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Wooly mammoths with other Ice Age megafauna. Mauricio Antón, from "Who Killed the Wooly Mammoth," <a href="http://en.wikipedia.org/wiki/PLoS_Biology">PLOS Biology</a>, 2008. CC, some rights reserved
Wooly mammoths with other Ice Age megafauna. Mauricio Antón, from "Who Killed the Wooly Mammoth," PLOS Biology, 2008. CC, some rights reserved

Natural Selections: Ice Age mammals

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During the last Ice Age North America was home to many varieties of "super-sized" mammals, megafauna. Giant beaver, 'possums, bears, sloths and other creatures joined the more familiar wooly mammoth in the land bridge migration. Dr Curt Stager and Martha Foley look at the question, "Why so big?"

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No one can know for sure why megafauna species were so big. However, scientists can speculate on this question by examining the body physiology of these animals.

“As you get bigger, you have more body mass per surface area, which means less of your innards are exposed to temperature change outside,” explained Dr. Curt Stager of Paul Smith’s College. This allows larger animals to maintain a consistent body temperature more easily. According to Stager, this makes sense for the wooly mammoths, big bears and other large Ice Age mammals.

“Most of them came over to North America from Asia by way of the Bering Strait land bridge,” said Stager. “It was pretty cold up there. It was during the Ice Age, sea levels went down because ice is trapped on the poles and it’s cooler so the ocean shrinks.”

The Bering Strait acted as a filter that only allowed the animals that tolerated cold very well to cross. While some of these mammals are more commonly known, others are less popular. Stager said, “There would be the regular versions, you could say, and then these giant versions of almost anything you can think of. So, the like nine foot long, 400 pound beavers in North America.”

There were also giant ground sloths that reached 10 feet long in North America, and 20 feet long in South America. Armadillos were six feet long and weighed approximately 600 pounds. The large and small versions of the species existed at the same time. Stager said, “There was this huge diversity of mammals, which is really neat, and it’s pretty much attributable to coming across that land bridge.”

According to Stager, far more species came over from Asia than went the other way. Horses were one of the only species to evolve in North America and then colonize Asia. “So if you walked around the plains of North America at the time, it would have been like the Serengeti in Africa. There were cheetahs and hyenas and lions and elephant-type things, the mammoths and mastodons, and zebras and camels and all kinds of stuff.”

Evidence suggests that these large Ice Age animals replaced much of North America’s native fauna. “It turns out, for example, in South America, the biggest predator before these mammals came over in the last two or three million years were these giant birds. They call them ‘terror birds.’ They were 10 feet tall, flightless, they had basketball-sized eggs, huge beaks; they could probably run 40 miles per hour.”

These birds were replaced by the naturally invasive large mammals caused by the lower sea levels. These mammals weren’t seen as exotic species since they were introduced naturally to their new environments. Paleontologists who study this invasion refer to it as The Great American Animal Interchange.

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