Pachycephalosauria (/ˌpækiˌsɛfələˈsɔːriə, -ˌkɛf-/; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. Genera include Pachycephalosaurus, Stegoceras, and Prenocephale. With the exception of two species, most pachycephalosaurs lived during the Late Cretaceous Period, dating between about 85.8 and 65.5 million years ago. They are exclusive to the Northern Hemisphere, all of them being found in North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. Skulls can be domed, flat, or wedge-shaped depending on the species, and are all heavily ossified. The domes were often surrounded by nodes and/or spikes. Partial skeletons have been found of several pachycephalosaur species, but to date no complete skeletons have been discovered. Often isolated skull fragments are the only bones that are found.
Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from Middle Jurassic Period strata of Kyrgyzstan and Stenopelix valdensis from Early Cretaceous strata of Germany, although R.M. Sullivan has doubted that either of these species are pachycephalosaurs. In 2017, a phylogenetic analysis conducted by Han and colleagues identified Stenopelix as a member of the Ceratopsia.
|Cast of a Pachycephalosaurus wyomingensis skull, Oxford University Museum of Natural History|
Maryańska & Osmólska, 1974
Gilmore, 1931 (conserved name)
Pachycephalosaurs were bipedal ornithischians characterized by their thickened skulls. They had a bulky torso with an expanded gut cavity and broad hips, short forelimbs, long legs, a short, thick neck, and a heavy tail. Large orbits and a large optic nerve point to pachycephalosaurs having good vision, and uncharacteristically large olfactory lobes indicate that they had a good sense of smell relative to other dinosaurs.They were fairly small dinosaurs, with most falling in the range of 2–3 meters (6.6-9.8 feet) in length and the largest, Pachycephalosaurus wyomingensis, estimated to measure 4.5 meters (14.8 feet) long and weigh 450 kilograms (990 pounds). The characteristic skull of pachycephalosaurs is a result of the fusion and thickening of the frontals and parietals, accompanied by the closing of the supratemporal fenestra. In some species this takes the form of a raised dome; in others, the skull is flat or wedge-shaped. While the flat-headed pachycephalosaurs are traditionally regarded as distinct species or even families, they may represent juveniles of dome-headed adults. All display highly ornamented jugals, squamosals, and postorbitals in the form of blunt horns and nodes. Many species are only known from skull fragments, and a complete pachycephalosaur skeleton is yet to be found.
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 Colbert 1955. This view was popularized in the 1956 science fiction story "A Gun for Dinosaur" by L. Sprague de Camp. Many paleontologists have since argued for the head-butting hypothesis, including Galton 1970 and Sues 1978. In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goats 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 a pachycephalosaur could make its head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur could 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 build 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 Goodwin & Horner 2004 argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage", the spongy bone structure could not sustain the blows of combat, and the radial pattern was simply an effect of rapid growth. Later biomechanical analyses by Snively & Cox 2008 and Snively & Theodor 2011 concluded, however, that the domes could withstand combat stresses. Lehman 2010 argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.
Goodwin & Horner 2004 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 whether or not color played a role in dome function.
Longrich, Sankey & Tanke 2010 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 have sometimes been incorrectly lumped together. This is unlike the case in ceratopsians and hadrosaurids, which had much more distinct cranial ornamentation. Longrich et al argued that instead the dome had a mechanical function, such as combat, one which was important enough to justify the resource investment.
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, 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. The frequency of trauma was comparable across the different genera in this family, 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 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 lend strong support to the hypothesis that pachycephalosaurids used their unique cranial structures for agonistic behavior.
The small size of most pachycephalosaur species and lack of skeletal adaptation indicates that they were not climbers and primarily ate food close to the ground. Mallon et al. (2013) examined herbivore coexistence on the island continent of Laramidia, during the Late Cretaceous and concluded that pachycephalosaurids were generally restricted to feeding on vegetation at, or below, the height of 1 meter. They exhibit heterodonty, having different tooth morphology between the premaxillary teeth and maxillary teeth. Front teeth are small and peg-like with an ovular cross section and were most likely used for grabbing food. In some species, the last premaxillary tooth was enlarged and canine-like. Back teeth are small and triangular with denticles on the front and back of the crown, used for mouth processing. In species in which the dentary has been found, mandibular teeth are similar in size and shape to those in the upper jaw. Wear patterns on the teeth vary by species, indicating a range of food preferences which could include seeds, stems, leaves, fruits, and possibly insects. A very wide rib cage and large gut cavity extending all the way to the base of the tail suggests the use of fermentation to digest food.
