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When the Isthmus closed about three million years ago, it made a dam between two oceans. But it was also a bridge between two continents. For the first time in tens of millions of years, North and South America were joined. The Isthmus became a bridge that animals used to travel between continents. It changed the wildlife in both places forever.
Once the land bridge formed, the Great American Interchange began. Animals started to move in both directions. A flood of saber-toothed cats, bears, deer, panthers, mice, horses, and mastodons moved south.

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David Stevensonand Greg Wenzel