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. 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 mega fauna had vanished. Thirty-five genera of mammals that once wandered this land, and nineteen genera of birds are now extinct. .
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.
