Rubeosaurus ovatus

Rubeosaurus ovatus is a useful example of how dinosaur names can change as fossils are re-examined. The name is anchored to USNM 11869, the rear part of a centrosaurine frill from the Two Medicine Formation of Montana. Charles Gilmore named it Styracosaurus ovatus in 1930 because its paired spikes curved inward. Andrew McDonald and John Horner moved it to the new genus Rubeosaurus in 2010. Later studies disagreed: one found its unusual frill within the variation of Styracosaurus albertensis, while another retained a distinct Styracosaurus ovatus. This page preserves the requested historical combination but marks it as disputed.

Its fossils occur between approximately 75.3 and 75 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Only the rear bar of the frill can be securely assigned to the holotype. It preserves paired elongated P3 and P4 processes, including strongly inward-directed spikes that inspired the species name ovatus. Other skull and body features often shown in Rubeosaurus reconstructions came from MOR 492, but that specimen was reassigned to the separate genus Stellasaurus in 2020. A complete Rubeosaurus face, nasal horn, limbs and torso therefore remain unknown; any full-body image must combine the type frill with general Styracosaurus-like anatomy.

• Partial rear parietal bar forming the secure holotype
• Paired elongate P3 and P4 frill processes
• Strongly inward-directed spikes on the rear frill
• Large openings inferred for the short ceratopsid frill
• Beak, teeth and body reconstructed from related centrosaurines

Charles W. Gilmore and his field party collected USNM 11869 in 1928 from the Two Medicine Formation near Landslide Butte on the Blackfeet Reservation. Gilmore described Styracosaurus ovatus in 1930. McDonald and Horner established Rubeosaurus in 2010 and referred the more complete skull MOR 492 to it. John Wilson and colleagues showed in 2020 that MOR 492 represented Stellasaurus ancellae instead, leaving Rubeosaurus ovatus known only from its original partial frill. Robert Holmes and colleagues separately argued that the holotype falls within the variation of Styracosaurus albertensis.

Discovery credit: Charles W. Gilmore and field party.

Naming authors: Charles W. Gilmore.

The upper Two Medicine Formation records inland river plains east of the growing Rocky Mountains. Volcanic ash falls, seasonal droughts and episodes of heavy runoff affected a landscape supporting conifers, flowering plants, ferns and horsetails. Einiosaurus, Achelousaurus and other dinosaurs occur through the formation, but its rock sequence spans time; they should not all be treated as one simultaneous community.

Nothing about social behaviour is directly preserved with the isolated frill. A Styracosaurus-like herbivore would have cropped plants with its beak and sliced them with dental batteries, but even that reconstruction relies on close relatives. The inward-curving spikes probably supported keratin and may have contributed to display or physical interaction. Their exact living shape and function cannot be tested from this one specimen.

• The same fossil has been called Styracosaurus ovatus and Rubeosaurus ovatus.
• The secure holotype is only the rear part of a skull frill.
• A more complete skull once assigned to Rubeosaurus became the holotype of Stellasaurus in 2020.
• Its paired frill spikes curve inward more strongly than in most classic Styracosaurus reconstructions.

A research repository is not necessarily a public exhibit. Loan and display status can change, so check with the institution before visiting.

1. 01
   Morphological variation and asymmetrical development in the skull of Styracosaurus albertensis

   Robert B. Holmes, Walter Scott Persons, Baltej Singh Rupal and 2 coauthors · Cretaceous Research · 2020

   Open source
2. 02
   On dinosaurian reptiles from the Two Medicine Formation of Montana

   Charles W. Gilmore · Proceedings of the United States National Museum 77(16) · 1930

   Open source
3. 03
   New material of “Styracosaurus” ovatus from the Two Medicine Formation of Montana

   Andrew T. McDonald, John R. Horner · New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium · 2010

   Open source
4. 04
   A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the ‘Styracosaurus-line’ dinosaurs

   John P. Wilson, Michael J. Ryan, David C. Evans · Royal Society Open Science 7, 200284 · 2020

   Open source
5. 05
   Styracosaurus ovatus Gilmore, 1930, holotype USNM 11869

   Smithsonian National Museum of Natural History

   Open source