Ceratosaurus, meaning “horn lizard”, is a genus of large predatory dinosaur from the Late Jurassic Period (Tithonian and Kimmeridgian ages). It is found in the Morrison Formation of North America, Tanzania and Portugal. It was discovered in the Cleveland Lloyd Dinosaur Quarry in central Utah and the Dry Mesa Quarry in Colorado. The type species, C. nasicornis, was described by O.C. Marsh in 1884 and then red scribed by Gilmore in 1920. Two more species, C. magnicornis and C. dentisulcatus, were described in 2000. In all, there are 6 subspecies which also includes C. ingens, C. stechowi, and C. meriani.

Ceratosaurus was characterized by large jaws with blade-like teeth. It had a large, blade-like horn on its snout and a pair of small horns over the eyes. It had short forelimbs, although they were very powerful. It had a row of small bony plates that ran down the middle of the back. The head was large and the hind legs were strong. It had a long tail. The skull was quite large in respect to the rest of the body. The jaws had twelve to fifteen long, flattened teeth on each maxilla.

The type specimen was estimated to be about 17.5 feet in length, yet it is unclear if the animal was full grown or not. The actual estimate of length of a Ceratosaurus was given by David B. Norman in 1985. He estimated that length was around 20 feet in adults. A specimen uncovered in the 1960s form the Cleveland-Lloyd Quarry, was estimated to be about 30 feet in length. Weight has been estimated at around 1,160 pounds, although some estimates go as high as 2,200 pounds. Later figures show that these estimates may be over-calculated and in 2007 John Foster estimated the actual weight to be between 606 and 996 pounds based on femur length.

Ceratosaurus lived during the age that included dinosaurs such as Allosaurus, Torvosaurus, Apatosaurus, Diplodocus, and Stegosaurus. It was a smaller than average carnivore of its time (Allosaurus and Torvosaurus were much bigger). It most likely occupied a different range than the larger predators to avoid competition. Being of a smaller size and having a flexible body, it is believed that Ceratosaurus was a good swimmer, thus giving it the ability to hunt aquatic prey, such as large fish and crocodiles, although it may have thrived more on large dinosaurs. It is believed that adults and juveniles sometimes ate together, although this evidence is debatable.

O.C. Marsh described the nasal horn of Ceratosaurus as being the “most powerful weapon” for both offensive and defensive purposes. Although other findings in the early 1900s concurred with this theory, today it is largely considered unlikely. Norman believed that the horn was more useful in combat between males competing for mating rights. Other studies state that the horn may be for visual display alone, and if so, it may have been brightly colored.

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