Pachycephalosaurus
Name:
Pachycephalosaurus
(thick headed lizard).
Phonetic: Pak-ee-sef-ah-low-sore-us.
Named By: Barnum Brown & Erich Maren
Schlaikjer - 1943.
Synonyms: Tylosteus, Troodon wyomingensis.
Possibly also Dracorex and Stygimoloch
if these two genera do indeed represent juvenile forms.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Pachycephalosauridae, Pachycephalosaurinae,
Pachycephalosaurini.
Species: P. wyomingensis
(type).
Diet: Herbivore/Omnivore?
Size: Uncertain due to lack of remains, but
comparison to more complete genera has yielded estimates of around
4.5 meters long.
Known locations: USA.
Time period: Maastrichtian of the Cretaceous.
Fossil representation: Skull remains.
Once again one of the most famous dinosaurs of all time is actually represented by some of the most incomplete fossil material. To date Pachycephalosaurus is only represented by skull material; the actual appearance of the body so often seen in restorations is actually based upon the common form seen in more complete relatives. Although it may seem strange that a dinosaur genus based upon such few remains should become so popular, there are other very famous genera that are also based upon largely incomplete remains, with Ankylosaurus and Spinosaurus being just two such examples.
Pachycephalosaurus
as a dinosaur
The
reason why Pachycephalosaurus has persevered to
become one of the most
famous and best loved dinosaurs is because of the very thick dome
that grew on top of its skull. This dome grew to be made of bone
almost as much as twenty-five centimetres thick and seems to have grown
to protect the brain. This has led to the popular depiction of
Pachycephalosaurus head butting one another during
dominance contests
and possibly even head butting predators to defend themselves. This
supposed behaviour is also part of the reason why Pachycephalosaurus
has become so popular, though today it is an idea that has been
questioned.
The
biggest source of the controversy regarding the head butting theory for
Pachycephalosaurus comes from a 2004 study by
Goodwin and Horner,
though there are other detractors to the head butting theory. One of
the main concerns is that the bone structure of the dome is not
entirely solid but kind of spongy inside. Although this would have
certainly reduced the weight of the dome structure and subsequent
stress supporting it, it is believed that it would have effectively
crumbled under the force of successive blows. The shape of the dome
has also been criticised for not being the right form for effective
head on head blows and although the skeleton of Pachycephalosaurus
is
not known, its relatives are all known to have poor neck and back
support for initiating head on head blows.
Despite
these misgivings however there have been studies that have lent further
support to the idea that Pachycephalosaurus used
its head as a weapon,
such as Snivley and Cox in 2008, Lehman in 2010 and Snivley
and Theodore in 2011. These studies centre on new analysis of
Pachycephalosaurus skulls including biomechanical
models that indicate
that the heads would have been quite effective at delivering blows.
However it should be pointed out at this point that the vast majority
of palaeontologists who believe that Pachycephalosaurus
was a head
butter do not think that it butted head to head.
A
reasonable alternative to the head butting theory is actually the flank
butting theory. What this means is that rather than bash heads, two
Pachycephalosaurus would square up to one another
and then try to use
their heads to hit each other in the sides of the body. This idea is
more popular amongst palaeontologist because it gives some explanation
as to the form of the dome while also taking into account its apparent
lack of suitability for striking hard objects.
Therefore
the most commonly accepted scenario for Pachycephalosaurus
is that an
individual would start life with a flat head, and then have the skull
changing form into that of the dome like structure that we know. This
would signal the sexual maturity of an individual and serve as a visual
display with perhaps the largest and most developed domes belonging to
the most mature individuals. When visual signals and perhaps audial
calls did not work to settle the rivalry, two individuals struck each
other in the sides with their domed heads until one of them gave in
and conceded victory to the other.
Because
of the lack of skeletal remains it is hard to know much more than what
we already do about Pachycephalosaurus. The size
of the skull however
does actually indicate that Pachycephalosaurus is
the largest currently
known pachycephalosaurid.
The precise feeding behaviour of
Pachycephalosaurus is also uncertain because of the
very small teeth.
The jaws however are also short and end in a beak which might suggest
that Pachycephalosaurus was more of a selective
browser, only picking
off the parts of plants that were the most palatable.
Pachycephalosaurus has also been considered to have
included
invertebrates like insects into its diet. Pachycephalosaurus
is also
believed to have had very good vision, with large forward facing eyes
that would have allowed Pachycephalosaurus to have
stereoscopic
vision. What this means is that Pachycephalosaurus
would have had the
ability to effectively gauge distances in front of it with a high
degree of accuracy.
Classification issues
Today
Pachycephalosaurus is the type genus of the
Pachycephalosauridae,
which means that all dinosaur genera similar in form to
Pachycephalosaurus are referred to as
pachycephalosaurids (though
further sub-divisions can and are sometimes made depending upon the
context of the study). However when Pachycephalosaurus
was first
named it was actually described as a troodont, a member of the
troodontidae. The type genus of this group was Troodon,
a genus
that at the time of the description of Pachycephalosaurus
was only
known by teeth. These teeth were similar to those seen in what were
to become pachycephalosaurids, so it seemed correct at the time.
Later
in the in the twentieth century however, the teeth of Troodon
were
found to be a precise match for another dinosaur genus called
Stenonychosaurus,
a saurischian theropod that
although bipedal was
very different to pachycephalosaurids. With the realisation that
Troodon was not a pachycephalosaurid it could no
longer be used to
establish the type genus of the group. With the next genus name
having seniority technically being Pachycephalosaurus,
this genus
became the type used to establish the new group Pachycephalosauridae
in 1974.
