Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Back to the Future” by J. Madeleine Nash (©2008 by J. Madeleine Nash).

The Sand Creek Divide is a high point in Wyoming’s Big Horn Basin. From it you can see the emerald patchwork of irrigated sugar beet and malt barley fields that hug the Big Horn River as well as the jagged mountain ranges that define the edges of this harsh mid-latitude desert.

But between 55 and 56 million years ago, says Scott Wing, a paleo-botanist at the Smithsonian’s Museum of Natural History, the big Horn Basin was a balmy, swampy Eden, teeming with flora and fauna that would be at home in today’s coastal Carolinas. And then, all of a sudden, things got a whole lot warmer. In a geological eye blink—less than 10,000 years, some think—global mean temperatures shot up by around 10 degrees Fahrenheit.

The Big Red, a sinuous ribbon of rose-colored rock, is the most vivid marker of this exceptionally torrid time—the Paleocene-Eocene Thermal maximum, or PETM, as most paleontologists call it. Even before it had a name, the PETM 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. At the time, mammals were rather simple, general-purpose creatures with few specializations. 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 in 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 and Lowell. Stott 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 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-milion-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, returning year after year to the Big Horn Basin.

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.”

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. In their place are the legumes, a family of plants, shrubs and trees 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.”