Acrotholus

Acrotholus (Greek for "highest dome"- akros meaning highest and tholos meaning dome) is an extinct genus of pachycephalosaurian dinosaur that lived during the Santonian of the late Cretaceous, in the Milk River Formation of Canada. The type species, A. 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, although the exact methods 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.