Pachycephalosauria
Encyclopedia
Pachycephalosauria is a clade
of ornithischian dinosaur
s. 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. In some fossils, the skull roof
is domed and several inches thick; in others it is flat or wedge-shaped. While traditionally regarded as distinct species or even families, the flat-headed pachycephalosaurs may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.
Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum
from the Middle Jurassic
Period of what is now Kyrgyzstan and Stenopelix valdensis
from the Early Cretaceous
Period of what is now Germany
, although Sullivan doubted that either of these species are pachycephalosaurs.
" by L. Sprague de Camp
. Many paleontologists have since argued for the head-butting hypothesis, including and . In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goat
s and musk oxen.
Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature. It has been suggested that pachycephalosaurs could make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur can the head, neck, and body be oriented in such a position. Instead, the cervical and anterior dorsal vertebrae of pachycephalosaurs show that the neck was carried in an "S"- or "U"-shaped curve.
Also, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Other possibilities include flank-butting, defense against predators, or both. The relatively wide width of pachycephalosaurs (which would protect vital internal organs from harm during flank-butting) and the squamosal horns of the Stygimoloch
(which would have been used to great effect during flank-butting) add credence to the flank-butting hypothesis.
A histological study conducted by argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage," the spongy bone structure couldn't sustain the blows of combat, and the radial pattern was simply an effect of rapid growth. Later biomechanical analyses by and concluded, however, that the domes could withstand combat stresses. argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.
instead argued that the dome functioned for species recognition. There is evidence that the dome had some form of external covering, and it is reasonable to consider the dome may have been brightly covered, or subject to change color seasonally. Due to the nature of the fossil record, however, it cannot be observed if color actually played a role in dome function.
argued that species recognition is an unlikely evolutionary cause for the dome because dome forms are not notably different between species. Because of this general similarity, several genera of Pachycephalosauridae were incorrectly lumped together. This is unlike the case in ceratopsia
ns and hadrosaurids, which had much more distinct cranial ornamentation. Longrich et al. argued that instead the dome had a mechanical function, one which was important enough to justify the resource investment, such as combat.
theropod
Majungasaurus
. And Yaverlandia
, another dinosaur initially described as a pachycephalosaurid, has also recently been reclassified as a coelurosaur (Naish in ). Further complicating matters are the diverse interpretations of ontogenetic and sexual features
in pachycephalosaurs.
A 2009 paper proposed that Dracorex and Stygimoloch were just early growth stages of Pachycephalosaurus, rather than distinct genera.
of the Order Ornithischia
by . They included within it only one family
, the Pachycephalosauridae. Later researchers, such as Michael Benton, have ranked it as an infraorder
of the Suborder Cerapoda
, which unites the ceratopsia
ns and ornithopods. In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts to restrict Maryańska and Osmólska naming of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by , in his phylogenetic studies, to re-define Pachycephalosauridae to include only "dome-skulled" species (including Stegoceras and Pachycephalosaurus), while leaving more "basal" species outside that family in Pachycephalosauria. Therefore, Sullivan's use of Pachycephalosauridae is equivalent to Sereno and Benton's use of Pachycephalosauria.
Sullivan diagnosed the Pachycephalosauridae based only on characters of the skull, with the defining character being a dome-shaped frontoparietal skull bone. According to Sullivan, the absence of this feature in some species assumed to be primitive led to the split in classification between domed and non-domed pachycephalosaurs; however, discovery of more advanced and possibly juvenile pachycephalosaurs with flat skulls (such as Dracorex hogwartsia) show this distinction to be incorrect. Sullivan also pointed out that the original diagnosis of Pachycephalosauridae centered around "flat to dome-like" skulls, so the flat-headed forms should be included in the family.
The following taxonomy
follows Sullivan's 2006 classification unless otherwise noted.
Note that Butler et al., 2011 reassigned Stenopelix
and Micropachycephalosaurus
to the Ceratopsia
.
presented here follows an analysis by Williamson and Carr, 2002.
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of ornithischian 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...
s. Well-known genera include Pachycephalosaurus
Pachycephalosaurus
Pachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few...
, Stegoceras
Stegoceras
Stegoceras is a genus of plant-eating pachycephalosaurid dinosaur that lived in what is now North America during the Late Cretaceous period....
, Stygimoloch
Stygimoloch
Stygimoloch is a putative genus of pachycephalosaurid dinosaur from the end of the Cretaceous period, roughly 65 million years ago...
, and 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...
. Most lived 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, 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...
and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some fossils, the skull roof
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, hence the alternative name dermatocranium...
is domed and several inches thick; in others it is flat or wedge-shaped. While traditionally regarded as distinct species or even families, the flat-headed pachycephalosaurs may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.
Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum
Ferganocephale
Ferganocephale was possibly an early pachycephalosaurian dinosaur. It was from the Middle Jurassic of Kyrgyzstan.The type species, Ferganocephale adenticulatum, was first described by Averianov, Martin, and Bakirov in 2005, and is based solely on teeth from the Callovian. The holotype is ZIN PH...
from the Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
Period of what is now Kyrgyzstan and Stenopelix valdensis
Stenopelix
Stenopelix is a genus of small ornithischian dinosaur from the Early Cretaceous of Germany. It was perhaps a basal ceratopsian from the Barremian Stage of the Cretaceous period, sometime between 130 and 125 million years ago...
from the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
Period of what is now Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, although Sullivan doubted that either of these species are pachycephalosaurs.
