| Summary: | fossils from the Golden Valley Formation in North Dakota and elsewhere in the upper Midwest indicate mean annual temperatures as high as 65 �F. In comparison, western North Dakota today has a cold dry grassland climate with a mean annual temperature of 41�F Unlike today's grasslands, dense, tropical forests grew in North Dakota during the early Eocene Epoch, similar to tropical Central America areas like Panama today. These greenhouse conditions extended into the Arctic. Palm trees and cycads grew in Alaska at this time, and alligators lived in the Canadian Arctic. Western North Dakota was a hot, humid densely forested swampy lowland transected by sluggish streams. It was likely frost-free. Several species of aquatic plants, ferns, herbs, shrubs, vines, and frees, including palms grew in this forested swampland. Fish, amphibians, reptiles including turtles, lizards, crocodiles, and alligators, birds, and mammals including insectivores, rodents, the early horse "Hyracotherium," and the large tapir-like pantodont Coryphodon, and carnivores inhabited the area. At this time there was a major immigration of mammals into North America from Asia including Coryphodon, which was the largest at up to eight feet long. Bulky and slow-moving, it was similar in some ways to the hippopotamus. Coryphodon was probably semi-aquatic and dug up swamp plants with its canine tusks, although it also browsed on forest vegetation. For its size, it had an extremely small brain, the smallest ratio of brain-to-body weight in any mammal. Life in the Paleocene Cannonball Sea: The Last Sea to Cover North Dakota During the Paleocene Epoch, the Cannonball Sea occupied much of central and eastern North Dakota, while forested swamplands covered the western part of the state. About 60 million years ago the Cannonball Sea receded from North Dakota. This was the last time that North Dakota was covered by marine waters. Sediments deposited in the Cannonball Sea are called the Cannonball Formation and consist mostly of sandstones and mudstones. The sandstones were laid down in the shallow marine beach and shoreline areas, the mudstone in deeper waters. Fossils found in these rocks provide information about the kinds of animals that inhabited the sea. Remains of large marine reptiles, such as mosasaurs and plesiosaurs that lived in the Cretaceous seas, are not found in the Cannonball Formation because they had become extinct at the end of the Cretaceous Period. The main predators in the Cannonball Sea were sharks. Fossils of several species of sharks have been found in the Cannonball Formation, the most common of which is Carchar�as. (Figure 15) Carchar�as, a sand-tiger shark, still lives today in shallow coastal waters in temperate to tropical oceans. They were one of the primary predators in the Cannonball Sea and were well adapted for ripping into flesh with their long, slender, sharp teeth. The ten-foot-long Carchar�as undoubtedly patrolled the shorelines of the Cannonball Sea, competing with other sharks for favorite prey-fish. Rays and raffish were the other common fish in the Cannonball Sea. Myliobatis 22 201 1-2013 North Dakota Blue Book
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