone Wars
The Bone Wars, also known as the Great Dinosaur Rush, was a period of intense and ruthlessly competitive fossil hunting and discovery during the Gilded Age of American history, marked by a heated rivalry between Edward Drinker Cope (of the Acade ...
between paleontologists Othniel Charles Marsh
Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of Paleontology in Yale College and President of the National Academy of Sciences. He was one of the preeminent scientists in the field of paleontology. Among h ...
and Edward Drinker Cope
Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...
, one of Marsh's collectors, John Bell Hatcher
John Bell Hatcher (October 11, 1861 – July 3, 1904) was an American paleontologist and fossil hunter known as the "king of collectors" and best known for discovering ''Torosaurus'' and ''Triceratops'', two genera of dinosaurs described by Ot ...
, collected a partial left squamosal ( YPM VP 335) later referred to ''Stygimoloch spinifer'' near Lance Creek, Wyoming
Lance Creek is a census-designated place (CDP) in Niobrara County, Wyoming, Niobrara County, Wyoming, United States. The population was 43 at the United States Census, 2010, 2010 census. Lance Creek is the namesake of the Lance Formation, a rock f ...
in the Lance Formation
The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69 - 66 Ma) rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the late ...
.[Greenfield, Tyler (2020-12-08).]
Armor for Agathaumas
. Incertae Sedis. Retrieved 2022-03-20.[Marsh, O. C. (1891). I.—The Gigantic Ceratopsidæ, or Horned Dinosaurs, of North America 1. ''Geological Magazine'', ''8''(5), 193-199.] Marsh described the squamosal along with the dermal armor of ''Denversaurus
''Denversaurus'' (meaning "Denver lizard") is a genus of panoplosaurin nodosaurid dinosaur from the Late Cretaceous (late Maastrichtian) of western North America. Although at one point treated as a junior synonym of ''Edmontonia'' by some t ...
'' as the body armor of ''Triceratops'' in 1892, believing that the squamosal was a spike akin to the plates on the Ornithischia
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
n ''Stegosaurus
''Stegosaurus'' (; ) is a genus of herbivorous, four-legged, armored dinosaur from the Late Jurassic, characterized by the distinctive kite-shaped upright plates along their backs and spikes on their tails. Fossils of the genus have been foun ...
''. The squamosal spike was even featured in Charles R. Knight
Charles Robert Knight (October 21, 1874 – April 15, 1953) was an American wildlife and paleoartist best known for his detailed paintings of dinosaurs and other prehistoric animals. His works have been reproduced in many books and are currently ...
’s painting of Cope’s ceratopsid ''Agathaumas
''Agathaumas'' (; "great wonder") is a dubious genus of a large ceratopsid dinosaur that lived in Wyoming during the Late Cretaceous (late Maastrichtian stage, 66 million years ago). The name comes from grc, αγαν - 'much' and - 'wonder'. I ...
,'' likely based on Marsh’s hypothesis. Marsh also named a species of the now-dubious
Doubt is a mental state in which the mind remains suspended between two or more contradictory propositions, unable to be certain of any of them.
Doubt on an emotional level is indecision between belief and disbelief. It may involve uncertainty ...
Ankylosaur ''Palaeoscincus
''Palaeoscincus'' (meaning "ancient skink" from the Greek παλαιός and σκίγγος) is a dubious genus of ankylosaurian dinosaur based on teeth from the mid-late Campanian-age Upper Cretaceous Judith River Formation of Montana. Like s ...
'' in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from Lance. The tooth was named ''Palaeoscinus latus'', but in 1990, Coombs found the tooth to be from a pachycephalosaurid
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
, possibly even ''Pachycephalosaurus'' itself. Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.
''P. wyomingensis'', the type and currently only valid species of ''Pachycephalosaurus'', was named by Charles W. Gilmore
Charles Whitney Gilmore (March 11, 1874 – September 27, 1945) was an American paleontologist who gained renown in the early 20th century for his work on vertebrate fossils during his career at the United States National Museum (now the N ...
in 1931. He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County, Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
. Gilmore assigned his new species to ''Troodon
''Troodon'' ( ; ''Troödon'' in older sources) is a wastebasket taxon and a dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late Cretaceous period (about 77 mya). It includes at least ...
