The first reported dinosaur discovery within the borders of Colorado was found during Ferdinand V. Hayden’s U.S. Government sponsored survey to explore the Territories of Colorado and New Mexico. In 1869 the survey team brought back to Washington D.C. a broken caudal vertebral bone collected in the high Rocky Mountains. The bone was given to Joseph Leidy Professor of Anatomy at the University of Pennsylvania who noted the fossil’s shared similarity to early dinosaur discoveries previously made in France and England. He formally named the fossil dinosaur; Antrodemus valens. Despite being the first dinosaur discovery within the borders of Colorado, the occurrence of this fossil fell into obscurity, as by the twentieth century paleontologists had recognized that the fossilized bone was of the better known theropod dinosaur Allosaurus. In the 150 years since, this mountainous region of Colorado has yielded few discoveries of dinosaurs and the location of this obscure initial discovery has remained a geological mystery. This paper reviews the available archival evidence regarding the events leading to the discovery of Colorado’s first dinosaur in an attempt to relocate its original discovery site.