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.
As the tracks began to disappear in the sagebrush and tall grass, I stopped the caravan and walked a ways to see if I could spot the road ahead. Rounding a small hill, I noticed an extensive bed of sandstone in the near distance and the elusive road right alongside it. Because sandstone in the Great Divide Basin and many other sedimentary basins in the American West often harbors fossils, I decided to spend some time searching these deposits before we resumed our trip to Salt Sage Draw. After about an hour of systematically scanning the rock on hands and knees, my then graduate students Tim Held and Justin Gish shouted that they had found a couple of nice mammal jaws. I eagerly joined them. Fossil jaws with teeth are prized because they contain enough information to identify the kind of animal they came from, even in the absence of other parts of the skeleton, and because they reveal what the animal ate.