'' as ''T. wyomingensis''. At the time, paleontologists thought that ''Troodon'', then known only from teeth, was the same as ''Stegoceras
''Stegoceras'' is a genus of pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous period, about 77.5 to 74 million years ago (mya). The first specimens from Alberta, Canada, were descri ...
'', which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae
Troodontidae is a clade of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil disco ...
, a misconception not corrected until 1945, by Charles M. Sternberg
Charles Mortram Sternberg (1885–1981) was an American-Canadian fossil collector and paleontology, paleontologist, son of Charles Hazelius Sternberg.
Late in his career, he collected and described ''Pachyrhinosaurus'', ''Brachylophosaurus'', ' ...
.
In 1943, Barnum Brown
Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. Named after the circus showman P. T. Barnum, he discovered the first documented remains of ''Tyrannosaurus'' during a career ...
and Erich Maren Schlaikjer
Erich Maren Schlaikjer (; November 22, 1905 in Newtown, Ohio – November 5, 1972) was an American geologist and dinosaur hunter. Assisting Barnum Brown, he co-described ''Pachycephalosaurus'' and what is now ''Montanoceratops''. Other discoveries ...
, with newer, more complete material, established the genus ''Pachycephalosaurus''. They named two species: ''Pachycephalosaurus grangeri'', the type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
of the genus ''Pachycephalosaurus'', and ''Pachycephalosaurus reinheimeri''. ''P. grangeri'' was based on AMNH
The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inter ...
1696, a nearly complete skull from the Hell Creek Formation
The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...
of Ekalaka
Ekalaka is a town in and the county seat of Carter County, Montana, United States. The population was 399 at the 2020 census.
History
Ekalaka was named after a Sioux girl, Ijkalaka, who was the wife of David Harrison Russell, a scout.
Ijka ...
, Carter County, Montana. ''P. reinheimeri'' was based on what is now DMNS 469, a dome and a few associated elements from the Lance Formation of Corson County, South Dakota. They also referred the older species "Troodon" ''wyomingensis'' to their new genus. Their two newer species have been considered synonymous with ''P. wyomingensis'' since 1983.
In 2015, some pachycephalosaurid material and a domed parietal attributable to ''Pachycephalosaurus'' were discovered in Scollard Formation
The Scollard Formation is an Upper Cretaceous to lower Palaeocene stratigraphic unit of the Western Canada Sedimentary Basin in southwestern Alberta. Its deposition spanned the time interval from latest Cretaceous to early Paleocene, and it inclu ...
, Alberta
Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
, Canada, implying dinosaurs of this era were cosmopolitan and didn't have discrete faunal provinces.
Description
The anatomy of ''Pachycephalosaurus'' is poorly known, as only skull remains have been described. ''Pachycephalosaurus'' is famous for having a large, bony dome atop its skull, up to thick, which safely cushioned its tiny brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. The spikes were probably blunt, not sharp.
The skull was short, and possessed large, rounded eye sockets that faced forward, suggesting that the animal had good eyesight and was capable of binocular vision
In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...
. ''Pachycephalosaurus'' had a small muzzle which ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck. Younger individuals of ''Pachycephalosaurus'' maybe have had flatter skulls, with larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out, as the dome grew.
''Pachycephalosaurus'' was probably biped
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...
al and was the largest of the pachycephalosaurid (bone-headed) dinosaurs. It has been estimated that ''Pachycephalosaurus'' was about long and weighed about . Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short fore limbs, a bulky body, long hind legs and a heavy tail, which was likely held rigid by ossified
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by Cell (biology), cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes ...
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
s.
Classification
''Pachycephalosaurus'' gives its name to the Pachycephalosauria
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
, a clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
of herbivorous
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
ornithischia
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
n ("bird hipped") dinosaurs which lived during the Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
Period in North America and Asia. Despite their bipedal stance, they were likely more closely related to the ceratopsian
Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...
s than the ornithopod
Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous wo ...
s.
