Open Paleo

Homalocephale

Maryańska & Osmólska, 1974 HOM-a-lo-SEF-a-lee

From Greek homalos (even, flat) and kephale (head); meaning 'even head' or 'flat-headed', referring to its flat skull roof unlike other pachycephalosaurs

72.2–66.0 million years ago Maastrichtian, Late Cretaceous
Late Cretaceous stages

Homalocephale is a genus of pachycephalosaurid dinosaur that lived during the Late Cretaceous period of what is now the Nemegt Formation, Mongolia. The genus was described in 1974 by Halszka Osmólska and Teresa Maryańska, and consists of a single species, H. calathocercos. Though Homalocephale has been regarded as a synonym (and juvenile form) of Prenocephale, juvenile specimens of the latter indicate that they were distinct. Homalocephale was 1.8 m (5.9 ft) long and possibly an omnivore.

Under dispute

Homalocephale has been proposed to be a juvenile or female of Prenocephale (Longrich and colleagues, 2010; still regarded as plausible by Longrich and Saitta, 2024), but Evans and colleagues (2018) showed that juvenile Prenocephale retain a parietosquamosal ornamentation pattern distinct from Homalocephale, supporting its validity. Its position is uncertain because the only specimen is immature (Evans and colleagues, 2011, 2021): Longrich and colleagues (2010) recovered it in a weakly supported Asian clade on the Pachycephalosaurus side of the tree, while Woodruff and colleagues (2025) left it unresolved within Pachycephalosauridae.

Diagnosis

  1. Cranial roof completely flat with distinct interfrontal and frontoparietal sutures (unique among Pachycephalosauridae)
  2. Supratemporal fenestra large
  3. Infratemporal fenestra low and elongated
  4. Orbit large and nearly circular
  5. Quadrate deflected posteriorly along upper half of length
  6. Foramen magnum and occipital condyle both large
  7. Basal tubera flat anteroposteriorly, ventral edges closely positioned to pterygoid wings
  8. Maxillary tooth crowns small with medial ridge and anterior/posterior ridges; worn on lingual surface
  9. Cranial roof ornamented with deep large pits peripherally, becoming denser toward midline

Species

Homalocephale calathocercos Maryańska & Osmólska, 1974

Type species

From Ancient Greek 'kalathos' meaning 'basket' and 'kerkos' meaning 'tail', referring to the caudal tendons arranged in a basket-like structure.

Holotype
MPC-D 100/1201 , Paleontological Center, Mongolian Academy of Sciences
Material
Partial skull and skeleton
Horizon
Nemegt Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
Nemegt, Western Sayr, site 8, Ömnögovi, Mongolia Gradzinski and colleagues (1969) map site 8 as a then-undescribed skull and partial postcranial skeleton. Their only coordinate for Nemegt is a range spanning some 40 km, covering all twelve numbered skeletons.

References

  1. Maryańska, T.; Osmólska, H. (1974). Pachycephalosauria, a new suborder of ornithischian dinosaurs. Palaeontologia Polonica 30: 45-102. Polish Academy of Sciences. maryańska1974a
  2. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the appearance features.

    jones2026a
  3. Longrich, N. R.; Sankey, J.; Tanke, D. (2010). Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA. Cretaceous Research 31(2): 274-284. Elsevier BV. doi:10.1016/j.cretres.2009.12.002

    Proposed Homalocephale is a juvenile or female of Prenocephale.

    longrich2010b
  4. Evans, D. C.; Brown, C. M.; Ryan, M. J.; Tsogtbaatar, K. (2011). Cranial ornamentation and ontogenetic status of Homalocephale calathocercos (Ornithischia: Pachycephalosauria) from the Nemegt Formation, Mongolia. Journal of Vertebrate Paleontology 31(1): 84-92. Informa UK Limited. doi:10.1080/02724634.2011.546287

    Finds the holotype immature and its parietosquamosal ornamentation distinct from that of Prenocephale prenes.

    evans2011a
  5. Evans, D. C.; Hayashi, S.; Chiba, K.; Watabe, M.; Ryan, M. J.; Lee, Y. N.; Currie, P. J.; Tsogtbaatar, K.; Barsbold, R. (2018). Morphology and histology of new cranial specimens of Pachycephalosauridae (Dinosauria, Ornithischia) from the Nemegt Formation, Mongolia. Palaeogeography, Palaeoclimatology, Palaeoecology 494: 121-134. Elsevier. doi:10.1016/j.palaeo.2017.11.029

    Showed juvenile Prenocephale differ from Homalocephale, supporting its validity.

    evans2018a
  6. Evans, D. C.; Brown, C. M.; You, H.; Campione, N. E. (2021). Description and revised diagnosis of Asia's first recorded pachycephalosaurid, Sinocephale bexelli gen. nov., from the Upper Cretaceous of Inner Mongolia, China. Canadian Journal of Earth Sciences 58(10): 981-992. Canadian Science Publishing. doi:10.1139/cjes-2020-0190

    Considers its phylogenetic position uncertain because the known material is juvenile.

    evans2021a
  7. Woodruff, D. C.; Horner, J. R.; Goodwin, M. B.; Evans, D. C. (2025). The first pachycephalosaurid from the Late Cretaceous Two Medicine Formation: effects of the Western Interior Seaway on North American pachycephalosaurid evolution. Zoological Journal of the Linnean Society 205(2). Oxford University Press (OUP). doi:10.1093/zoolinnean/zlaf087

    Leaves it unresolved within Pachycephalosauridae, paired with Goyocephale.

    woodruff2025a
  8. Longrich, N. R.; Saitta, E. T. (2024). Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) — A Distinct Taxon of Small-Bodied Tyrannosaur. Fossil Studies 2(1): 1-65. doi:10.3390/fossils2010001

    Still regards the synonymy with Prenocephale as plausible.

    longrich2024d
  9. Gradziński, R.; Kaźmierczak, J.; Lefeld, J. (1969). Geographical and geological data from the Polish-Mongolian Palaeontological Expeditions. Palaeontologia Polonica 19: 33-82. Polska Akademia Nauk, Instytut Paleobiologii.

    Maps the holotype as site 8 at Nemegt, the numbering the describing paper cites, and gives the locality only as a bounding range.

    gradziński1969a