American Mastodon, Paleontology Hall, by Unknown, c. 1917, Smithsonian Archives - History Div, The skeleton of an American Mastodon with long tusks stands in the Paleontology Hall of the United States National Museum, now the National Museum of Natural History, c. 1917. The American Mastodons occupied woodland areas of North American from coast to coast and from Alaska to central Mexico, but are now extinct. At the time of this picture the exhibit was called the "Hall of Extinct Monsters.".