''Pachycephalosaurus'' is the most famous member of the Pachycephalosauria (though not the best-preserved member). The clade also includes ''Stenopelix
''Stenopelix'' (meaning "narrow pelvis") is a genus of small marginocephalian dinosaur, possibly a basal ceratopsian, from the Early Cretaceous of Germany. It lived in the late Berriasian faunal stage, Stage of the Cretaceous period, approximatel ...
'', ''Wannanosaurus
''Wannanosaurus'' (meaning " Wannan lizard", named after the location where it was discovered) is a genus of basal pachycephalosaurian dinosaur from the Maastrichtian Upper Cretaceous Xiaoyan Formation, about 80 million years ago ( mya) i ...
'', ''Goyocephale
''Goyocephale'' is an extinct genus of pachycephalosaurian ornithischian that lived in Mongolia during the Late Cretaceous about 76 million years ago. It was first described in 1982 by Altangerel Perle, Teresa Maryańska and Halszka Osmólska f ...
'', ''Stegoceras
''Stegoceras'' is a genus of pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous period, about 77.5 to 74 million years ago (mya). The first specimens from Alberta, Canada, were descri ...
'', ''Homalocephale
''Homalocephale'' (from Ancient Greek, Greek ὁμαλός, ''homalos'', "even", and κεφαλή, ''kephalē'', "head") is a genus of pachycephalosaurid dinosaur that lived during the Late Cretaceous period of what is now the Nemegt Formation, M ...
'', ''Tylocephale
''Tylocephale'' (meaning "swollen head", from the Greek ''τυλη'' meaning 'callus' or 'hard swelling' and ''κεφαλη'' meaning 'head') is a genus of pachycephalosaurid dinosaur from the Late Cretaceous period. It was a herbivorous dinosau ...
'', ''Sphaerotholus
''Sphaerotholus'' is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States and Canada. To date, three species have been described: the type species, ''S. goodwini'', from the Den-na-zin Member of the Kirtla ...
'' and ''Prenocephale
''Prenocephale'' (meaning "sloping head") is a genus of small pachycephalosaurid dinosaur from the Late Cretaceous Nemegt Formation of Mongolia. It was similar in many ways to its close relative, ''Homalocephale''.
Description
Adult ''Prenoceph ...
''. Within the tribe Pachycephalosaurini, ''Pachycephalosaurus'' is most closely related to ''Alaskacephale
''Alaskacephale'' was a genus of pachycephalosaurid dinosaur from the Prince Creek Formation, that lived in the early Maastrichtian stages of the Late Cretaceous (around 71 to 69 million years ago).
Discovery
''Alaskacephale'' was named by Ro ...
''. ''Dracorex'' and ''Stygimoloch'' have been synonymized with ''Pachycephalosaurus''.
In 2010, Gregory S. Paul proposed that while ''Stygimoloch'' and ''Dracorex'' possibly represent a juvenile stage of ''Pachycephalosaurus'', ''Stygimoloch'' might represent a different species, ''P. spinifer''.[ This idea has been regarded as a way of interpretation by Mark Witton and Thomas Holtz.] A phylogenetic analysis from 2021 by Evans and his colleagues accepted the validity of the genus ''Stygimoloch'' on the basis of it being found in later rock layers than ''Pachycephalosaurus'', but agreed with the consensus that ''Dracorex'' represents an ontogimorph rather than a distinct taxon. However, David Evans himself noted in his twitter post that he and his colleagues would also consider ''Stygimoloch'' as ''P. spinifer''.
Below is a cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
modified from Evans ''et al.'', 2013.
Below is a cladogram from Evans ''et al.'', 2021.
Paleobiology
Growth
Aside from ''Pachycephalosaurus'' itself, two other pachycephalosaurs were described from the latest Cretaceous of the northwestern United States: ''Stygimoloch spinifer'' ("thorny Moloch
Moloch (; ''Mōleḵ'' or הַמֹּלֶךְ ''hamMōleḵ''; grc, Μόλοχ, la, Moloch; also Molech or Molek) is a name or a term which appears in the Hebrew Bible several times, primarily in the book of Leviticus. The Bible strongly co ...
of the Styx
In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, whic ...
