Paleontology and geology
During most of the Early Carboniferous (Mississippian), North Dakota was covered by shallow seas and experienced a subtropical to tropical climate. Sands, shales, carbonates, and evaporites were deposited in this sea, which was inhabited by crinoids, brachiopods, bivalves, bryozoans, cephalopods, corals, and trilobites. By the end of the period the seas had receded from the area.
At the beginning of the Late Carboniferous (Pennsylvania) North Dakota was dry land. Later in the period, shallow seas inundated the state again, and sand, silt, mud, and carbonates were deposited. Brachiopod, gastropod, crinoid, and shark fossils have been recovered from these rocks. Carboniferous rocks are not exposed at the surface, but information about them comes from drill cores.