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• Language: unknown
• Subjects: Arctic; Arctic Ocean
• Online Access: http://cdm16921.contentdm.oclc.org/cdm/ref/collection/ndbb/id/13415

Life in North Dakota's Cretaceous Underwater World During the Cretaceous Period 85 million to 65 million years ago, North Dakota was either completely or partially covered by shallow, subtropical to warm-temperate oceans that were similar to the oceans that had covered the State during the Paleozoic Era. These oceans occupied what is referred to as the Western Interior Seaway, and at times were so extensive that they connected the Arctic Ocean with the Gulf of Mexico, which split the North American continent in two. In North Dakota, these Cretaceous Period seas were probably never more than about 500 feet deep. Fine- grained sediments, mostly silt and clay that were deposited on the floor of those oceans, have become rock and are now referred to as the Carlile, Niobrara, and Pierre Formations. Sandstones and siltstones of the Fox Hills Formation have indicated the existence in Central North Dakota of shallow estuaries, lagoons and marine shorelines. These Cretaceous Period marine rocks are the oldest rocks exposed at the surface in North Dakota, and embedded in them are fossils of the animals and plants that inhabited these ancient oceans. It is known that these oceans were shallow and warm, primarily because of the kinds of animals that lived in them, as indicated by the presence of their fossilized remains. At times, these shallow-water communities were teeming with life. An example of a fossil site containing the remains of animals that lived in the shallow, warm ocean named the Pierre Sea is the 75-million-year-old site in Griggs County near Cooperstown. It is here where fossils of corals; gastropods (Trachytriton); bivalves (Nemodon, Inoceramus); cephalopods (Didymoceras, Solenoceras, and Baculites); shrimp (Callianassa); crabs; echinoderms, such as starfish and sea urchins; Bryozoa; and brachiopods (Lingula) are found weathering out of the Pierre Formation. The most spectacular fossils found at the Cooperstown site are the remains of the giant marine lizards called mosasaurs, along with sea turtles, sharks, and five-foot tall flightless sea birds. A nearly complete 23-foot-long skeleton of the mosasaur Plioplatecarpus was collected from this site by the North Dakota Geological Survey, and is now displayed at the North Dakota Heritage Center. (Figure 3) Mosasaurs were huge marine lizards, some up to 40 feet or more in length, that inhabited the world's oceans during the Cretaceous Period. The name mosasaur means "Meuse Reptile," and refers to the initial discovery of fossils of these animals in the 1770s along the Meuse River near the town of Maastricht in the Netherlands. Mosasaurs are related to monitor lizards, such as the Komodo dragon that lives in Indonesia today. Unlike those of their terrestrial lizard relatives, mosasaurs had paddles or flippers for limbs. They swam by moving the back part of their bodies and flattened tails from side to side much like crocodiles do today. Their paddles were used primarily for steering rather than for propulsion. Mosasaurs were active predators and were among the main carnivores in the Cretaceous Period oceans. The structure of their skull indicates that they had a good sense of sight and a poor sense of smell. They probably preyed on other 6 201 1-2013 North Dakota Blue Book