Pachycephalosaurus lived over 66 million years ago in the Late Cretaceous and today is remembered for its distinctive skull plate— inches thick bone designed to withstand serious blunt force. With a ring of stubby horns around the plate, Pachycephalosaurs was capable of cracking its skull against surfaces with little damage to itself. This made it one of the most terrifying dinosaurs to come across in its time.

The exact use of the hard-headed skull isn't clear. Theories range from defensive attacks against predators, butting heads with rival males, and using it as an intimidation display. One thing is clear though: you wouldn't want to find yourself on the wrong end of Pachycephalosaurus's battering ram.

This specimen is a fossilized piece of a Pachycephalosaurus skull dome. The material was recovered on private land in the Hell Creek Formation of South Dakota and is estimated to be over 66,000,000 years old.

Each specimen comes in a glass-topped riker display box. The box measures 4 1/2" x 3 1/2". A small information card is also included, which serves as the certificate of authenticity.

Both Small and Medium sized pieces are available. Smalls come in a sealed gem jar container and measure around 5 x 5mm. Medium sized pieces are shipped in bubble wrap and measure at least 1/2 inch in length.

Across the tens of millions of years that dinosaurs roamed the face of the Earth, evolutionary forces produced many unique adaptations in the reptile group. One of the most unique of these adaptations was the iconic Pachycephalosaurus’s bone helmet. This incredible dinosaur had a thick skull roof protruding from its head with several spikes around the base. This dome’s exact use remains a matter of debate, with explanations ranging from its use in combat, males competing for mates, and intimidation displays.

Named for its distinctive headgear, Pachycephalosaurus (“thick-headed lizard”) walked the Earth during the Late Cretaceous Period, over 66 million years ago, up until the Chicxulub impact. The dinosaur was a herbivore and was likely bipedal, with two powerful back legs and shorter arms. Very few remains of Pachycephalosaurus have been discovered beyond its distinctive skulls, leading to much speculation on the specifics of the animal’s behavior and appearance.

Pachycephalosauria and the horned ceratopsia (such as triceratops) together form a group of dinosaurs called the Marginocephalia, all of whom have a bony ridge or shelf at the back of the skull.

Pachycephalosaurus’s dome was tinged with knobs and spikes. Some surviving skulls have stress-induced fractures, suggesting their use in combat, but these domes’ use were not limited to fighting. They were also likely used in what’s called agonistic behavior: the posturing and movements used as displays of threats to rival males.

Beautifully articulated skeletons suggest that in Asia the earliest of these bipedal herbivores lived in a Sahara-like desert with ephemeral streams and small drainage basins. Contrast this with North America, where the dinosaur's remains are found in sediments from a Cretaceous coastal plain deposited during a temperate climate. The frequency of finding isolated skull caps suggests water transport down rivers draining from the rising Rocky Mountains. This implies that Pachycephalosaurus likely lived in the foothills of mountains which were supplying sediment to the coast at that time.

Much remains unknown about Pachycephalosaurus. Owing to the limited number of remains found, much of what we know about the dinosaur relies on a degree of speculation. But what we do know is a reminder on the breadth of evolutionary adaptation to produce something as unique as the Pachycephalosaurus’s bone helmet.