Pachycephalosaurus is a genus of pachycephalosaurid dinosaurs. The type
species, P. wyomingensis, is the only known species. 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 primarily known from a single skull and a few extremely thick
skull roofs, though more complete fossils have been found in recent years.
Pachycephalosaurus was one of the last non-avian dinosaurs before the
Cretaceous–Paleogene extinction event. Another dinosaur, Tylosteus of western
North America, has been synonymized with Pachycephalosaurus.
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal
omnivore with an extremely thick skull 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.
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 25 cm 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 vision and was capable of binocular vision. 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.
Pachycephalosaurus was probably bipedal and was the largest of the
pachycephalosaurid dinosaurs. It has been estimated that Pachycephalosaurus
was around 4.5 metres long and weighed 450 kilograms. 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. Classification and systematics
Pachycephalosaurus gives its name to the Pachycephalosauria, a clade of
herbivorous ornithischian dinosaurs which lived during the Late Cretaceous
Period in North America and Asia. Despite their bipedal stance, they were
likely more closely related to the ceratopsians than the ornithopods.
Pachycephalosaurus is the most famous member of the Pachycephalosauria. The
clade also includes Stenopelix, Wannanosaurus, Goyocephale, Stegoceras,
Homalocephale, Tylocephale, Sphaerotholus and Prenocephale. Within
the tribe Pachycephalosaurini, Pachycephalosaurus is most closely
related to Dracorex and Stygimoloch, although these may be juvenile forms of
Pachycephalosaurus. Below is a cladogram modified from Evans
et al., 2013. Discovery and history
Remains attributable to Pachycephalosaurus may have been found
as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand
Vandiveer 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.
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. It
became known as Tylosteus. Its actual nature was not found until Baird
restudied it over a century later and identified it as a squamosal 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 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
information. 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. P. wyomingensis, the type and currently
only valid species of Pachycephalosaurus, was named by Charles
W. Gilmore in 1931. He coined it for the partial skull USNM 12031, from the Lance
Formation of Niobrara County, Wyoming. Gilmore assigned his new species to
Troodon as T. wyomingensis. At the time, paleontologists thought that Troodon,
then known only from teeth, was the same as Stegoceras, which had similar teeth.
Accordingly, what are now known as pachycephalosaurids were assigned to the
family Troodontidae, a misconception not corrected until 1945, by Charles M.
Sternberg. In 1943, Barnum Brown and Erich Maren
Schlaikjer, with newer, more complete material, established the genus
Pachycephalosaurus. They named two species: Pachycephalosaurus grangeri,
the type species of the genus Pachycephalosaurus, and
Pachycephalosaurus reinheimeri. P. grangeri was based on AMNH 1696, a
nearly complete skull from the Hell Creek Formation of Ekalaka, Carter
County, Montana. P. reinheimeri was based on what is now DMNH 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. Paleobiology
Nearly all Pachycephalosaurus fossils have been recovered from the Lance
Formation and Hell Creek Formation of the western United States.
Pachycephalosaurus possibly co-existed alongside additional pachycephalosaur
species of the genera Dracorex and Stygimoloch, though these may represent
juveniles of Pachycephalosaurus itself. Other dinosaurs that shared its time and
place include Thescelosaurus, the hadrosaurid Edmontosaurus, ceratopsid
Triceratops, Torosaurus, ankylosaurid Ankylosaurus, and the theropods
Ornithomimus, Struthiomimus, Troodon, and Tyrannosaurus.
Scientists once suspected that Pachycephalosaurus and its relatives
were the bipedal equivalents of bighorn sheep or musk oxen; that male
individuals would ram each other headlong. It was also believed that they
would make their head, neck, and body horizontally straight, in order to
transmit stress during ramming. However, it is now believed that the
pachycephalosaurs could not have used their domes in this way.
Foremost, the skull roof could probably not have adequately sustained impact
associated with such ramming. Also, there is no definitive evidence of scars
or other damage on fossilized Pachycephalosaurus skulls, although
recent findings may contradict this. 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 direct head-butting.
Lastly, the rounded shape of the skull would lessen the contacted surface area
during head-butting, resulting in glancing blows. However, CT scan
comparsions of the skulls of Stegoceras validum, Prenocephale prenes and several
head-striking artiodactyls supported pachycephalosaurids being well-equipped
for head-butting. Alternatively, Pachycephalosaurus and
other pachycephalosaurid genera engaged in flank-butting in 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 width of most
pachycephalosaurs, a trait that would have protected vital organs from harm.
The flank-butting theory was first proposed by Sues in 1978, and expanded
upon by Ken Carpenter in 1997. = Growth=
The pachycephalosaurs Dracorex and Stygimoloch may be specimens of
Pachycephalosaurus in which the dome and horns are not well-developed, either
because the animal was a juvenile or a female. This consideration was supported
at the 2007 annual meeting of the Society of Vertebrate Paleontology. 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. 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, lead them to conclude that Dracorex and
Stygimoloch were simply juvenile Pachycephalosaurus, which lost spikes
and grew domes as they aged. A 2010 study by Nick Longrich and colleagues
also supported the hypothesis that all flat-skulled pachycephalosaurs were
juveniles, suggesting that flat-skulled forms like Goyocephale and Homalocephale
represent juveniles of dome-skulled adults.
= Paleopathology= 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 artifacts and bone
absorptions may instead have been due to such behavior. Peterson et al. 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. 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. 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. 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.
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. = Diet=
Scientists do not yet know what these dinosaurs ate. Having very small, ridged
teeth they could not have chewed tough, fibrous plants as effectively as other
dinosaurs of the same period. It is assumed that pachycephalosaurs lived on
a mixed diet of leaves, seeds, fruit and insects. The sharp, serrated teeth would
have been very effective for shredding plants.
See also Timeline of pachycephalosaur research
References External links
Pachycephalosaurus in the Dinodictionary Pachycephalosaurus wyomingensis from
National Geographic Online TEDx talk by Jack Horner on
shape-shifting dinosaur skulls and dinosaur misclassification.