Homalocephale: what a flat skull can – and cannot – tell us

Homalocephale calathocercos is a small pachycephalosaur from the Nemegt Formation of southern Mongolia. Its name means roughly “even-headed,” a reference to the low skull roof that first made the animal distinctive. The type is more informative than an isolated skull: it includes substantial postcranial material and part of an unusual ossified tail structure. Yet the same specimen raises a major limitation. Bone development shows that it had not finished growing, so its skull cannot be treated as a complete picture of the adult.

Quick facts

Discovery and the type specimen

Polish-Mongolian expeditions collected the holotype in the Nemegt Basin during work in the Gobi Desert. The specimen was originally catalogued as G.I. SPS 100/51 and is now MPC-D 100/1201. Maryańska and Osmólska named the genus and species in 1974. The fossil comes from the Nemegt Formation, a succession of fluvial and floodplain deposits whose age is generally placed in the Late Cretaceous, although correlations within the formation continue to be refined.

The preserved material includes cranial bones, vertebrae, pelvis and hindlimb elements. The skeleton is incomplete, but it allows researchers to compare more than the outline of the skull. Several parts of the tail are preserved in position, including ossified tendons. The specimen is still one individual, however; it cannot establish the full range of variation within the species or the adult proportions of the animal.

Why the flat skull became controversial

Pachycephalosaurs are famous for thickened skull roofs, yet the top of the Homalocephale skull is relatively flat. Older classifications treated flat-headed and domed forms as distinct evolutionary types. Later studies found that skull roof thickness can change as pachycephalosaurs grow. That raised the possibility that some flat-headed named forms were juveniles of domed genera, or that their apparent differences reflected growth rather than separate lineages.

Evans and colleagues examined cranial ornamentation and vertebral fusion in the Homalocephale holotype. The degree of neurocentral suture closure indicated that the animal was immature. Their result makes the simple evolutionary story – a permanently flat-headed primitive form – uncertain. It also does not prove that Homalocephale grew a dome. The available specimen leaves adult development unresolved, so both a flat adult and later cranial thickening remain hypotheses.

A tail framed by ossified tendons

The holotype preserves a lattice-like arrangement of ossified tendons beside part of the tail. Such tendons occur in several ornithischian dinosaurs and can stiffen sections of the vertebral column. In Homalocephale, the preserved caudal basket is unusually informative because parts remain articulated. It shows how the mineralised structures were arranged around the tail vertebrae rather than merely listing isolated rods.

The fossil does not record the animal's movement directly. A stiffened tail could have helped resist bending or transmitted forces through the body, but those are biomechanical interpretations. The structure is not evidence by itself for a particular courtship display, fighting style or social behaviour. Those claims would require functional analyses and comparisons, and even then would remain inferences rather than observed actions.

Body, diet and locomotion

The known individual was a small, bipedal animal, probably around the scale of a large dog to a small person in body length depending on how the incomplete skeleton is restored. Exact mass figures are model-based and should be treated cautiously. The hindlimbs and pelvis support terrestrial bipedal locomotion. The forelimbs were shorter and did not carry the body during ordinary walking, as in other pachycephalosaurs.

Its diet is not directly recorded. Teeth and jaw anatomy suggest plant material was important, but a mixed diet cannot be excluded from the type alone. No stomach contents, tooth-wear study tied to a broad sample, or associated plant remains establishes a specific menu. Similarly, the skeleton does not reveal whether Homalocephale lived in groups or cared for its young. Behavioural claims should not be copied from better-known dinosaurs simply because they belonged to the same broad clade.

What the fossils leave open

Homalocephale is often presented as a clear example of an ancestral flat-headed pachycephalosaur. The evidence supports a more careful statement: one immature animal had a low skull roof and a distinctive set of cranial and postcranial features. Whether the adult had a dome, how the species varied and how it related to other Nemegt pachycephalosaurs remain questions for additional fossils and better-sampled analyses.

The Nemegt Formation preserves several other large and small dinosaurs. Placing Homalocephale beside them in the dinosaur catalogue helps compare anatomy and age without implying that every reconstruction is equally complete. The flat skull is valuable precisely because it invites a testable question about growth; it is not a licence to fill the missing adult stage with certainty.