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Illinois State Museum: The Midwestern United States 16,000 Years Ago
What did Northern America look like 16,000 years ago? Were the plants the same? What happened to the mastodons and the saber-toothed cats? Here's a short website that is easy-to-understand and supplies at least some of the answers.
PBS
Nova: America's Stone Age Explorers: End of the Big Beasts
Three opposing views about the disappearance of the "megafauna" in North America. What really happened to the wooly mammoth, mastadon, saber-toothed tiger, among other animals?