11.2" Placenticeras Ammonite Fossil - South Dakota

This is giant 11.2" wide Placenticeras costatum ammonite from the Pierre Shale of South Dakota. The wonderful suture patterns are well defined on the inner whorls. It has been nicely prepared and the rock around it has been sculpted to create a nice base, allowing it to display upright.

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.

As with many of these large ammonites there are some areas of restoration along a crack repair.