As recently as 11,000 years ago--"near time" to geologists--mammoths, mastodons, gomphotheres, ground sloths, giant armadillos, native camels and horses, the dire wolf, and many other large mammals roamed North America. In what has become one of science's greatest riddles, these large animals vanished in North and South America around the time humans arrived at the end of the last great ice age. Part paleontological adventure and part memoir, Twilight...