") and ''Dracorex hogwartsia'' ("dragon king of Hogwarts
Hogwarts School of Witchcraft and Wizardry () is a fictional Scotland, Scottish boarding school of Magic in Harry Potter, magic for students aged eleven to eighteen, and is the primary setting for the first six books in J. K. Rowling's ''Harry Pot ...
"). The former is only known from a juvenile skull with a reduced dome and large spikes, while the latter, also known from only a juvenile skull, had a seemingly flat head with short horns. Due to their unique head ornamentation, they were seen as separate species for a number of years, but in 2007, they were proposed to be juvenile or female morphologies of ''Pachycephalosaurus''. At that year's meeting of the Society of Vertebrate Paleontology, Jack Horner of Montana State University
Montana State University (MSU) is a Public university, public land-grant university, land-grant research university in Bozeman, Montana. It is the state's largest university. MSU offers baccalaureate degrees in 60 fields, master's degrees in 6 ...
presented evidence, from analysis of the skull of the ''Dracorex'' specimen, that it may well be a juvenile form of ''Stygimoloch''. In addition, he presented data that indicates that both ''Stygimoloch'' and ''Dracorex'' may be juvenile forms of ''Pachycephalosaurus''. Horner and M.B. Goodwin published their findings in 2009, showing that the spike/node and skull dome bones of all three "species" exhibit extreme plasticity and that both ''Dracorex'' and ''Stygimoloch'' are known only from juvenile specimens while ''Pachycephalosaurus'' is known only from adult specimens. These observations, in addition to the fact that all three forms lived in the same time and place, led them to conclude that ''Dracorex'' and ''Stygimoloch'' were simply juvenile ''Pachycephalosaurus'', which lost spikes and grew domes as they aged.[Horner J.R. and Goodwin, M.B. (2009). "Extreme cranial ontogeny in the Upper Cretaceous Dinosaur ''Pachycephalosaurus''." ''PLoS ONE'', 4(10): e7626]
Online full text
A 2010 study by Nick Longrich and colleagues also supported the hypothesis that all flat-skulled pachycephalosaur species were juveniles of the dome-headed adults, such as ''Goyocephale'' and ''Homalocephale''.
The discovery of baby skulls assigned to ''Pachycephalosaurus'' that were described in 2016 from two different bone beds in the Hell Creek Formation has been presented as further evidence for this hypothesis. The fossils, as described by David Evans and Mark Goodwin ''et al'' are identical to all three supposed genera in the placement of the rugose knobs on their skulls, and the unique features of ''Stygimoloch'' and ''Dracorex'' are thus instead morphologically consistent features on a ''Pachycephalosaurus'' growth curve.
It has been noted that morphological differences between ''Stygimoloch'' and ''Pachycephalosaurus'' may also partly be due to slight stratigraphic differences. The few ''Stygimoloch'' specimens that have reliable stratigraphic data were all collected from the upper part of the Hell Creek Formation, whereas ''Pachycephalosaurus'' morphs were all collected from the lower part. This has also led to suggestions that ''Stygimoloch'' might represent its own species, ''P. spinifer''.[ In their 2021 redescription of '']Sinocephale
''Sinocephale'' (meaning "Chinese head") is a genus of pachycephalosaurid dinosaur that lived in Inner Mongolia, China during the Cretaceous period. The only species, ''Sinocephale bexelli'', was originally named as a species of the genus ''Troo ...
bexelli'', Evans and his colleageues treated ''Stygimoloch'' (but not ''Dracorex'') as a separate taxon based on their phylogenetic analysis. However, Evans himself has noted that he and his colleagues support the idea of ''P. spinifer''.[
]
Dome function
It has been commonly hypothesized that ''Pachycephalosaurus'' and its relatives were the bipedal equivalents of bighorn sheep
The bighorn sheep (''Ovis canadensis'') is a species of sheep native to North America. It is named for its large horns. A pair of horns might weigh up to ; the sheep typically weigh up to . Recent genetic testing indicates three distinct subspec ...
or musk oxen
Musk ( Persian: مشک, ''Mushk'') is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial sub ...
