Description and Classification

Homalocephale was a relatively small, plant-eating dinosaur. It grew to be about 1.8 meters (6 feet) long and likely weighed around 40 to 45 kilograms (90 to 100 pounds). It walked on two long hind legs, which suggests it could move quite quickly. Its tail was likely held stiffly off the ground to help with balance.

The most studied feature of Homalocephale is its skull. The top of its skull was thick, solid bone, but instead of forming a high dome like its relative Stegoceras, it was remarkably flat and wide. The surface of this thickened area was ornamented with small bumps and pits. Homalocephale also had large eye sockets, indicating it probably had good eyesight.

Homalocephale belongs to the group Pachycephalosauria, often called “bone-headed” dinosaurs, which are part of the larger Ornithischia group (bird-hipped dinosaurs). Within Pachycephalosauria, it is usually placed in the family Pachycephalosauridae. The flat nature of its skull has led to some scientific discussion. For a time, some researchers thought that flat-headed pachycephalosaurs like Homalocephale might be young individuals or females of dome-headed species. However, the type specimen of Homalocephale shows features of an adult animal, such as fused bones, supporting the idea that it is a distinct genus. It is often considered a more primitive type of pachycephalosaur compared to those with highly developed domes.

Distinguishing Features

• A broad, relatively flat, and thickened skull roof, different from the prominent domes of many other pachycephalosaurs.
• The surface of the skull roof was decorated with a pattern of small knobs and pits.
• Unusually large openings (supratemporal fenestrae) on the top rear of its skull, which in more highly domed pachycephalosaurs are often closed over by bone growth.
• A relatively wide pelvis (hip bones).
• Long hind limbs suitable for running.

Paleoenvironment and Diet

Fossils of Homalocephale were found in the Nemegt Formation in Mongolia’s Gobi Desert. During the Late Cretaceous, this area was not a desert but a lush floodplain with many rivers, forests, and open woodlands. This environment supported a wide variety of dinosaurs. Homalocephale shared its habitat with large predators like Tarbosaurus, giant long-necked sauropods like Nemegtosaurus, duck-billed dinosaurs such as Saurolophus, and other pachycephalosaurs including Prenocephale.

As a herbivore, Homalocephale would have eaten plants. Its small, leaf-shaped teeth lining its jaws were likely adapted for shredding leaves, ferns, fruits, and seeds from low-growing vegetation. The relatively broad shape of its muzzle might suggest it was not a very selective feeder.

Significance and Ongoing Research

Homalocephale is an important dinosaur for paleontologists because its type specimen is one of the most complete skeletons known for any pachycephalosaur. This allows for a better understanding of the anatomy of these unique dinosaurs, especially the flat-headed varieties. It provides crucial evidence in the study of how pachycephalosaur skulls evolved, particularly the development of their characteristic cranial thickening.

Ongoing research continues to explore the relationships between different pachycephalosaur species, including whether flat-headed forms like Homalocephale directly represent an ancestral state or a different growth stage or sex of domed forms. While most current evidence points to Homalocephale being a distinct adult animal, the full story of pachycephalosaur headgear development is still being pieced together. The function of the thickened skulls in pachycephalosaurs—whether for display, defense, or head-butting contests (like modern goats or sheep)—is also a major topic of study. The flat skull of Homalocephale might have been used differently in any such behaviors compared to the domed skulls of dinosaurs like Pachycephalosaurus. Studies of its bone structure and comparisons with relatives like Goyocephale help scientists learn more about this fascinating group of dinosaurs.