The ammonites of South Dakota were residents of the Pierre Seaway. The Pierre Seaway expanded and contracted throught the Cretaceous Period (144 - 66.5 million years ago). The Pierre shales, which we find most of the ammonites in, were laid down during the Campion and Maastrichtian Stages (82 - 69 million years ago).
The Pierre Seaway extended from the Gulf of Mexico to the Arctic Ocean. It covered parts of Texas, New Mexico, Oklakoma, Colorado, Kansas, Nebraska, Wyoming, Montana, South Dakota, North Dakota, and Canada. Depths ranged from about 50 meters to 200-300 meters at its deepest points.
This inland seaway accounts for the variety of fish, reptiles, mollusks, and other aquatic animals and plants that we find in the sedementary deposits of these states.