, where male individuals would ram each other headlong, and that they would horizontally straighten their head, neck, and body in order to transmit stress during ramming. However, there have also been alternative suggestions that the pachycephalosaurs could not have used their domes in this way.
The primary argument that has been raised against head-butting is that the skull roof may not have adequately sustained impact associated with ramming, as well as a lack of definitive evidence of scars or other damage on fossilized ''Pachycephalosaurus'' skulls (however, more recent analyses have uncovered such damage; see below). Furthermore, the cervical and anterior dorsal vertebrae show that the neck was carried in an "S"- or "U"-shaped curve, rather than a straight orientation, and thus unfit for transmitting stress from direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.[
]
Alternatively, ''Pachycephalosaurus'' and other pachycephalosaurid genera may have engaged in flank-butting during intraspecific combat. In this scenario, an individual may have stood roughly parallel or faced a rival directly, using intimidation displays to cow its rival. If intimidation failed, the ''Pachycephalosaurus'' would bend its head downward and to the side, striking the rival pachycephalosaur on its flank. This hypothesis is supported by the relatively broad torso of most pachycephalosaurs, which would have protected vital organs from trauma. The flank-butting theory was first proposed by Sues in 1978, and expanded upon by Ken Carpenter in 1997.[
]
In 2012, a study showed that cranial pathologies in a ''P. wyomingensis'' specimen were likely due to agonistic behavior. It was also proposed that similar damage in other pachycephalosaur specimens previously explained as taphonomic
Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...
artifacts and bone absorptions may instead have been due to such behavior. Peterson et al. (2013) studied cranial pathologies among the Pachycephalosauridae and found that 22% of all domes examined had lesions that are consistent with osteomyelitis
Osteomyelitis (OM) is an infection of bone. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The long bones of the arms and legs are most commonly involved in children e.g. the femur and humerus, while the ...
, an infection of the bone resulting from penetrating trauma, or trauma to the tissue overlying the skull leading to an infection of the bone tissue. This high rate of pathology lends more support to the hypothesis that pachycephalosaurid domes were employed in intra-specific combat. ''Pachycephalosaurus wyomingensis'' specimen BMR P2001.4.5 was observed to have 23 lesions in its frontal
Front may refer to:
Arts, entertainment, and media Films
* ''The Front'' (1943 film), a 1943 Soviet drama film
* ''The Front'', 1976 film
Music
* The Front (band), an American rock band signed to Columbia Records and active in the 1980s and e ...
bone and ''P. wyomingensis'' specimen DMNS 469 was observed to have 5 lesions. The frequency of trauma was comparable across the different genera in the pachycephalosaurid family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
, despite the fact that these genera vary with respect to the size and architecture of their domes, and fact that they existed during varying geologic periods.[ These findings were in stark contrast with the results from analysis of the relatively flat-headed pachycephalosaurids, where there was an absence of pathology. This would support the hypothesis that these individuals represent either females or juveniles, where intra-specific combat behavior is not expected.
Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar bone which contains ]fibroblasts
A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells o ...
that play a critical role in wound healing, and are capable of rapidly depositing bone during remodeling. Peterson et al. (2013) concluded that taken together, the frequency of lesion distribution and the bone structure of frontoparietal domes, lends strong support to the hypothesis that pachycephalosaurids used their unique cranial structures for agonistic behavior.[ CT scan comparisons of the skulls of ''Stegoceras validum'', ''Prenocephale prenes'', and several head-striking ]artiodactyls
The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
have also supported pachycephalosaurids as being well-equipped for head-butting.
Diet
Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth, they could not have chewed tough, fibrous plants like flowering shrubs as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
, seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s, and fruit
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particu ...
s. The sharp, serrated teeth would have been very effective for shredding plants. It is also suspected that the dinosaur may have included meat in its diet. The most complete fossil jaw shows that it had serrated blade-like front teeth, reminiscent of those of carnivorous theropods.
