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Introducing Acrotholus, A New Dome-Headed Dinosaur from Alberta

May 6, 2013

Study of pachycephalosaur fossil record hints at higher diversity of small-bodied ornithischians

Life reconstruction of the new pachycephalosaur from the Milk River Formation, Alberta

Life reconstruction of the new pachycephalosaurid from the Milk River Formation, Alberta.  Courtesy J. Csotonyi

Today, my research group reported on a new species of pachycephalosaur from Alberta, Canada, in the current issue of the Nature Communications (link here). Acrotholous audeti ( Ack-RHO-tho-LUS) was identified from both newly discovered and historically collected fossils. Approximately 6 feet long and weighing about 40 kgs in life, the newly identified plant-eating dinosaur represents the oldest definitive pachycephalosaur from North America, and possibly the World.  Acrotholus provides a wealth of new information on the evolution of dome-headed dinosaurs. Although it is one of the earliest known members of this group, it’s thickened skull dome is surprisingly well-developed for its geological age. More importantly, detailed study of the unique fossil record of dome-headed dinosaurs highlights biases against the preservation of small dinosaur species, and suggests they were probably far more diverse than we currently know.

Caleb Brown photographed during his discovery of Acrotholus audeti

Caleb Brown photographed during his discovery of Acrotholus audeti in 2008

Acrotholus means “tall dome,” referring to the dome-shaped skull, which is composed of solid bone over 2 inches thick. The species name honors Alberta rancher Roy Audet, on whose land the best specimen was discovered.  Acrotholous walked on two legs, and had a greatly thickened domed skull above its eyes, which was used as a visual signal to other members of its species, and may have also been used in head-butting contests. Acrotholous lived about 85 million years ago. The new dinosaur is based on two skull ‘caps’ from the Milk River Formation of southern Alberta. One of these was collected by the Royal Ontario Museum over 50 years ago, but the best specimen was found in 2008 by University of Toronto graduate student Caleb Brown during a field expedition organized by myself and Dr. Michael Ryan of the Cleveland Museum of Natural History.

Small mammals and reptiles can be very diverse and abundant in modern ecosystems, but small dinosaurs (<100 kg) are considerably less common than large ones in the fossil record. Whether this pattern is a true reflection of dinosaur ecosystems, or is an artefact related to the greater potential for small bones to be destroyed by carnivores and natural decay, has been debated. The massively constructed skull domes of pachycephalosaurs are resistant to destruction, and are much more common than their relatively delicate skeletons-–which otherwise closely resemble those of other small plant-eating dinosaurs. Therefore, we argue, the pachycephalosaur fossil record might provide insight into the diversity of small, plant-eating dinosaurs as a whole.

See Acrotholus in 3D via Digimorph:

Read the Press Release here.

Full Reference:

Evans, D. C., R. Schott, D. Larson, C. Brown, and M. J. Ryan. 2013. The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs. Nature Communications 4:1828. doi:10.1038/ncomms2749

Badlands of the Milk River Formation exposed at Roy Audet's ranch in Southern Alberta

Badlands of the Milk River Formation exposed at Roy Audet’s ranch in Southern Alberta


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