Pachycephalosaurus

About Pachycephalosaurus

Characteristics: A Body Shrouded in Mystery

Pachycephalosaurus was the largest member of the Pachycephalosauridae family, boasting a thick dome-shaped parietal bone reaching up to 25cm in thickness. Although it represents the epitome of "stone-headed" dinosaurs, surprisingly, only skull fossils have been discovered so far.

Therefore, the skeletal structure of its body and limbs is inferred from close relatives such as Stegoceras.

Recent research has revealed that their bodies evolved specifically to withstand the intense impact of "head-butting". The joints of the spine were firmly locked by a complex structure called "double ridge and groove", providing high rigidity to prevent bending from impact. It is also thought that they possessed a broad pelvis, which not only housed their digestive organs but also served to anchor powerful hind leg muscles to stabilize their posture.

Growth Stages of Pachycephalosaurus: "Dracorex" and "Stygimoloch"

One of the major achievements in dinosaur research during the 2000s concerned the growth of Pachycephalosaurus. It is now widely accepted that Dracorex and Stygimoloch, which were once considered separate dinosaurs, are actually the juvenile and subadult stages of Pachycephalosaurus.

In 2009, histological analysis by paleontologists Jack Horner and Mark Goodwin revealed that Pachycephalosaurus underwent dramatic changes in head shape during its growth.

• Juvenile Stage = "Dracorex": Lacked a dome and had a completely flat skull roof, adorned with long, sharp spikes on the back of the head and snout.
• Subadult Stage = "Stygimoloch": A small dome began to rise, and the spikes on the back of the head were still present but became somewhat shorter and thicker.
• Adult Stage = "Pachycephalosaurus": An extremely thick and massive dome was completed, and the long spikes were absorbed into the bone, turning into small, blunt nodes.

It is explained that the long spikes, which functioned as a visual deterrent and defense against predators during the juvenile (Dracorex) stage, were absorbed back into the bone tissue from their bases as the dome rapidly expanded during the subadult (Stygimoloch) stage, eventually transforming into blunt nodes in adulthood.

However, while the view that "Dracorex is a juvenile Pachycephalosaurus" is almost universally agreed upon, the status of Stygimoloch is still debated. Some researchers argue that "it is unnatural for magnificent spikes to shrink as an animal grows," suggesting that "Stygimoloch might be an independent genus."

Pachycephalosaurus: The Role of the Stone Head

For many years, there was a theory that this extremely thick stone head was "just a visual display and not used for head-butting," based on the hypothesis that it "could not withstand the impact of a headbutt." However, recent research has disproven this view.

Biomechanical simulations using computers (finite element analysis) have proven that the dome of Pachycephalosaurus was structured to protect the brain by dispersing the intense shock of a collision through its arch-like shape. Furthermore, it is known that the surface of the dome during life was covered with a thick keratin sheath, which acted as a cushion.

Additionally, discoveries from "paleopathology (the study of ancient diseases and injuries in fossils)" also support the head-butting theory. Detailed examinations of adult dome fossils revealed crater-like depressions near the apex in a surprisingly high proportion—about 22%. These were traces of "osteomyelitis" (bone infection), caused by bacteria infecting tissue torn by severe blows.

From these pieces of evidence, it is now considered certain that adult Pachycephalosaurus engaged in "intraspecific combat," violently butting heads with one another over mates or territory.

Superior Sensory Functions of Pachycephalosaurus

Although its brain was relatively small, research utilizing CT scanning technology has revealed that Pachycephalosaurus possessed exceptionally advanced sensory functions.

Particularly noteworthy is its "sense of smell." By examining the brain structure (the size of the olfactory bulbs), it was discovered that the olfactory capability of Pachycephalosaurus far exceeded that of other herbivorous dinosaurs, ranking second only to the carnivorous Tyrannosaurus. Furthermore, with massive eye sockets facing forward, it also seemed to have excellent binocular vision and depth perception.

It is thought that these highly developed senses of smell and sight functioned as sensors to sniff out nutritious food in the forest and to quickly detect approaching predators to escape.

Diet Revision: From Herbivore to Omnivore

For a long time, Pachycephalosaurus was thought to be a herbivore (selective browser) that plucked leaves and fruits with its beak. However, recently, there has been a discovery that drastically changed our image of its diet.

A juvenile skull fossil reported in 2018 preserved the previously undiscovered "very front of the mouth (premaxilla)." Shockingly, it contained a row of "sharp, pointed teeth suitable for tearing meat," much like those of a carnivorous dinosaur.

Building a massive stone dome as it grew required immense energy. Therefore, it is speculated that, especially during its youth, it actively hunted small animals with high nutrient absorption efficiency to accelerate its growth. This is considered a survival strategy where its diet changed as it grew.

Paper Description and Specimen ANSP 8568

Pachycephalosaurus head-dome fossils have often been found isolated in a "rounded and worn state." Because of this, a "transport hypothesis" was once believed, which stated that "they lived in highlands such as deep mountains where fossils are unlikely to be preserved, and after death, they were washed down rivers and tumbled downstream, wearing away."

However, geological surveys have revealed that much of the wear on the domes was due to "head-butting damage" and "cracking caused by the freezing and thawing of rocks after fossilization." They had not been transported long distances, but had fossilized very close to their natural habitat of coastal plains and river floodplains. They were dinosaurs living in lowland forests alongside Triceratops and others.

Pachycephalosaurus was described as a genus in 1943. It was described by American paleontologist Barnum Brown and others based on specimen number AMNH 1696.

In fact, a fossil discovered much earlier in the 1850s, specimen number ANSP 8568, was mistakenly thought to be "the dermal armor of an armadillo-like reptile." It took about 70 years to realize this fossil was a bone of a pachycephalosaur, and even later in 2006, it was pointed out that it "might be Dracorex," making it a specimen that has been passed around between genera.