Paleoecology
Nearly all ''Pachycephalosaurus'' fossils have been recovered from the Lance Formation
The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69 - 66 Ma) rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the late ...
and Hell Creek Formation
The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...
of the western United States.[ ''Pachycephalosaurus'' possibly coexisted alongside additional pachycephalosaur species of the genera '']Sphaerotholus
''Sphaerotholus'' is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States and Canada. To date, three species have been described: the type species, ''S. goodwini'', from the Den-na-zin Member of the Kirtla ...
'', as well as ''Dracorex'' and ''Stygimoloch'', though these last two genera may represent juveniles of ''Pachycephalosaurus'' itself.[ Other dinosaurs that shared its time and place include '']Thescelosaurus
''Thescelosaurus'' ( ; ancient Greek - (''-'') meaning "godlike", "marvellous", or "wondrous" and (') "lizard") was a genus of small neornithischian dinosaur that appeared at the very end of the Late Cretaceous period in North America. It was ...
'', the hadrosaur
Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which includ ...
id ''Edmontosaurus
''Edmontosaurus'' ( ) (meaning "lizard from Edmonton") is a genus of hadrosaurid (duck-billed) dinosaur. It contains two known species: ''Edmontosaurus regalis'' and ''Edmontosaurus annectens''. Fossils of ''E. regalis'' have been found in rocks ...
'' and a possible species of ''Parasaurolophus
''Parasaurolophus'' (; meaning "near crested lizard" in reference to '' Saurolophus)'' is a genus of herbivorous hadrosaurid ornithopod dinosaur that lived in what is now North America and possibly Asia during the Late Cretaceous Period, abou ...
'', ceratopsia
Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...
ns like ''Triceratops
''Triceratops'' ( ; ) is a genus of herbivore, herbivorous Chasmosaurinae, chasmosaurine Ceratopsidae, ceratopsid dinosaur that first appeared during the late Maastrichtian stage of the Late Cretaceous Period (geology), period, about 68 m ...
'', ''Torosaurus
''Torosaurus'' ("perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous ceratopsid dinosaur that lived during the late Maastrichtian stage of the Cretaceous period, between 68 and 66 million years ago, th ...
'', ''Nedoceratops
''Nedoceratops'' (meaning "insufficient horned face") is a controversial genus of ceratopsid dinosaur from the Late Cretaceous period Lance Formation of North America. It is known only from a single skull discovered in Wyoming. Its status is the ...
'', ''Tatankaceratops
''Tatankaceratops'' (meaning "Bison horn face") is a controversial genus of herbivorous ceratopsian dinosaur. It is a small chasmosaurine ceratopsian which lived during the Late Cretaceous period (latest Maastrichtian stage) in what is now South ...
'', and ''Leptoceratops
''Leptoceratops'' (meaning 'Thin-horned face' and derived from Greek ''lepto-/λεπτο-'' meaning 'small', 'insignificant', 'slender', 'meagre' or 'lean', ''kerat-/κερατ-'' meaning 'horn' and ''-ops/ωψ'' meaning face), is a genus of l ...
'', the ankylosaurid
Ankylosauridae () is a family of armored dinosaurs within Ankylosauria, and is the sister group to Nodosauridae. The oldest known Ankylosaurids date to around 122 million years ago and went extinct 66 million years ago during the Cretaceous–Pal ...
''Ankylosaurus
''Ankylosaurus'' is a genus of armored dinosaur. Its fossils have been found in geological formations dating to the very end of the Cretaceous Period, about 68–66 million years ago, in western North America, making it among the last of the n ...
'', nodosaurids
Nodosauridae is a family of ankylosaurian dinosaurs, from the Late Jurassic to the Late Cretaceous period in what is now North America, South America, Europe, and Asia.
Description
Nodosaurids, like their close relatives the ankylosaurids, were ...
''Denversaurus
''Denversaurus'' (meaning "Denver lizard") is a genus of panoplosaurin nodosaurid dinosaur from the Late Cretaceous (late Maastrichtian) of western North America. Although at one point treated as a junior synonym of ''Edmontonia'' by some t ...
