Pachycephalosauria

Pachycephalosauria is a group of dinosaurs known for their remarkably thick skulls.

The name comes from Greek words meaning "thick headed lizards." These animals lived during the Cretaceous period. They lived only in the Northern Hemisphere. Most fossils are found in North America and Asia. They belong to a larger group called Marginocephalia. This group also includes the Ceratopsia, which are the horned dinosaurs. Pachycephalosaurs were bipedal, meaning they walked on two legs. They were mostly herbivores or omnivores. This means they ate plants or a mix of plants and meat.

The bodies of these dinosaurs were built for life on the ground. They had long legs and short forelimbs, or arms. Their torsos were bulky with wide hips and a large gut cavity. This large space helped them use fermentation to digest food. They had a short, thick neck and a heavy tail. Their heads were special because of the frontoparietal bone. This bone would fuse and thicken to create a dome. In some species, the skull was a raised dome. In other species, the skull was flat or wedge-shaped. Some even had blunt horns or nodes on their heads.

Most pachycephalosaurs were relatively small animals. Most lived in a length range of 2 to 3 meters. The largest species was Pachycephalosaurus wyomingensis. This dinosaur was estimated to be 4.5 meters long. It could weigh as much as 450 kilograms.

They had very good vision due to large orbits and optic nerves. They also had a great sense of smell. This was because of their uncharacteristically large olfactory lobes. These senses helped them find food and stay safe.

Finding complete fossils of these dinosaurs is very difficult. Most researchers only find isolated skull fragments. To this day, no one has discovered a complete pachycephalosaur skeleton. This makes it hard to identify new species. Sometimes, scientists name a bone as a pachycephalosaur by mistake. For example, Majungatholus was once thought to be one. We now know it was actually a Majungasaurus, which is a theropod.

Another animal, Yaverlandia, was also reclassified as an archosaur of uncertain affinities.

There is a big debate about how these dinosaurs used their heads. One popular idea is that they used them for head-butting. This theory suggests they rammed each other like bighorn sheep.

Some scientists found evidence of strong neck muscles and spinal rigidity. However, other scientists disagree. They noted that pachycephalosaurs had an "S" or "U" shaped neck. This shape might not allow them to hold their bodies straight for ramming. Some suggest they might have used flank-butting instead. This means they might have hit each other on the side.

Another idea is that the dome was used for species recognition. Some researchers think the domes might have been brightly colored. This could help them recognize their own kind. However, other scientists argue that the domes look very similar across different species. Because of this, they think the dome must have a mechanical purpose. This could be for combat or defense.

Some studies even looked at bone health. They found that about 22% of the domes had lesions. These lesions are consistent with osteomyelitis, which is a bone infection.

Scientists have worked hard to organize the history of these animals. Maryańska and Osmólska first named the group in 1974. Later, researchers like Michael Benton changed how they were classified. Some scientists, like Robert Sullivan, studied how the skulls grew. He argued that flat-headed versions might just be young, growing dinosaurs. For example, Dracorex and Stygimoloch might actually be young Pachycephalosaurus.

This idea helps explain why we see different skull shapes. It shows that these dinosaurs changed a lot as they aged.