Pachycephalosaurs lived exclusively in Laurasia, being found in western North America and central Asia. Pachycephalosaurs originated in Asia and had two major dispersal events, resulting in the two separate waves of pachycephalosaur evolution observed in Asia. The first, occurring before the late Santonian or early Campanian, involved a migration from Asia to North America, most likely by way of the Bering Land Bridge. This migration was by a common ancestor of Stygimoloch, Stegoceras, Tylocephale, Prenocephale, and Pachycephalosaurus. The second event occurred before the middle Campanian, and involved a migration back into Asia from North America by a common ancestor of Prenocephale and Tylocephale. Two species originally reported to be pachycephalosaurs discovered outside this range, Yaverlandia bitholus of England and Majungatholus atopus of Madagascar, have recently been shown to actually be theropods.
The Asian and North American species of pachycephalosaurs lived in markedly different environments. Asian specimens are normally more of intact, indicating they were not transported far from their place of death before fossilization. They likely lived in a large desert region in central Asia with a hot and arid climate. North American specimens are typically found in rocks that were formed by erosion from the Rocky Mountains. Specimens are far less intact; usually only skull caps are recovered, and those found regularly exhibit surface exfoliation and other signs that they were transported long distances by water before fossilization. It is assumed that they lived in the mountains in a temperate climate and were carried by erosion after death to their final resting place.
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 abelisaurid theropod Majungasaurus. And Yaverlandia, another dinosaur initially described as a pachycephalosaurid, has also recently been reclassified as a coelurosaur (Naish in Sullivan 2006). 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.
The Pachycephalosauria was first named as a suborder of the order Ornithischia by Maryańska & Osmólska 1974. They included within it only one family, the Pachycephalosauridae. Later researchers, such as Michael Benton, have ranked it as an infraorder of a suborder Cerapoda, which unites the ceratopsians and ornithopods. In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts by Maryańska and Osmólska to restrict the definition of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by Sereno 1986, 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.
Aquilops is an early herbivorous ceratopsian dinosaur dating from the Early Cretaceous of North America, approximately 108 million to 104 million years ago. The type species is A. americanus.Bagaceratopidae
Bagaceratopidae is a family of neoceratopsian dinosaurs. It was named by Alifanov in 2003 but no definition has been proposed. Because of lacking of the definition, Bagaceratopidae was considered inactive by Paul Sereno in 2005. Alifanov in 2003 classified to this family four genera: Bagaceratops, Breviceratops, Lamaceratops and Platyceratops and in a publication from 2008 he included in Bagaceratopidae also Magnirostris and newly described genus Gobiceratops. Alifanov suggested also that Bagaceratopidae, unlike other neoceratopsian families, is of Paleoasiatic origin.Bagaceratopids existed during the late Cretaceous period, between about 85.8 and 70.6 million years ago.Cerapoda
Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.Chaoyangsauridae
Chaoyangsauridae is a family of ceratopsian dinosaurs. They are among the earliest known marginocephalian dinosaurs, with remains dating to about 160 million years ago, during the Late Jurassic period. Members of this group had sharp beaks for snipping off leaves to eat, and a very small frill.
