One of the key ways scientists have been able to identify climate change is through the discovery of fossils. During the last ice age, for instance, animals indigenous to cold habitats lived much farther south than would be possible today, such as reindeer and wolverine. Likewise, scientists know that musk ox, a cold-climate species that currently lives in the Arctic, roamed as far south as Mexico City during the last ice age. A mastodon tooth was discovered off the coast of New Jersey. This type of proxy data tells scientists two things: (1) a species found in cold climates existed as far south as New Jersey, and (2) if the fossil was found offshore, it was not covered by ocean at the time the mastodon was there, which indicates that the ocean level was lower because much of the water was locked up in glacial ice and ice caps.

Fossil remains of wooly mammoths have been found in sand and gravel glacial deposits in North Dakota. Bison latifrons were recently discovered in North Dakota from the ice age, Pleistocene giant bison. Larger than today's bison, their horn cores spanned more than 7 feet (2 m), compared to the bison of today with a 2-foot (0.6 m) span. Paleontologists have also found walrus remains (an arctic animal) off the coast of Virginia, which means that walruses migrated that far south during the last ice age.

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