Paleobiology
The adaptive significance of the skull dome has been heavily debated. The popular hypothesis among the general public that the skull was used in head-butting, as sort of a dinosaurian battering ram, was first proposed by . This view was popularized in the 1956 science fiction story "A Gun for DinosaurA Gun for Dinosaur
"A Gun for Dinosaur" is a science fiction story written by L. Sprague de Camp as part of his Rivers of Time series. It was first published in the magazine Galaxy Science Fiction for March, 1956, and first appeared in book form in the anthology The World That Couldn't Be and 8 Other SF Novelets...
" by L. Sprague de Camp
L. Sprague de Camp
Lyon Sprague de Camp was an American author of science fiction and fantasy books, non-fiction and biography. In a writing career spanning 60 years, he wrote over 100 books, including novels and notable works of non-fiction, including biographies of other important fantasy authors...
. Many paleontologists have since argued for the head-butting hypothesis, including and . In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goat
Mountain goat
The Mountain Goat , also known as the Rocky Mountain Goat, is a large-hoofed mammal found only in North America. Despite its vernacular name, it is not a member of Capra, the genus of true goats...
s and musk oxen.
Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature. It has been suggested that pachycephalosaurs could make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur can the head, neck, and body be oriented in such a position. Instead, the cervical and anterior dorsal vertebrae of pachycephalosaurs show that the neck was carried in an "S"- or "U"-shaped curve.
Also, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Other possibilities include flank-butting, defense against predators, or both. The relatively wide width of pachycephalosaurs (which would protect vital internal organs from harm during flank-butting) and the squamosal horns of the Stygimoloch
Stygimoloch
Stygimoloch is a putative genus of pachycephalosaurid dinosaur from the end of the Cretaceous period, roughly 65 million years ago...
(which would have been used to great effect during flank-butting) add credence to the flank-butting hypothesis.
A histological study conducted by argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage," the spongy bone structure couldn't sustain the blows of combat, and the radial pattern was simply an effect of rapid growth. Later biomechanical analyses by and concluded, however, that the domes could withstand combat stresses. argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.
instead argued that the dome functioned for species recognition. There is evidence that the dome had some form of external covering, and it is reasonable to consider the dome may have been brightly covered, or subject to change color seasonally. Due to the nature of the fossil record, however, it cannot be observed if color actually played a role in dome function.
argued that species recognition is an unlikely evolutionary cause for the dome because dome forms are not notably different between species. Because of this general similarity, several genera of Pachycephalosauridae were incorrectly lumped together. This is unlike the case in ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
ns and hadrosaurids, which had much more distinct cranial ornamentation. Longrich et al. argued that instead the dome had a mechanical function, one which was important enough to justify the resource investment, such as combat.
Classification
Most pachycephalosaurid remains are not complete, usually consisting of portions of the frontoparietal bone that forms the distinctive dome. This can make taxonomic identification a difficult task, as the classification of genera and species within Pachycephalosauria relies almost entirely on cranial characteristics. Consequently, improper species have historically been appointed to the clade. For instance, Majungatholus, once thought to be a pachycephalosaur, is now recognized as a specimen of the abelisauridAbelisauridae
Abelisauridae is a family of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous Period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are found on the modern continents of Africa and South America, as well as on the Indian...
theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
Majungasaurus
Majungasaurus
Majungasaurus is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 65.5 million years ago, at the end of the Cretaceous Period. Only one species has been identified...
. And Yaverlandia
Yaverlandia
Yaverlandia is a genus of theropod dinosaur. Known from a partial fossil skull found in Lower Cretaceous strata on the Isle of Wight, it was described as the earliest known member of the pachycephalosaurid family, but recent research by Darren Naish shows it to have actually been a theropod,...
, another dinosaur initially described as a pachycephalosaurid, has also recently been reclassified as a coelurosaur (Naish in ). Further complicating matters are the diverse interpretations of ontogenetic and sexual features
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
in pachycephalosaurs.
A 2009 paper proposed that Dracorex and Stygimoloch were just early growth stages of Pachycephalosaurus, rather than distinct genera.
Taxonomy
The Pachycephalosauria was first named as a suborderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of the Order 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'...
by . They included within it only one family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, the Pachycephalosauridae. Later researchers, such as Michael Benton, have ranked it as an infraorder
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of the Suborder Cerapoda
Cerapoda
Neornithischia is a clade of the dinosaur order Ornithischia. They are the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth...
, which unites the ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
ns and ornithopods. In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts to restrict Maryańska and Osmólska naming of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by , in his phylogenetic studies, to re-define Pachycephalosauridae to include only "dome-skulled" species (including Stegoceras and Pachycephalosaurus), while leaving more "basal" species outside that family in Pachycephalosauria. Therefore, Sullivan's use of Pachycephalosauridae is equivalent to Sereno and Benton's use of Pachycephalosauria.
