Mean atmospheric O2
Geologic Time Scale
Fanged bony fish of the genus Enchodus, six-foot-long predators from the late Cretaceous period about 75 million years ago, swim in a estuary in the Western Interior Seaway of North America. On the shore to the right are duck-billed Hadrosaurs and on the left are a pair of ostrich-like ornithomimid dinosaurs of the genus Struthiomimus. In the center a flock of feathered Ichthyornis take to the air.
During the mid to late Cretaceous the continent of North America was divided by waters from the Arctic Ocean and the Gulf of Mexico. Also known as the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, much of the North American Midwest was under water for about 30 million years. In some places the water may have been as deep as 3,000 feet.
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