65 million years ago, an asteroid collided with the Earth, causing the end of the Age of Reptiles. Prior to this point in time, dinosaurs were the dominant form of terrestrial life, with few other creatures able to even compete against them in terms of size or diversity. One particular lineage of dinosaur was emerging just as the extinction was about to happen: the ceratopsids. Areas of the oldest ceratopsids are especially valuable to understanding the evolution of horned dinosaurs, as it sheds light on how these dinosaurs diversified and gained prominence during the Cretaceous Period.

In the past, only one known specimen from the Upper Cretaceous Wahweap Formation in Southern Utah has been named, Diabloceratops eatoni. This dinosaur lived between 77-81 million years ago, as all dinosaurs known from the formation are. Erik K. Lund and a team of paleontologists have identified and published a scientific paper on a new early ceratopsid from this area, Machairoceratops cronusi, which will help paleontologists improve their understanding of the diversity of ceratopsids during the late Cretaceous Period up until their extinction 65 million years ago. 

There are a number of things which distinguish M. cronusi as a species separate from D. eatoni. The squamosal, a bone found by the lower back of a ceratopsian cranium, is fan-shaped, rather than rectangular as seen in Diabloceratops. It also contains two anterodorsally curved epiparietal ornamentations, or decorative bones located at the top of the frills which curve toward the dinosaur’s back. It is positioned higher in the sediments of the formation than Diabloceratops, indicating that it may have lived more recently than its relative.

Though M. cronusi is distinguished enough from D. eatoni to be classified as its own species, there are a number of similar morphological features the two share, suggesting that they have a common ancestor. The two share robust, elongate supraorbital ornamentation, or decoration above the eye socket, triangular parietosquamosal frills, or triangular frills between the middle and lower half of the back of the skull, and elongate spike-resembling epiparietal loci ornamentation, or ornamentation along the tips of the spikes on the frill.  

Parsimony-based analysis has placed it as a centrosaurine ceratopsid, and from a Bayesian approach, scientists were able to identify it as a member of a sister clade to that of D. eatoni and other similar species of ceratopsians from the time. In order to complete these analyses, scientists compared M. cronosi to other centrosaurs and non-centrosaurine ceratopsians, giving them the ability to confirm that the newest specimen was a centrosaur and that it is a new species of dinosaur. More research may be needed in order to better classify this new species.

The evolution of ceratopsid cranial ornamentation can help shed insight on how natural selection was put into play as dinosaurs continued to evolve and change, even towards the end of their life on Earth. Casey Chan, an AAST freshmen, provided her opinion on how this specimen may contribute to centrosaurine diversification in the Late Cretaceous. “I believe that the discovery of a new centrosaur is very important in our understanding of centrosaurine diversification,” she said. “Since this centrosaur has a new type of frill, perhaps we could use it to act as a precedent in order to further our knowledge of these dinosaurs’ ornamentation. I find it really interesting that this specific horned dinosaur has frill decorations, and it must have had some sort of evolutionary purpose. This leads me to question whether the frill decorations of the centrosaurians changed over time, perhaps to be frillier than before?”

Today’s scientists suspect that ceratopsids used cranial ornamentation for sexual selection purposes. However, because identifiable centrosaurine remains are so rare, it may be difficult to study what characteristics were generally given evolutionary favor across all ceratopsids, or whether favorable traits depended on the species itself. As further specimens are named and identified, this may shed further insight into centrosaurine and ceratopsid evolution overall.