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Temporal range: Late Cretaceous, 70–66 Ma PreꞒ O S D C P T J K Pg N


Scientific classification PachycephalosaurusPachycephalosaurus_header_cell_0_1_0
Kingdom:Pachycephalosaurus_cell_0_2_0 AnimaliaPachycephalosaurus_cell_0_2_1
Phylum:Pachycephalosaurus_cell_0_3_0 ChordataPachycephalosaurus_cell_0_3_1
Clade:Pachycephalosaurus_cell_0_4_0 DinosauriaPachycephalosaurus_cell_0_4_1
Order:Pachycephalosaurus_cell_0_5_0 OrnithischiaPachycephalosaurus_cell_0_5_1
Family:Pachycephalosaurus_cell_0_6_0 PachycephalosauridaePachycephalosaurus_cell_0_6_1
Tribe:Pachycephalosaurus_cell_0_7_0 PachycephalosauriniPachycephalosaurus_cell_0_7_1
Genus:Pachycephalosaurus_cell_0_8_0 †Pachycephalosaurus

Brown & Schlaikjer, 1943Pachycephalosaurus_cell_0_8_1


Pachycephalosaurus (/ˌpækɪˌsɛfələˈsɔːrəs/; meaning "thick-headed lizard," from Greek pachys-/παχύς- "thick", kephale/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid dinosaurs. Pachycephalosaurus_sentence_0

The type species, P. wyomingensis, is the only known species. Pachycephalosaurus_sentence_1

It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Pachycephalosaurus_sentence_2

Remains have been excavated in Montana, South Dakota, Wyoming and Alberta. Pachycephalosaurus_sentence_3

It was a herbivorous creature which is primarily known from a single skull and a few extremely thick skull roofs, though more complete fossils have been found in recent years. Pachycephalosaurus_sentence_4

Pachycephalosaurus was one of the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event. Pachycephalosaurus_sentence_5

Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex in recent studies. Pachycephalosaurus_sentence_6

Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. Pachycephalosaurus_sentence_7

It possessed long hindlimbs and small forelimbs. Pachycephalosaurus_sentence_8

Pachycephalosaurus is the largest-known pachycephalosaur. Pachycephalosaurus_sentence_9

The thick skull domes of Pachycephalosaurus and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. Pachycephalosaurus_sentence_10

This hypothesis has been disputed in recent years. Pachycephalosaurus_sentence_11

History of discovery Pachycephalosaurus_section_0

Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. Pachycephalosaurus_sentence_12

As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandeveer Hayden, 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 in southeastern Montana. Pachycephalosaurus_sentence_13

This specimen, now ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal. Pachycephalosaurus_sentence_14

It became known as Tylosteus. Pachycephalosaurus_sentence_15

Its actual nature was not found until Baird restudied it over a century later and identified it as a squamosal (bone from the back of the skull) of Pachycephalosaurus, including a set of bony knobs corresponding to those found on other specimens of Pachycephalosaurus. Pachycephalosaurus_sentence_16

Because the name Tylosteus predates Pachycephalosaurus, according to the International Code of Zoological Nomenclature Tylosteus would normally be preferred. Pachycephalosaurus_sentence_17

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 information. Pachycephalosaurus_sentence_18

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. Pachycephalosaurus_sentence_19

The issue is of uncertain importance, though, if Dracorex actually represents a juvenile Pachycephalosaurus, as has been recently proposed. Pachycephalosaurus_sentence_20

P. wyomingensis, the type and currently only valid species of Pachycephalosaurus, was named by Charles W. Gilmore in 1931. Pachycephalosaurus_sentence_21

He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County, Wyoming. Pachycephalosaurus_sentence_22

Gilmore assigned his new species to Troodon as T. wyomingensis. Pachycephalosaurus_sentence_23

At the time, paleontologists thought that Troodon, then known only from teeth, was the same as Stegoceras, which had similar teeth. Pachycephalosaurus_sentence_24

Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae, a misconception not corrected until 1945, by Charles M. Sternberg. Pachycephalosaurus_sentence_25

In 1943, Barnum Brown and Erich Maren Schlaikjer, with newer, more complete material, established the genus Pachycephalosaurus. Pachycephalosaurus_sentence_26

They named two species: Pachycephalosaurus grangeri, the type species of the genus Pachycephalosaurus, and Pachycephalosaurus reinheimeri. Pachycephalosaurus_sentence_27

