The Largest Ceratosaurus

How many species of this rare, ornamented genus were there?

Ceratosaurus nasicornis at the Smithsonian National Museum of Natural History
Ceratosaurus nasicornis at the Smithsonian National Museum of Natural History Photo by the author

Eastern Utah’s Cleveland-Lloyd dinosaur quarry is a treasure trove of predatory dinosaurs. In addition to elements from more than 46 individual Allosaurus, this fossil-rich pocket has yielded remains of rarer predators that lived in the region 150 million years ago, including the little-known Marshosaurus and the tyrannosaur Stokesosaurus. The charismatic, well-ornamented predator Ceratosaurus has been uncovered from these deposits, too, but the particular individual found in the Jurassic quarry might belong to a species that was only recently recognized.

Since the late 19th century, the Ceratosaurus genus has been best represented by one species: Ceratosaurus nasicornis. Paleontologist O.C. Marsh included a beautiful reconstruction of this dinosaur in a kangaroo-like pose in his essential 1896 tome The Dinosaurs of North America. In 2000, however, paleontologists James Madsen and Samuel Welles named two additional species in their detailed monograph on the osteology of Ceratosaurus. One, represented by an articulated skeleton found in Colorado’s Fruita Paleontological Area, was named Ceratosaurus magnicornis, and the unusual Cleveland-Lloyd specimen was dubbed Ceratosaurus dentisulcatus.

A reconstruction of Ceratosaurus at the Museum of Ancient Life. Photo by the author.

The Cleveland-Lloyd species was not found all together in a single, articulated skeleton. Work over many years turned up the scattered remains of what Madsen and Welles considered to be a single Ceratosaurus individual. When the isolated parts were viewed together, the paleontologists were struck by the size of the dinosaur. This Ceratosaurus was significantly larger than any found before. (I have seen these fossils myself in the Natural History Museum of Utah collections, and compared to the skeleton on display at the Smithsonian National Museum of Natural History, the Cleveland-Lloyd Ceratosaurus is huge.) What Masen and Welles called Ceratosaurus dentisulcatus also differed in various anatomical aspects such as larger, more recurved teeth and a nasal opening set lower down at the front of the skull. Sadly, the portions of the skull which preserved the dinosaur’s ornaments were not found, so we don’t know how this species might have differed from others in this respect.

It’s difficult to say how large this individual actually was. The Cleveland-Lloyd Ceratosaurus was much larger than the roughly 17.5-foot specimen that formed the basis of previous anatomical descriptions, and informal estimates have placed the larger species at about 28 feet. Yet, given the new interest in dinosaur growth, I have to wonder if Ceratosaurus dentisulcatus really represents a bigger, badder species than Ceratosaurus nasicornis. Ceratosaurus is a relatively rare dinosaur, so much so that we still don’t have a good idea of how individuals varied from one to another, nor do we have a solid understanding of Ceratosaurus growth. Maybe the Cleveland-Lloyd Ceratosaurus is just an older, and therefore larger, individual of Ceratosaurus nasicornis in the same way that the dinosaur often called Saurophaganax might be an older or particularly large variant of Allosaurus. Even though the dinosaurs of the Morrison Formation have been known for a long time and seem familiar, there is much we still don’t know about their biology.


Madsen JH, Welles SP. Ceratosaurus (Dinosauria, Therapoda), a Revised Osteology. Miscellaneous Publication. Utah Geological Survey.

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