Sullivan diagnosed the Pachycephalosauridae based only on characters of the skull, with the defining character being a dome-shaped frontoparietal skull bone. According to Sullivan, the absence of this feature in some species assumed to be primitive led to the split in classification between domed and non-domed pachycephalosaurs; however, discovery of more advanced and possibly juvenile pachycephalosaurs with flat skulls (such as Dracorex hogwartsia) show this distinction to be incorrect. Sullivan also pointed out that the original diagnosis of Pachycephalosauridae centered around "flat to dome-like" skulls, so the flat-headed forms should be included in the family.
The following taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
follows Sullivan's 2006 classification unless otherwise noted.
- Family Pachycephalosauridae
- AlaskacephaleAlaskacephaleAlaskacephale was a genus of pachycephalosaurid dinosaur that lived in the late Campanian stage of the Late Cretaceous ....
- AmtocephaleAmtocephaleAmtocephale is a genus of pachycephalosaurid dinosaur from early Late Cretaceous deposits of southern Gobi Desert, Mongolia....
- ColepiocephaleColepiocephaleColepiocephale 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...
- GoyocephaleGoyocephaleGoyocephale is a genus of pachycephalosaurid dinosaur which lived during the Late Cretaceous period. It lived in what is now Mongolia.Goyocephale probably weighed 10–40 kg. The type species, Goyocephale lattimorei, was formally described by Perle, Maryañska, and Osmólka from an incomplete...
- GravitholusGravitholusGravitholus was a genus of dinosaur from the late Cretaceous period . It was a pachycephalosaur, a type of dinosaur with a thick skull made of hardened bone. It lived in what is now Alberta, Canada, and was described in 1979 by W. P. Wall and Peter Galton.The type species is Gravitholus albertae...
- HanssuesiaHanssuesiaHanssuesia 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....
- HomalocephaleHomalocephaleHomalocephale is a genus of dinosaur belonging to the pachycephalosaurid family, which lived during the late Cretaceous period of what is now Mongolia, 80 million years ago. The genus was described in 1974 by Osmólska & Maryañska, and consists of a single species, H...
- possible juvenile form of Prenocephale - PrenocephalePrenocephalePrenocephale 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...
- SphaerotholusSphaerotholusSphaerotholus 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...
- StegocerasStegocerasStegoceras is a genus of plant-eating pachycephalosaurid dinosaur that lived in what is now North America during the Late Cretaceous period....
(incl. Ornatotholus) - TexacephaleTexacephaleTexacephale is a genus of basal pachycephalosaurid dinosaur from the Campanian stage of the Late Cretaceous. The type species is Texacephale langstoni; its fossils were discovered in the Aguja Formation and described in 2010 by Longrich, Sankey and Tanke...
- TylocephaleTylocephaleTylocephale is a genus of pachycephalosaurid dinosaur from the Late Cretaceous Period. It was a herbivorous dinosaur estimated to have been about 1.4 metres in length...
- WannanosaurusWannanosaurusWannanosaurus is a genus of basal pachycephalosaurian dinosaur from the Campanian-age Upper Cretaceous Xiaoyan Formation, about 80 million years ago in what is now Anhui, China...
- Tribe Pachycephalosaurini
- DracorexDracorexDracorex 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...
- possible juvenile form of Pachycephalosaurus - PachycephalosaurusPachycephalosaurusPachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few...
- StygimolochStygimolochStygimoloch is a putative genus of pachycephalosaurid dinosaur from the end of the Cretaceous period, roughly 65 million years ago...
- possible juvenile form of Pachycephalosaurus
- Dracorex
- Alaskacephale
- Nomina dubia (doubtful names)
- FerganocephaleFerganocephaleFerganocephale was possibly an early pachycephalosaurian dinosaur. It was from the Middle Jurassic of Kyrgyzstan.The type species, Ferganocephale adenticulatum, was first described by Averianov, Martin, and Bakirov in 2005, and is based solely on teeth from the Callovian. The holotype is ZIN PH...
- HeishansaurusHeishansaurusHeishansaurus, meaning "Heishan lizard" after the area in China where it was discovered, is the name given to a dubious genus of dinosaur. First described in 1953 as a pachycephalosaur, this dinosaur may actually be an ankylosaur. The fossils, dating from the Late Cretaceous , are fragmentary, and...
(probably an ankylosaur) - “Stegoceras” bexelli
- Ferganocephale
Note that Butler et al., 2011 reassigned Stenopelix
Stenopelix
Stenopelix is a genus of small ornithischian dinosaur from the Early Cretaceous of Germany. It was perhaps a basal ceratopsian from the Barremian Stage of the Cretaceous period, sometime between 130 and 125 million years ago...
and Micropachycephalosaurus
Micropachycephalosaurus
Micropachycephalosaurus is a monotypic genus of ornithischian dinosaur. It lived in Shandong Province, China during the Late Cretaceous period . The incomplete skeleton of the single specimen was found on a cliff southwest of Laiyang...
to the Ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
.
Phylogeny
The cladogramCladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
presented here follows an analysis by Williamson and Carr, 2002.