Even before it had a name, the Paleocene-Eocene Thermal Maximum was starting to fascinate Wing. For some time, it had been clear to paleontologists studying the evolution of mammals that the transition between the Paleocene and the Eocene was marked by the kind of innovative burst that implies sweeping ecological change. Yet no hint of such a change had appeared in any of the fossil leaves Wing had collected. He would stare at leaves from the Paleocene and leaves from the Eocene, but see almost no difference between them. "It was getting to be annoying," he recalls.

The Paleocene is the geological epoch that started 65 million years ago, right after a wayward asteroid or comet crashed into the planet, ending the reign of the dinosaurs. At the time, mammals were rather simple, general-purpose creatures with few specializations: Their teeth, their ankle bones and joints all look extremely primitive. Then, barely 10 million years later, at the dawn of the Eocene, the first relatives of deer abruptly appear, along with the first primates and first horses. 

"You can literally draw a line through the rock," says Philip Gingerich, a vertebrate paleontologist at the University of Michigan. "Above it there are horses; below it there aren't." In fact, where Gingerich works — at Polecat Bench, in the northern sector of the Big Horn Basin — you can actually see the line, in the form of a band of light gray sandstone. Oddly enough, many fossil mammals commonly found above this line, including those first horses, were abnormally small. Typically, Gingerich says, Eocene horses grew to the size of modern-day cocker spaniels, but these horses were "about the size of Siamese cats." 

In 1991, as Gingerich and others were marveling over the miniature mammals of Polecat Bench, oceanographers James Kennett of the University of California, Santa Barbara, and Lowell Stott of the University of Southern California investigated a major extinction of small, shelly creatures that, during the late Paleocene, lived on the sea floor off the coast of Antarctica. This massive die-off, they found, coincided with a steep rise in deep ocean temperatures and a curious spike in atmospheric carbon.

Less than a year later, paleontologist Paul Koch and paleo-oceanographer James Zachos, both now at the University of California, Santa Cruz, teamed up with Gingerich to show that this geochemical glitch had also left its calling card on land. The trio established this indirectly by measuring the carbon content of fossilized teeth and nodules plucked from the Big Horn Basin's 55.5 million-year-old rocks.

To Wing, it began to seem increasingly implausible that plant communities could have segued through the PETM unaffected. So in 1994, he started a methodical search for the fossils that he was all but sure he had missed, returning year after year to the Big Horn Basin. He started in its southeastern corner and then moved north to explore Polecat Bench and the Clarks Fork Basin. Yet it wasn't until 2003, when he reached the Worland area, that he began to meet with success.

At first, he found just a smattering of leaves, too few to suggest any pattern. Then, in 2005, at the end of a long day, he slid his shovel into a grayish mound and pulled out a tiny leaf. "I knew immediately that this was totally different from anything I'd seen before, that this was really dramatic, so I got down on my knees and poked the shovel in again and then again. In every shovelful, there were more leaves coming out. First I started to laugh; then I started to cry. And then I looked up."

Staring down at Wing was a new field assistant. "He had a look on his face that said, 'Now I'm going to die.' I understood what he must have been thinking. 'Here I am, I've just graduated from college. I've never camped before. I've never been on a paleontological expedition before. It's 6 o'clock in the evening. It's still 100 degrees. I don't know where I am. And it looks like the boss has gone completely nuts!' So I said to him, 'Really, it's OK. I'm not crazy. It's just that I've been looking for this since you were 10 years old!' "

From that one site, Wing went on to extract more than 2,000 leaf fossils representing 30 different species. Missing from the mix are the cypresses and other conifers that were so common during the Paleocene; gone also are the distant cousins of broadleaf temperate zone trees like sycamores, dogwoods, birches. In their place are the legumes, a family of plants, shrubs and trees — think of acacias and mimosas —that thrive today in seasonally dry tropical and subtropical areas.

"What you see is almost a complete changeover from what was growing here before," Wing marvels. "What this means is that you could have stood in this one spot in Wyoming, surrounded by a forest, and everything would have looked pretty much the same for millions of years. And then, over a few tens of thousands of years, almost all the plants you're familiar with disappear and are replaced by plants you've never seen before in your life." At least some of the newcomers migrated north from as far away as the Mississippi Embayment, precursor of the Gulf of Mexico. With them came a wave of small but voracious predators: Many of the fossil leaves are peppered with the scars left by chewing, sucking, mining and boring insects.