'', and ''Edmontonia
''Edmontonia'' is a genus of panoplosaurin nodosaurid dinosaur from the Late Cretaceous Period. It is part of the Nodosauridae, a family within Ankylosauria. It is named after the Edmonton Formation (now the Horseshoe Canyon Formation in Canada) ...
'', and the theropods
Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ca ...
''Acheroraptor
''Acheroraptor'' is an extinct genus of dromaeosaurid theropod dinosaur known from the latest Maastrichtian Hell Creek Formation of Montana, United States. It contains a single species, ''Acheroraptor temertyorum''. ''A. temertyorum'' is ...
'', ''Dakotaraptor
''Dakotaraptor'' (meaning “thief from Dakota”) is a potentially chimaeric genus of large dromaeosaurid theropod dinosaur that lived in North America during the Late Cretaceous period. The remains have been found in the Maastrichtian stage of ...
'', ''Ornithomimus
''Ornithomimus'' (; "bird mimic") is a genus of ornithomimid dinosaurs from the Late Cretaceous Period of what is now North America. ''Ornithomimus'' was a swift bipedal theropod which fossil evidence indicates was covered in feathers, equippe ...
'', ''Struthiomimus
''Struthiomimus'' (meaning "ostrich mimic", from the Ancient Greek, Greek στρούθειος/''stroutheios'' meaning "of the ostrich" and μῖμος/''mimos'' meaning "mimic" or "imitator") is a genus of Ornithomimidae, ornithomimid dinosaurs ...
'', ''Anzu Anzu may refer to:
*Anzû, a divine storm-bird in several Ancient Mesopotamian religion, Mesopotamian religions
*Anzu (dinosaur), ''Anzu'' (dinosaur), a genus of theropod dinosaur containing the species ''Anzu wyliei''
As a given name
is a Japan ...
'', '' Leptorhynchos'', ''Pectinodon
''Pectinodon'' is a genus of troodontid dinosaurs from the Late Cretaceous period (66 mya). It currently contains a single valid species, ''Pectinodon bakkeri'' (sometimes classified as ''Troodon bakkeri''), known only from teeth.
In 1982, ...
'', ''Paronychodon
''Paronychodon'' (meaning "beside claw tooth") was a theropod dinosaur genus. It is a tooth taxon, often considered dubious because of the fragmentary nature of the fossils, which include "buckets" of teeth from many disparate times and places b ...
'', ''Richardoestesia
''Richardoestesia'' is a morphogenus of theropod dinosaur teeth, originally described from the Late Cretaceous of what is now North America. It currently contains two species, ''R. gilmorei'' and ''R. isosceles''. It has been used as a morphota ...
'', and ''Tyrannosaurus
''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...
''.[Weishampel, David B.; Barrett, Paul M.; Coria, Rodolfo A.; Le Loeuff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth, M.P.; and Noto, Christopher R. (2004). "Dinosaur Distribution". In: D.B. Weishampel, P. Dodson, and H. Osmólska (eds.) ''The Dinosauria'' (2nd edition). 517–606. .]
See also
* Timeline of pachycephalosaur research
This timeline of pachycephalosaur research is a chronological listing of events in the history of paleontology focused on the pachycephalosaurs, a group of dome-skulled herbivorous marginocephalian dinosaurs. One of the first major events related ...
References
External links
''Pachycephalosaurus'' in the Dinodictionary
from ''National Geographic'' Online
TEDx talk by Jack Horner
on shape-shifting dinosaur skulls and dinosaur misclassification.
*
*
{{Portal bar, Dinosaurs, Cretaceous, United States
Late Cretaceous dinosaurs of North America
Pachycephalosaurs
Taxa named by Erich Maren Schlaikjer
Fossil taxa described in 1943
Taxa named by Barnum Brown
Lance fauna
Hell Creek fauna
Paleontology in Montana
Paleontology in South Dakota
Paleontology in Wyoming
Maastrichtian genus first appearances
Maastrichtian genus extinctions
Ornithischian genera