The dinosaur most famously recognized for its hard head is the Pachycephalosaurus. Its name, derived from Greek, literally means “thick-headed lizard,” a direct reference to its most prominent feature. This dinosaur sported a massive, bony dome on the top of its skull, which could be up to 10 inches (25 cm) thick. This dome was often bordered by bony knobs and short, blunt spikes that projected from its snout and the rear of the skull, contributing to its distinctive appearance.
Pachycephalosaurus was a bipedal herbivore. Its diet likely consisted of soft plants, fruits, and seeds, given its small, ridged teeth. This dinosaur lived during the Late Cretaceous period, approximately 76 to 65 million years ago. Fossils of Pachycephalosaurus have primarily been discovered in western North America, including areas now known as Montana, South Dakota, Wyoming, and Alberta, Canada. It was a relatively large pachycephalosaurid, estimated to be around 15 feet (4.5 meters) long and weighing between 820 to 990 pounds (370 to 450 kilograms).
Unraveling the Mystery of the Dome’s Function
The purpose of the Pachycephalosaurus’s exceptionally thick skull dome has been a long-standing subject of scientific debate. One prominent theory suggests the dome was primarily used for intraspecific combat, specifically head-butting, similar to modern bighorn sheep or musk oxen. In this scenario, males might have engaged in head-on collisions to establish dominance, secure mating rights, or defend territory.
Evidence supporting this head-butting hypothesis includes the dome’s robust structure, which appears capable of withstanding considerable impact forces. Studies have also identified lesions and signs of infection, consistent with trauma, on the fossilized domes of some pachycephalosaurids, suggesting they did engage in such high-impact behaviors. However, some paleontologists have questioned this theory, pointing to the potential for glancing blows due to the dome’s rounded shape and the S- or U-shaped curve of the neck, which might not have been suitable for absorbing direct head-on impacts.
An alternative explanation proposes that the dome served as a visual display for sexual selection or species recognition. This display could have helped individuals identify members of their own species or signal health and fitness to potential mates. Another possibility is flank-butting, where individuals would strike each other’s sides rather than head-on, a behavior supported by the broad torso of pachycephalosaurs that could have protected vital organs. It is also conceivable that the dome had multiple functions, combining both combat and display.
Beyond the Most Famous: Other Dome-Headed Dinosaurs
While Pachycephalosaurus is the most widely recognized, it was part of a larger group of dinosaurs known as Pachycephalosaurids. This family of dinosaurs, primarily found in North America and Asia, is characterized by their thick skulls, which could be domed, flat, or wedge-shaped depending on the species. These dinosaurs lived during the Late Cretaceous period, ranging from approximately 85.8 to 66 million years ago.
Other notable pachycephalosaurids include Dracorex and Stygimoloch, both of which also possessed unique cranial adaptations. Dracorex was characterized by a flat skull adorned with numerous spikes and bumps, resembling a dragon. Stygimoloch featured a smaller, narrower dome and prominent spikes at the back of its head.
There has been an ongoing scientific discussion regarding the relationship between these three genera. Some paleontologists propose that Dracorex and Stygimoloch might represent juvenile and sub-adult growth stages of Pachycephalosaurus, respectively, with their cranial features changing as they matured. However, this idea remains a subject of debate within the scientific community, with some researchers suggesting they may be distinct genera or species. Regardless of their precise taxonomic relationship, Pachycephalosaurus stands as the most extreme and well-known example of the thick-skulled dinosaurs.