Large Hoploscaphites (Ammonite), Pierre Shale, North Dakota.

2.62" (6.7 cm) long Hoploscaphites (Ammonite) from the Pierre Shale of South Dakota. The Pierre Shale Dates to 73 to 70 Million Years Old.

Hoploscaphites is a species of Scapphite, which have the signature characteristic of having small spikes on the shell.