They were discovered by Fielding Bradford Meek and Henry Martyn Bannister while they were looking for fossil shells in the Lance Formation near the Black Butte and Bitter Creek.

Meek and Bannister were employed by Ferdinand Vandeveer Hayden's Geological Survey of the Territories and notified paleontologist Edward Drinker Cope of the find.

Cope himself searched the ridge near Black Butte and re-discovered Meek's site, finding huge bones protruding from the rocks near a coal vein.

Later in 1872, Cope published a description and name for the animal, Agathaumas sylvestris, or "marvelous forest-dweller," in reference to its great size and the environment revealed in the same rocks as its bones.

Several years later, with the discovery of the giant sauropod dinosaurs of the Morrison Formation, it became clear to him that British forms such as Cetiosaurus and Pelorosaurus were land animals.

[1] Cope originally did not know to what group Agathaumas belonged, though he noted that some of the remains were similar to the British reptile Cetiosaurus[4] and very different from the corresponding elements of Hadrosaurus and Dryptosaurus (Laelaps).