Knightia fossil fish, framed. Measures 12.25″w x 10.5″t Green River Formation, Wyoming These are the fossil remains of a fish who lived during the Eocene Epoch, over 50 million years ago. As the fish died, the remains settled on the bottom of the lakebed and were covered with fine sediment. Every fin and scale was covered so gently that every outline has been preserved. Over millions of years, more mud and pressure transformed the remains in to fossils. Eventually weathering eroded and exposed the fossils.