MEANING: Thick-headed lizard
PERIOD: Late Cretaceous
CONTINENT: North America
Pachycephalosaurus is a bipedal ornithischian dinosaur with an extremely thick skull roof. The largest of the pachycephalosaurid dinosaurs, it was about 4.5 m long, and weighed around 400 kg. Pachycephalosaurus is most well known for the large bony dome atop its skull. The dome was up to 25 cm thick and gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat, though this hypothesis has been disputed in recent years.
Pachycephalosaurus is from the Cretaceous. The Cretaceous is a geological period that lasted from about 145 to 66 million years ago. It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, "chalk", which is abundant in the latter half of the period.
The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct flora and fauna, while dinosaurs continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.
The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.
Pachycephalosaurus is a pachycephalosaur. Pachycephalosauria, meaning "thick headed lizards" is a group of ornithischian dinosaurs. Along with the ceratopsians, they make up the clade Marginocephalia. They almost all lived during the Late Cretaceous Period, and are exclusive to the Northern Hemisphere. Pachycephalosaurians are bipedal, herbivorous animals with thick skulls. These bony skulls can be domed, flat, or wedge-shaped depending on the species, and were often surrounded by nodes and spikes. There is some debate why pachycephalosaurians had such heavily armored heads, but one leading hypothesis is that they were used in intra-species combat, similar to modern-day goats.