Ice Age Extinctions of the Megafauna

Lisa Gantenbein
March 15, 2000


During the last Ice Age before humans arrived, the North American continent belonged to various forms of enormous, fantastic creatures. By the end of the Ice Age, most of these large animals had become extinct. Numerous attempts have been made to explain the disappearance of these animals, but there has yet to be a consensus. Among the theories that have been debated, two are predominant. They are the climatic change theory, and the overkill theory. The climatic change theory advocates the idea that the global warming, which brought about the end of the Ice Age, caused the animals to die off. The other is the overkill theory, which maintains that humans, by over hunting the animals, are responsible for the extinctions. It is likely that many of these animals became extinct at the end of the Ice Age due to a combination of climatic change and overkill.
The Ice Age, which began about one and a half million years ago, is also referred to as the Pleistocene Epoch. Much of the Pleistocene falls within the geological time period called the Quaternary stage. We are still in the Quaternary today. The Ice Age consisted of a series of glacial advances and retreats called glaciations and deglaciations. There have been a total of at least twenty glaciations, including four or five that were significant, in the last 2.4 million years.
In the most recent glaciation, thirty percent of the earth was covered in sheets of ice over two miles thick. The last glaciation in North America is called the Wisconsin. The Wisconsin glaciaciation began 122,000 years ago. The advance of the Late Wisconsin began 35,000 years ago. The ice sheets, called the Laurentide and the Cordilleran glaciers stretched southward to the middle of the continent, covering most of what is now Canada and the area in the Pacific Northwest from Alaska to Tacoma. On the eastern half of the continent, the ice covered the area to about the middle of where New York is today. Diverse environmental conditions south of the ice sheets remained as they were, but in the northern half of the continent, regions that had once been surfaced with dense forests, fertile land, and abundant game, turned cold, and barren.
The expanding glaciers in the Ice Age forced sea levels to drop between 330 and 490 feet, opening a passage between Siberia in northeastern Asia and Alaska in North America. This passage was a strip of land, called the Bering Land Bridge. The land bridge still exists today, but is immersed under water. When sea levels were lowered by 330 feet, a 1000 mile wide strip of land was exposed. It surfaced and submerged many times throughout the Ice Age, and is believed to have been exposed for much of the time between 75,000 and 14,400 years ago. New research shows that the land bridge may have even been exposed until as late as 11,000 years ago.
This land bridge was the route of migration for a large variety of mammals crossing between Asia and North America. Some of the animals arrived long before the glaciation of North America. The animals that migrated to this continent adapted to their new surroundings. Many of the animals living on the continent were herbivorous and extremely large compared to the present fauna. These huge, plant eating animals are called the megafauna.
Some of the most well known megafauna are the Probocidea. Animals with trunks, which include the mastodon (Mammut) and the mammoth (Mammuthus) of the Ice Age, and the two modern species of elephants, belong in the order Proboscidia. The mastodon probably traveled alone rather than in a herd, browsing on Spruce limbs in open woodlands. The mammoths migrated into North America by at least 1.7 million years ago (Haynes, 1991). They traveled in herds and generally grazed in vast grasslands. The types of vegetation that these animals consumed, can be determined bye observing the structures of their teeth. A new mammoth species evolved from the original mammoths as they migrated from Eurasia, over the Bering Land Bridge, leaving substantial populations in Beringia as they progressed southward. This new species was called the Columbian mammoth (Mammuthus columbi). The Columbian was a larger mammoth well adapted