| Summary: | this climate cooling caused extensive glaciation in Antarctica. Open grasslands with riparian forests, similar to African savannas today, were established in North Dakota. Average annual rainfall was only about 14 to 18 inches, about what it is in western North Dakota today. Little is known about life in North Dakota during this cool and dry period because most of the rocks deposited during that time, and their entombed fossils have been removed by erosion. Also, at this time land mammal diversity in the mid-continent was at an all time low. Remains of a few grassland mammals including the oreodont Merychyus, horse Miohippus, and an unusual burrowing beaver, Palaeocastor have been found in the remnant Miocene Epoch rocks. Palaeocastor is one of the earliest known beavers. Unlike today's aquatic beaver, Palaeocastor was terrestrial. It was about a foot long - the size of a muskrat. It excavated and lived in two-foot-long corkscrew-shaped burrows. Scratch marks on the walls of these burrows indicate that the beavers dug them by using their teeth and scraping with movement of their heads. Their helical-shaped burrows are preserved as trace fossils and are called "Devil's corkscrews." They have been given the scientific name Daemonelix. There is almost no record of prehistoric life in North Dakota from about 20 million years ago during most of the Miocene and all of the Pliocene Epochs, until about 50,000 years ago, during the Pleistocene Epoch. This is because rocks deposited during that time and the fossils that would have been found in those rocks have been removed by millennia of erosion. This was a time of global climatic flux from warm conditions in the early Miocene to cooling in the middle Miocene when glaciation again occurred in Antarctica, and the modern East Antarctic ice sheet began to form. Occasionally isolated fossils have been found in North Dakota that provide tantalizing hints of what life was like during this mysterious time. One of these finds is the tooth of the "shovel-tusked" gomphothere, Amebelodon. Gomphotheres were elephant-like animals that migrated to North America from Eurasia across the Bering land bridge during the Miocene about 16.5 million years ago. Amebelodon was one of the gompthotheres about 10 feet tall at the shoulder, and was somewhat similar to a modern elephant in appearance. Its skull and tusks were, however, very different than today's elephant. Long, flattened spade-like tusks projected from its lower jaws. These flattened three-foot-long tusks would have been used like a shovel to dig up rooted water plants in rivers and ponds. It was the largest herbivore that roamed the North Dakota plains during the Miocene. Life in North Dakota During the Great Ice Age The last Great Ice Age, the Pleistocene Epoch, began about 2.6 million years ago, although climate cooling and the development of ice caps at both poles began at the end of the Pliocene Epoch about 3.5 million years ago. It was a harsh time in North Dakota, because the geology and life of the state was dramatically affected by the cold climate and events associated with glaciation. During the colder intervals, glaciers, some several hundred feet thick, advanced into North Dakota from Canada on numerous occasions. During these times of glacial advance, worldwide Chapter One - History of Ancient Life in North Dakota 31
|
|---|