Ammonite And Orthoceras Fossil Dish In Black SeabedAmmonite And Orthoceras Fossil Dish In Black Seabed. Orthoceras cephalopods are the fossilized remains of marine animals which had long conical shaped shells. Orthoceras proliferated within Earth's oceans approximately 400 million years ago! They have a lovely glistening quality when polished. Their shells were preserved in black marble, and this beautiful combination makes for unique display pieces and jewelry.