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Acrotholus (Greek for "highest dome"- akros meaning highest and tholos meaning dome) is an extinct genus of pachycephalosaur dinosaur that lived during the Santonian of the late Cretaceous, in the Milk River Formation of Canada. The type species, audeti, was named after Roy Audet allowing access to his ranch leading to the discovery of the species. The discovery of this specimen lead to several new revelations in the fossil records questioning the preservation of small-bodied organisms along with the evolution of early pachycephalosaurs. The iconic cranial dome found on Acrotholus makes it one of the earliest indisputable known members of the pachycephalosaur family.

Like others of its clade, Acrotholus was a bipedal herbivore characterized by a dome-shaped head. The dome had often been associated with intra-species combat though exact method of contact have been debated.

History of discovery[]

The holotype was found in the Deadhorse Coulee Member of the Milk River Formation in Southern Alberta, Canada in 2008. This formation has been known to expose organisms from the Late Santonian. The fossil consists of a nearly complete frontoparietal dome along with the anterior half of the frontoparietal dome. Pachycephalosaurs in general are unique in dinosaur fossil records due to their relatively small size in relation to most dinosaurs in their time period, 40 kg or less. It has been suggested due to smaller bone being susceptible to destruction by carnivores and weathering, preservation bias exist for smaller species. Pachycephalosaurs are unique in this regard due to their recognizable head dome which is resistant towards pre-depositional destruction. Among known fossil specimens, approximately 66% are known from only cranial remains. Acrotholus' discovery further implicates the diversity of small bodied dinosaurs which may not have survived the fossil record, suggesting a much more complex ecology in the late Mesozoic era. A similar conclusion have been made with Asian species of pachycephalosaurs, with their ontogeny under debate due to their flat domed shapes.



Unlike most known pachycephalosaurs, A. audeti's dome on the skull is oval in shape having a maximum thickness of 55 millimetres (2.2 in) above the cerebral fossa. The lack of tesserae or tubercles indicate the specimen was past the juvenile stage of its development. Computed tomography (CT) scans reveal a low vascularity, high density, and fully fused internally fused frontal-frontal and frontoparietal sutures. Peripheral bones are high and well developed on the dome indicating the peripherals were incorporated within the dome. The incorporation of supraorbitals in the dome are similar if not greater than more derived pachycephalosaur. Compared to Stegoceras validum and later Campanian pachycephlosaurids, the dorsally convex frontonasal boss is short and not separated with grooves from the anterior supraorbital lobe. This region is approximately 50% of the thickness to the cerebral fossa (55 mm). Orbital fosse are only slightly concave and pierced by small foramina. Three depression can be seen on the ventral surface of the frontoparietal dome, the orbital cavity, endocranial fossa, and temporal fossa. Comparisons of the skull finds A. audeti to be distinctly different from other pachycephalosaurs in relation to dome. Most notably, the supraorbital region and incorporation of the supraorbital into the dome identify the specimen from other pachycephalosaurs. Very little soft tissue covering have been suggested over the dome due to the structural anatomy and hypothesized use.


A. audeti was recovered as the sister taxon to Prenocephale prenes. Acrotholus is the earliest known specimen of pachycephalosaur with derived traits from the fossil records, predating later known species of flat headed species in Asia. The discovery of this species further suggests pachycephalosaur head features were well established by the Santonian predating later species thought to have primitive traits such as Homalocephale and Stegoceras validum.

Below is a cladogram modified from Evans et al., 2013.


Wannanosaurus yansiensis


Colepiocephale lambei

Hanssuesia sternbergi

Stegoceras novomexicanum

Stegoceras validum

Goyocephale lattimorei

Homalocephale calathocercos

Tylocephale gilmorei

Foraminacephale brevis

Amtocephale gobiensis

Acrotholus audeti

Prenocephale prenes

Alaskacephale gangloffi

Pachycephalosaurus wyomingensis

Sphaerotholus buchholtzae

Sphaerotholus goodwini