United States in the form of partial skull fragments and as an articulated postcranial skeleton both found in 1871. Originally named Hadrosaurus agilis (Marsh, 1872), it was placed in a new genus and renamed Claosaurus agilis in 1890 when major differences between the specimen and Hadrosaurus came to light.

In 1892, Marsh named a second species, C. annectens. It was later reassigned to Anatosaurus and then Edmontosaurus, where it is currently. G. R. Wieland named third species C. affinis in 1903,