Paleontology and geology
Shallow, subtropical seas covered North Dakota during most of the Silurian. Carbonates were the primary sediments deposited in these shallow marine environments. Corals, trilobites and other invertebrates inhabited the seas. Near the end of the Silurian, the seas receded and karst topography developed on the eroding land surface. Silurian rocks are exposed at the surface only in a small area in the northeastern part of the state.