Scientific Classification
Kingdom: Animalia
Phylum: Chordata
SuperOrder: Dinosauria
Order: Ornithischia
Family: Pachycephalosauridae
Genus: Pachycephalosaurus

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The Pachycephalosaurus was medium sized dinosaur that lived on the earth in the late Cretaceous period. Its fossils have been recovered from North America, in Lance Formation region of Montana. These remains date back some 72 to 66 million years ago. This period lies in the Maastrichtian age, right at the boundary of the Cretaceous and Paleogene periods. This means that could have been present during the extinction event at the end of the Cretaceous, to be eliminated because of it.
This dinosaur is known by its extremely thick cranial bones. They were possibly flat in its growing years and formed a dome shape as it matured. The function of this adaptation is debated by scientists even today.

The size of the Pachycephalosaurus was possible moderate; since it is only known by a skull fossils, its length can only be calculated based on comparisons with other similar dinosaurs. But researchers have estimated its length to be 14 to 16 feet and its weight about 500 kilos. This would make its size comparable to modern day bulls. It is still the largest known pachycephalosaurid.

The feeding habits of the dinosaur are not well established. It possibly was an herbivore or an omnivore. But it certainly could not have been a meat eating dinosaur.

Etymology

The name Pachycephalosaurus has three different components. The term ‘pachy’ is the Greek word of ‘dense’ or ‘thick’. The term ‘cephalo’ is derived from the Greek word ‘kefali’ which means ‘head’; this term is used frequently in medical terminology for denoting the head or brain. The suffix ‘saurus’ is again derived from the Greek word for ‘lizard’, ‘sauros’. Thus, the name of this dinosaur translates to ‘the lizard with the thick head’. It makes reference to the hard cranial dome that the dinosaur possessed.

The specific name P. wyomingensis is a nod to the state of Wyoming where the remains of the dinosaur were first discovered.

The generic name was coined by Barnum Brown and Erich Schlaikjer while the specific name was coined by Charles Gilmore. The dinosaur was first described by the latter however.

Discovery of and nature of fossils

• The holotype of the Pachycephalosaurus was discovered way back in 1859, by a fossil collector Ferdinand Vandiveer Hayden. He found it near the Missouri River, in a location that currently falls under the Lance Formation. It consisted of a portion of the squamosal bone of the skull.

• It was thought to belong to an armadillo like reptile at that time, as dinosaurs were not well defined them. Joseph Leidy termed it a part of the skin armor of a reptile, and named this reptile the Tylosteus.

• In 1930, a partial skull recovered from the Niobrara County of Wyoming was described by Charles Gilmore. He classified it under the genus Troodon and gave it the epithet wyomingensis.

• In 1943, Brown and Schlaikjer obtained newer skulls and partial skeletons from the Hell Creek Formation, Carter County in Montana and the Lance Formation, Corson County, South Dakota. They named these specimen the Pachycephalosaurus grangeri and reinheimeri respectively. They also reclassified the Troodon bones of Gilmore in this genus.

• Both the species named by Brown and Schlaikjer are considered synonymous with the Pachycephalosaurus today.

• It was Donald Baird that noticed the resemblance between the Tylosteus and the Pachycephalosaurus. But since the genus Tylosteus was named first, ideally the name Pachycephalosaurus should have been considered its junior synonym. But Baird petitioned that the latter name be upheld as it was more widely known and the former was not used in almost a century.

Classification

The Pachycephalosaurus is classified under order Ornithischia, family Pachycephalosauridae and tribe Pachycephalosaurini. It is clear that this dinosaur is the type genus for both the above mentioned family and tribe.

It is related to the ceratopsid dinosaurs distantly, and not the Troodon, which was a theropod.

The characteristics of the dome

The dome on the skull of the Pachycephalosaurus was about 25 cms thich and was composed of temporal bone. Most scientists believe that this dome developed as the animal reached adulthood. The dinosaurs Dracorex and Stygimoloch were similar to the Pachycephalosaurus, but were smaller in size and consisted of flattened skulls. It is theorized that both these genera are actually juvenile specimens of the Pachycephalosaurus, with the dome being underdeveloped.

Paleontologists also believe that the dome was made up of fibro-lamellar bone, which would heal very quickly after being damaged.

The function of the dome

Based on the healed fractures found on them, researchers believe that the dome was used in intraspecific combat. Some scientists believe that the Pachycephalosaurus individual butted head during fights. But the interior of the dome was spongy and hence may not have withstood such trauma. Hence, a new flank butting hypothesis is put forth, which stated that the dinosaurs targeted each other’s sides with the dome.

Some pachycephalosaurid specimens have no pathological lesions on their skull, indicating that they were either juvenile or female.

Physical features

• As very little fossil evidence is available about the Pachycephalosaurus, most of its phenotypic characteristics are extrapolated from its peers. It was about 4 to 4.5 meters long and weighed around 1000 pounds.

• The teeth of this dinosaur were extremely small and ridged.

• It had short and moderately flexible neck

• The forelimbs of the dinosaur were slender and short while its hind legs were much longer.

• The tail of this dinosaur is believed to be stiff at the base like other pachycephalosaurids.

Habits and habitat

The Pachycephalosaurus was a bipedal dinosaur. It was terrestrial and derived its nutrition from leaves, seeds and insects. It teeth were not large enough for it to be a carnivore and were not sharp enough for it be able to tear off thick vegetation. Hence it is believed that the Pachycephalosaurus was omnivorous or insectivorous.

Related and coexisting species

The Pachycephalosaurus was related to the Sphaerotholus, Alaskacephale and Amtocephale. The Dracorex and Stygimoloch could have been its juveniles.

It shared its environment with many other well-known animals, namely the Tyrannosaurus rex, Triceratops, Torosaurus, Troodon, Ankylosaurus, Ornithomimus etc.

The concluding notes

The Pachycephalosaurus is still very poorly represented by fossils. Hence, much of the information about it is derived from other similar dinosaurs. Until a better preserved skeletons are discovered, many of the characteristics of the Pachycephalosaurus shall remain unknown.

Just for fun we have a soundclip available for you to hear what a Pachycephalosaurus could've sounded like. Click to the Dinosaur Sounds area to hear it. Please note that the dinosaur sounds are only for entertainment and are not an actual fact.