On a broiling day in July 2009, a caravan of four-wheel-drive vehicles traveled a faint, two-track dirt road in southwestern Wyoming's Great Divide Basin. The expedition was headed for an area known as Salt Sage Draw in search of buried treasure: fossils dating to between 55 million and 50 million years ago, at the start of the Eocene epoch, when the ancestors of many modern orders of mammals were beginning to replace the more archaic mammals that had existed during the earlier Paleocene epoch. One of us (Anemone) had been leading field crews of anthropologists, paleontologists and geologists to the basin since 1994, and Salt Sage Draw had proved a fruitful hunting ground over the years, yielding fossils at several localities. Yet this time I was having trouble finding the site. It dawned on me that the road we were on was not the one we had used in previous years. My error would turn out to be very fortunate indeed.