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Ice Age Extinctions of the Megafauna
March 15, 2000
ICE AGE EXTINCTIONS OF THE MEGAFAUNA
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 for the North American grasslands.
While the Proboscidea originated in Asia and Africa and migrated to North America, other animals evolved in the New World and traveled across the land bridge in the opposite direction from North America to Eurasia. Among the genera that originated in the New World was the horse (Equus). Horses migrated to Asia and Europe during a period when the land bridge was exposed. Before the mass extinctions, the horse became one of the most populous mammals in North America. By the end of the Ice Age, the horses had completely died off in the New World, but survived in Eurasia. The horse exists, now and even abounds in the New World, because it was reintroduced in the sixteenth century by Europeans. Other Ice Age animals include such varied genera as the glyptodont (glyptotherium), the ground sloth (Eremotherium), and the giant beaver (Castoroides ohioensis). What is notable and extremely interesting about these animals is that they have many similarities, but are much larger than their modern day counterparts. For example, the glyptodont, similar to a turtle, would have been comparable in size to a Volkswagen Beetle. The ground sloth reached weights of up to three tons, about the size of a mammoth, and the giant beaver was about the size of a black bear (Grayson, 1993).
About 18,000 years ago, the Wisconsin glacier began its retreat. Buried lands began to reveal themselves. Strong winds spread seeds planting various types of new vegetation along the banks of the glaciers. By the time of 10,000 years ago, the glaciers retreated to the size they are today. This point marks the end of the Pleistocene and the beginning of the Holocene.
Such an extensive variety of animals, large and small, thrived on the continent until the end of the Pleistocene. At this point most of the megafauna had vanished. Thirty-five genera of mammals that once wandered this land, and nineteen genera of birds are now extinct. Archeological research shows that none of these extinct animals existed after 10,000 years ago. Of the thousands of archeological sites in North America, not one has contained the remains of any extinct animal that proves to be younger than 10,000 years old (Grayson, 1993). It was once assumed by many people that all thirty-five genera became extinct in the short period of time between 10,000 and 12,000 years ago. This presumption emerged when six of the genera were proven to have disappeared in the span of two-thousand years. That was in 1969, and since then, only the extinctions of three more genera have been placed in the same time period (Grayson, 1993). It is possible that the other twenty-six genera became extinct at the same time, but if that is the case, we have a long way to go before it will be proven. The other feasible possibility is that the extinction’s happened thousands, or even millions of years ago.
The climatic change brought about the end of the Ice Age. It resulted in a number of physical environmental changes. The temperature of the earth grew increasingly warm, resulting in a greater difference in seasonal temperatures. The glaciers melted away, causing sea levels to rise and shore lines to recede. The general hypothesis of the climatic change theory is that a mixture of factors, such as environmental and ecological changes that resulted from the warming climate, wiped out the megaphone. Could this climate change of the last glacial retreat be the cause of these extinction’s? For decades, scientists have deliberated the question which is surrounded by controversy. They have considered many different models of explanation that comply with the general hypothesis of the climatic change theory. One idea comes from Peter D. Ward as follows:
As the glaciers receded, a profound reorganization occurred in hydrological, biological, and sedimentological systems. The world’s ice coverage changed from 30% of its surface to the current level of about 10%. This phase change of so much water from solid to liquid was the driving force behind much of the subsequent geological, climatic, and biotic transformation. As the ice melted, water and sediment choked the rivers; they altered the drainage of great lakes and, in breaking the ice dams that had formed some of these lakes, caused cataclysmic floods that themselves changed the face of North America (the channeled scab lands of Washington State and Idaho are examples). The continental shelves changed positions and depth as the shorelines migrated inward under rapidly rising seas. Rainfall patterns shifted; the seasons themselves were lengthened or shortened. Temperatures rose all over the earth. And of course, in the wake of so much physical perturbation to the environment, biotic systems changed. These great changes in the physical environment are at the heart of many scientists’ belief that the large-animal extinction at the end of the Ice Age was brought about by physical or environmental changes, not by human hunting. The extinction of many species in a broad environment must involve many variables in both biotic and physical spheres, because ecosystems are complex. Physical changes may bring about the death of one species, and its disappearance then affects other species, such as its predators, prey, and parasites. In this way even a single extinction causes effects that ripple through the ecosystem. At the end of the age of glaciation, many species faced environmental changes, and surely many succumbed before they could adapt or migrate. Habitat destruction is probably the major cause of physically induced extinction.
One may wonder why, if the climatic change ended the existence of the megafauna, did it not affect the smaller animals in such dramatic ways. The likely answer that large animals have more dif
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