In
the above paragraph it was stated that Pachycephalosaurus
has technical
seniority to be used as the type genus of the group. This is because
the oldest Pachycephalosaurus fossils were actually
first described
by Joseph Leidy in 1872 as a different genus called Tylosteus.
Tylosteus was based upon the description of what
would later be
identified as the squamosal bone from a Pachycephalosaurus
skull, but when
first described it was believed to have been a piece of dermal armour
from some kind of armoured reptile. Fast forward to the late
twentieth century and a new study of this bone by Donald Baird
revealed its real identity, but also raised a troubling prospect.
Under standard rules and procedures governed by the ICZN
(International Commission on Zoological Nomenclature) the first name
established usually has priority over any subsequent names; however
Baird actually managed to get a special exception made for
Pachycephalosaurus.
There
is a special clause where if it can be proven that the second name
for a genus has been in regular scientific use, while the first
name has not (in essence being almost forgotten) then an
application for protected status can be made for the second name. At
the time of this discovery Pachycephalosaurus was
already one of the
most popular dinosaurs talked about and was now the defining type
genus of the Pachycephalosauridae. Tylosteus was
virtually unknown
with perhaps only a handful of paleontologists and museum curators
knowing about it and none of them writing about it. For these reasons
the ICZN granted Pachycephalosaurus protected
status as a nomen
protectum (literally ‘name protected’) in 1985. Although this
is a very rare sequence of events, exactly the same argument was
successfully made for the most famous dinosaur genus of all time,
Tyrannosaurus,
when it was realised that Tyrannosaurus was
actually
the second name given to this genus.
Pachycephalosaurus
will remain a valid genus, however it is possible that other
pachycephalosaurid genera may end up getting absorbed into it.
The main two genera in question are Stygimoloch
named in 1983 and
Dracorex named in 2006. Neither Stygimoloch
nor Dracorex have the
large domes of the skull of Pachycephalosaurus,
but there is
compelling speculation that explains how all of these genera are
related. First of all, all three of these genera are known to have
lived at the same time and locations as one another. Dracorex
and
Stygimoloch are both represented only by the remains
of juveniles,
while Pachycephalosaurus however is only known by
the remains of
adults. Wider study of pachycephalosaurids in general seems to point
to a continuing trait where all juveniles had flat heads, while the
dome like structure seen in adults only developed when an individual
became fully mature. This is backed up by further study that the
bones of pachycephalosaurid skulls could change considerably in form as
an individual matured.
Unfortunately
while many palaeontologists are of the opinion that both Stygimoloch
and Dracorex should be regarded as juvenile Pachycephalosaurus,
there
are not currently any transitional fossil forms that could conclusively
prove this without doubt. To blur things even further, the
squamosal bone fragment once described as Tylosteus
has since been
considered to actually belong to Dracorex. If
correct then this would
make the protected name status for Pachycephalosaurus
unnecessary,
but it could also be a moot point if Dracorex is
indeed based upon
juveniles of Pachycephalosaurus.
Further reading
- A new species of troodont dinosaur from the Lance Formation of
Wyoming - Proceedings of the United States National Museum 79 (9): 1–6
- Charles W. Gilmore - 1931.
- A study of the tro�dont dinosaurs with the description of a new
genus and four new species, Barnum Brown & Erich Maren
Schlaikjer - 1943.
- New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia)
from North America - Canadian Journal of Earth Sciences 25: 462–472 -
Peter M. Galton & Hans-Dieter Sues - 1983.
- Pachycephalosaurus Brown & Schlaikjer,
1943 and Troodon
wyomingensis Gilmore, 1931 (Reptilia, Dinosauria): Conserved
- Bulletin
of Zoological Nomenclature, 43 (1) - ICZN Opinion 1371 - 1986.
- Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a
new look at head-butting behavior - Contributions to Geology 32 (1):
19–25 - Kenneth Carpenter - 1997.
- Cranial histology of pachycephalosaurs (Ornithischia:
Marginocephalia) reveals transitory structures inconsistent with
head-butting behavior - Paleobiology 30 (2): 253–267 - Mark Goodwin
& John R. Horner - 2004.
- Structural mechanics of pachycephalosaur crania permitted
head-butting behavior, E. Snivley & A. Cox - 2008.
- Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur
Pachycephalosaurus - PLoS ONE 4(10) - J. R. Horner & M. B.
Goodwin - 2009.
- Pachycephalosauridae from the San Carlos and Aguja Formations
(Upper Cretaceous) of west Texas, and observations of the
frontoparietal dome, T. M. Lehmen - 2010.
- Common Functional Correlates of Head-Strike Behavior in the
Pachycephalosaur Stegoceras validum
(Ornithischia, Dinosauria)
and Combative Artiodactyls, E. Snivley & J. M.
Theodor - 2011. K. Carpenter. ed.
- Cranial Pathologies in a Specimen of Pachycephalosaurus
- In Farke,
Andrew A. PLoS ONE 7 (4) - J. E. Peterson & C. P. Vittore -
2012.
- Distributions of Cranial Pathologies Provide Evidence for
Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae) - PLoS ONE
8(7) - J. E. Peterson, C. Dischler & N. R. Longrich - 2013.
- The early expression of squamosal horns and parietal ornamentation
confirmed by new end-stage juvenile Pachycephalosaurus
fossils from the
Upper Cretaceous Hell Creek Formation, Montana. - Journal of Vertebrate
Paleontology. 36 (2). - Mark B. Goodwin & David C. Evans -
2016.
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