Paleontology and geology
North Dakota was dry land during the Early Ordovician, but by the middle of the period, seas had inundated the state. Sands, clays, and limy sediments were deposited in these shallow, warm seas. Diverse assemblages of invertebrate animals including corals, cephalopods, trilobites, brachiopods, bryozoans, and graptolites inhabited the shallow marine environments. Ordovician rocks are exposed at the surface only in a small area at the eastern edge of the state.