P. grangeri was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka, Carter County, Montana. Pachycephalosaurus_sentence_28

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. Pachycephalosaurus_sentence_29

They also referred the older species "Troodon" wyomingensis to their new genus. Pachycephalosaurus_sentence_30

Their two newer species have been considered synonymous with P. wyomingensis since 1983. Pachycephalosaurus_sentence_31

In 2015, some pachycephalosaurid material and a domed parietal attributable to Pachycephalosaurus were discovered in Scollard Formation, Alberta, Canada, implying dinosaurs of this era were cosmopolitan and didn't have discrete faunal provinces. Pachycephalosaurus_sentence_32

Description Pachycephalosaurus_section_1

The anatomy of Pachycephalosaurus is poorly known, as only skull remains have been described. Pachycephalosaurus_sentence_33

Pachycephalosaurus is famous for having a large, bony dome atop its skull, up to 25 cm (10 in) thick, which safely cushioned its tiny brain. Pachycephalosaurus_sentence_34

The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. Pachycephalosaurus_sentence_35

The spikes were probably blunt, not sharp. Pachycephalosaurus_sentence_36

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. Pachycephalosaurus_sentence_37

Pachycephalosaurus had a small muzzle which ended in a pointed beak. Pachycephalosaurus_sentence_38

The teeth were tiny, with leaf-shaped crowns. Pachycephalosaurus_sentence_39

The head was supported by an "S"- or "U"-shaped neck. Pachycephalosaurus_sentence_40

Younger individuals of Pachycephalosaurus maybe have had flatter skulls, with larger horns projecting from the back of the skull. Pachycephalosaurus_sentence_41

As the animal grew, the horns shrunk and rounded out, as the dome grew. Pachycephalosaurus_sentence_42

Pachycephalosaurus was probably bipedal and was the largest of the pachycephalosaurid (bone-headed) dinosaurs. Pachycephalosaurus_sentence_43

It has been estimated that Pachycephalosaurus was about 4.5 metres (14.8 ft) long and weighed about 450 kilograms (990 lb). Pachycephalosaurus_sentence_44

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 tendons. Pachycephalosaurus_sentence_45

Classification Pachycephalosaurus_section_2

Pachycephalosaurus gives its name to the Pachycephalosauria, a clade of herbivorous ornithischian ("bird hipped") dinosaurs which lived during the Late Cretaceous Period in North America and Asia. Pachycephalosaurus_sentence_46

Despite their bipedal stance, they were likely more closely related to the ceratopsians than the ornithopods. Pachycephalosaurus_sentence_47

Pachycephalosaurus is the most famous member of the Pachycephalosauria (though not the best-preserved member). Pachycephalosaurus_sentence_48

The clade also includes Stenopelix, Wannanosaurus, Goyocephale, Stegoceras, Homalocephale, Tylocephale, Sphaerotholus and Prenocephale. Pachycephalosaurus_sentence_49

Within the tribe Pachycephalosaurini, Pachycephalosaurus is most closely related to Alaskacephale. Pachycephalosaurus_sentence_50

Dracorex and Stygimoloch have been synonymized with Pachycephalosaurus. Pachycephalosaurus_sentence_51

Below is a cladogram modified from Evans et al., 2013. Pachycephalosaurus_sentence_52

Paleobiology Pachycephalosaurus_section_3

Growth Pachycephalosaurus_section_4

Dracorex and Stygimoloch were first proposed to be juvenile or female morphologies of Pachycephalosaurus at the 2007 annual meeting of the Society of Vertebrate Paleontology. Pachycephalosaurus_sentence_53

Jack Horner of Montana State University presented evidence, from analysis of the skull of the single existing Dracorex specimen, that this dinosaur may well be a juvenile form of Stygimoloch. Pachycephalosaurus_sentence_54

In addition, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus. Pachycephalosaurus_sentence_55

Horner and M.B. Pachycephalosaurus_sentence_56

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. Pachycephalosaurus_sentence_57

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. Pachycephalosaurus_sentence_58

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. Pachycephalosaurus_sentence_59

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. Pachycephalosaurus_sentence_60

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. Pachycephalosaurus_sentence_61

Dome function Pachycephalosaurus_section_5

It has been commonly hypothesized that Pachycephalosaurus and its relatives were the bipedal equivalents of bighorn sheep or musk oxen, 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. Pachycephalosaurus_sentence_62