Four dinosaur genera, Chaoyangsaurus, Xuanhuaceratops, Yinlong and Hualianceratops, are usually considered to belong to the Chaoyangsauridae. All four animals are more primitive (or basal) than both Psittacosaurus and neoceratopsians.Foraminacephale
Foraminacephale (meaning "foramina head") is a genus of pachycephalosaurid dinosaur from Late Cretaceous (Campanian stage) deposits of Canada.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 Perle, Teresa Maryańska and Osmólska for a disarticulated skeleton with most of a skull, part of the forelimb and hindlimb, some of the pelvic girdle, and some vertebrae. Perle et al. named the remains Goyocephale lattimorei, from the Mongolian goyo, meaning "decorated", and the Ancient Greek kephale, for head. The species name honours Owen Lattimore.Gravitholus
Gravitholus (meaning 'heavy dome') was a genus of dinosaur from the late Cretaceous period (Campanian stage, around 75 million years ago). 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. There is some debate amongst paleontologists as to whether the animal represents a distinct genus, or if it may be synonymous with Stegoceras. Recent publications indicate it may be a valid genus.Helioceratops
Helioceratops is a genus of neoceratopsian dinosaur from the Middle Cretaceous of China. The type species is H. brachygnathus, described in 2009 by a group of paleontologists led by Jin Liyong. Helioceratops was discovered in the Quantou Formation of China's eastern Jilin province and is known mostly from skull fragments. It reached a length of around 1.3 m (4.3 ft) and may have shared its habitat with Changchunsaurus.Homalocephale
Homalocephale (from Greek ωμαλος, homalos, "even", and κεφαλή, kephalē, "head") 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 & Teresa Maryańska, and consists of a single species, H. calathocercos, though this may be a synonym (and juvenile form) of Prenocephale. Homalocephale was 1.8 metres (6 ft) long and herbivorous.Marginocephalia
Marginocephalia (/mär′jə-nō-sə-făl′ē-ən/ Latin: margin-head) is a clade of ornithischian dinosaurs that is characterized by a bony shelf or margin at the back of the skull. These fringes were likely used for display. There are two clades included in Marginocephalia: the thick-skulled Pachycephalosauria and the horned Ceratopsia. All members of Marginocephalia were primarily herbivores. They basally used gastroliths to aid in digestion of tough plant matter until they convergently evolved tooth batteries in Neoceratopsia (or "new Ceratopsia") and Pachycephalosauria. Marginocephalia first evolved in the Jurassic Period and became more common in the Cretaceous. They are basally small facultative quadrupeds while derived members of the group are large obligate quadrupeds. Primitive marginocephalians are found in Asia, but the group migrated upwards into North America.Pachycephalosaurs, or "thick-headed reptiles", have primitive features that include basally small sized bodies, obligate bipedalism, and simple teeth with one row in operation at a time that are replaced as they are worn down. As they evolved, pachycephalosaurs evolved much thicker and advanced skull roofs including dome forms with horn-like ornamentation. Some research suggests these domes were used like helmets for protection while head-butting members in intraspecific combat. Some research suggests their necks were not strong enough to support such an impact. Flat-headed pachycephalosaur speciments have been found in Asia, and there is great controversy on the meaning of these flat heads. Recent research suggests the flat heads could be a juvenile state before developing the dome shape in the adult stage. It could also be evidence of sexual dimorphism with the female being more flat-headed.Ceratopsians, or "horned-faces", differ from pachycephalosaurs in the presence of a rostral bone, or beak. They are also known for having a jugal horn and a thin parietal-squamosal shelf that extends back and up into a frill. This frill could have been used for anchoring jaw muscles, as well as for display. The horns were likely used for establishing dominance, or defending territories. It is also possible they were a factor in sexual display and species recognition. One of the basalmost members of this group is Psittacosaurus, which is one of the most species-rich dinosaur genera from Asia. Ceratopsians later evolved into very large quadrupeds with elaborate facial horns such as Triceratops, Styracosaurus, and Centrosaurus. There was no change in richness of species throughout the Cretaceous before the Cretaceous-Paleogene extinction.Micropachycephalosaurus
Micropachycephalosaurus (meaning "small thick-headed lizard") is a monotypic genus of ceratopsian dinosaur. It lived in China during the Late Cretaceous period. The skeleton of the single specimen was found on a cliff southwest of Laiyang, Shandong Province. It was a bipedal and herbivorous dinosaur.Neornithischia
Neornithischia ("new ornithischians") 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. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs. Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Parksosauridae; in addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.Pachyostosis
Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner cavities. This joint occurrence is called pachyosteosclerosis. However, especially in the older literature, "pachyostosis" is often used loosely, referring to all osseous specializations characterized by an increase in bone compactness and/or volume. It occurs in both terrestrial and, especially, aquatic or semi-aquatic vertebrates.In aquatic animals, such as seacows (manatees and dugongs), Thalassocnus, and plesiosaurs, it provides (or provided) ballast as an adaptation for an aquatic existence.
