5" Placenticeras Ammonite - Pierre Shale, SD
It's Late Cretaceous in age (Upper Campanian Stage) or approximately 75 million years old. Back at this time South Dakota was covered by a shallow inland sea. These ammonites are found in hard concretions, or balls of rock which form around the fossil. These concretions are split open to reveal the ammonites which then must be expertly prepared from the surrounding rock, a very time consuming task requiring a lot of skill.
The concretion was sculpted in a way that it creates a free-standing display for the ammonite.