Fossil Horse Quarry Near Hagerman, Idaho, Worked by National Museum, 1930, Smithsonian Institution Archives, Smithsonian's fossil horse quarry near Hagerman, Idaho, as it appeared in 1930 from a vantage point across a small canyon. (The quarry is on the upper slope of the hill in the foreground of the image.) The Smithsonian excavated in the area between 1929 and 1934, and eventually recovered more than 130 skulls, numerous associated jaws, and enough bones for 10 complete skeletons of the Pliocene fossil horse Plesippus shoshonensis (now known as Equus simplicidens)..