However, there have also been alternative suggestions that the pachycephalosaurs could not have used their domes in this way. Pachycephalosaurus_sentence_63

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). Pachycephalosaurus_sentence_64

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. Pachycephalosaurus_sentence_65

Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Pachycephalosaurus_sentence_66

Alternatively, Pachycephalosaurus and other pachycephalosaurid genera may have engaged in flank-butting during intraspecific combat. Pachycephalosaurus_sentence_67

In this scenario, an individual may have stood roughly parallel or faced a rival directly, using intimidation displays to cow its rival. Pachycephalosaurus_sentence_68

If intimidation failed, the Pachycephalosaurus would bend its head downward and to the side, striking the rival pachycephalosaur on its flank. Pachycephalosaurus_sentence_69

This hypothesis is supported by the relatively broad torso of most pachycephalosaurs, which would have protected vital organs from trauma. Pachycephalosaurus_sentence_70

The flank-butting theory was first proposed by Sues in 1978, and expanded upon by Ken Carpenter in 1997. Pachycephalosaurus_sentence_71

In 2012, a study showed that cranial pathologies in a P. wyomingensis specimen were likely due to agonistic behavior. Pachycephalosaurus_sentence_72

It was also proposed that similar damage in other pachycephalosaur specimens previously explained as taphonomic artifacts and bone absorptions may instead have been due to such behavior. Pachycephalosaurus_sentence_73

Peterson et al. Pachycephalosaurus_sentence_74

(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. Pachycephalosaurus_sentence_75

This high rate of pathology lends more support to the hypothesis that pachycephalosaurid domes were employed in intra-specific combat. Pachycephalosaurus_sentence_76

Pachycephalosaurus wyomingensis specimen BMR P2001.4.5 was observed to have 23 lesions in its frontal bone and P. wyomingensis specimen DMNS 469 was observed to have 5 lesions. Pachycephalosaurus_sentence_77

The frequency of trauma was comparable across the different genera in the pachycephalosaurid 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. Pachycephalosaurus_sentence_78

These findings were in stark contrast with the results from analysis of the relatively flat-headed pachycephalosaurids, where there was an absence of pathology. Pachycephalosaurus_sentence_79

This would support the hypothesis that these individuals represent either females or juveniles, where intra-specific combat behavior is not expected. Pachycephalosaurus_sentence_80

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. Pachycephalosaurus_sentence_81

Peterson et al. Pachycephalosaurus_sentence_82

(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. Pachycephalosaurus_sentence_83

CT scan comparisons of the skulls of Stegoceras validum, Prenocephale prenes, and several head-striking artiodactyls have also supported pachycephalosaurids as being well-equipped for head-butting. Pachycephalosaurus_sentence_84

Diet Pachycephalosaurus_section_6

Scientists do not yet know what these dinosaurs ate. Pachycephalosaurus_sentence_85

Having very small, ridged teeth, they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. Pachycephalosaurus_sentence_86

It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, and fruits. Pachycephalosaurus_sentence_87

The sharp, serrated teeth would have been very effective for shredding plants. Pachycephalosaurus_sentence_88

It is also suspected that the dinosaur may have included meat in its diet. Pachycephalosaurus_sentence_89

The most complete fossil jaw shows that it had serrated blade-like front teeth, reminiscent of those of carnivorous theropods. Pachycephalosaurus_sentence_90

Paleoecology Pachycephalosaurus_section_7

Nearly all Pachycephalosaurus fossils have been recovered from the Lance Formation and Hell Creek Formation of the western United States. Pachycephalosaurus_sentence_91

Pachycephalosaurus possibly coexisted alongside additional pachycephalosaur species of the genera Sphaerotholus, as well as Dracorex and Stygimoloch, though these last two genera may represent juveniles of Pachycephalosaurus itself. Pachycephalosaurus_sentence_92

Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus and a possible species of Parasaurolophus, ceratopsids like Triceratops, Torosaurus, Nedoceratops, Tatankaceratops and Leptoceratops, ankylosaurids Ankylosaurus, nodosaurids Denversaurus and Edmontonia, and the theropods Acheroraptor, Dakotaraptor, Ornithomimus, Struthiomimus, Anzu, Leptorhynchos, Pectinodon, Paronychodon, Richardoestesia and Tyrannosaurus. Pachycephalosaurus_sentence_93

See also Pachycephalosaurus_section_8


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