Most giant deer showed pronounced pachyostosis of the mandible and skull. It has been suggested that this served to store minerals for antler growth. Many Pachycephalosauria and most members of the Dinocephalia clade of therapsids had thickened skull bones, probably used in head-butting contests.Prenocephale
Prenocephale was a small pachycephalosaurid dinosaur genus from the Late Cretaceous (Campanian) of Mongolia and was similar in many ways to its close relative, Homalocephale, which may simply represent Prenocephale juveniles.Prenoceratops
Prenoceratops, (meaning 'bent or prone-horned face' and derived from Greek prene-/πρηνη- meaning 'bent forwards' or 'prone', cerat-/κερατ- meaning 'horn' and -ops/ωψ meaning 'face') is a genus of ceratopsian dinosaur from the Late Cretaceous Period. Its fossils have been found in the upper Two Medicine Formation in the present-day U.S. state of Montana, in Campanian age rock layers that have been dated to 74.3 million years ago.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 Barremian Stage of the Cretaceous period, sometime between 130 and 125 million years ago. The genus is based on a partial skeleton lacking the skull, and its classification is based on characteristics of the hips.Teresa Maryańska
Teresa Maryańska is a Polish paleontologist who has specialized in Mongolian dinosaurs, particularly pachycephalosaurians and ankylosaurians. Peter Dodson (1998 p. 9) claims that in 1974 Maryanska together with Halszka Osmólska were among the first "women to describe new kinds of dinosaurs". She is considered not only as one of Poland's but also one of the world's leading experts on dinosaurs.A member of the 1964, 1965, 1970, and 1971 Polish–Mongolian expeditions to the Gobi Desert, she has described many finds from these rocks, often with Halszka Osmólska. Among the dinosaurs she has described are:
Saichania and Tarchia (1977)
with Osmólska, Homalocephale, Prenocephale, and Tylocephale (and Pachycephalosauria) (1974), Bagaceratops (1975), and Barsboldia (1981)
and with Osmόlska and Altangerel Perle, Goyocephale (1982).Alan Feduccia notes that Maryanska and her colleagues (Osmólska and Wolsan) produced in 2002 the "most impressive analysis of the oviraptorosaurs".Amongst her many publications are contributions to three chapters of the 2nd edition of the highly respected The Dinosauria: the chapters on the Therizinosauroidea, the Ankylosauria and on the Pachycephalosauria.As of 2004, she was affiliated with the Muzeum Ziemi of the Polska Akademia Nauk. She was assistant director of Muzeum Ziemi.Texacephale
Texacephale 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. The generic name means Texas + "head" (kephale in Greek) in reference to its place of discovery, and the specific name honors Wann Langston.Wannanosaurus
Wannanosaurus (meaning "Wannan lizard", named after the location where it was discovered) is a genus of basal pachycephalosaurian dinosaur from the Campanian-age Upper Cretaceous Xiaoyan Formation, about 80 million years ago (mya) in what is now Anhui, China. The type species, Wannanosaurus yansiensis, was described by Hou Lian-Hai in 1977.It is known from a single partial skeleton, including a partial skull roof and lower jaw, an femur and tibia, part of a rib, and other fragments. Because it has a flat skull roof with large openings, it has been considered primitive among pachycephalosaurs. Sometimes it has been classified as a member of the now-deprecated family Homalocephalidae, now thought to be an unnatural assembly of pachycephalosaurians without domed skulls.Although its remains are from a very small individual, with a femur length of ~8 centimeters (3.1 in) and an estimated overall length of about 60 cm (2 ft), the fused bones in its skull suggest that it was an adult at death. Like other pachycephalosaurians, it was probably herbivorous or omnivorous, feeding close to the ground on a variety of plant matter, and possibly insects as well.