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	Sometimes, after an animal dies, its body is covered with earth before it has had time to decay. A leaf may fall into a lake and become covered with mud. Footprints on a beach may harden in the sun and later become covered with another layer of sand.

					woolly mammoth


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	About 65 million years ago, all of the dinosaurs died. No one knows for sure why. After this happened, their place was taken by animals with fur, called mammals. Most of the large animals alive today are mammals.

				dinosaur tracks, northern New Mexico

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Sometimes, after an animal dies, its body is covered with earth before it has had time to decay. A leaf may fall into a lake and become covered with mud. Footprints on a beach may harden in the sun and later become covered with another layer of sand.