Inside a cavernous tent bathed in golden late-afternoon light, the resident team of vertebrate paleontologists pores over the day's haul, emitting sporadic whoops of surprise. "That might be a eureka," exclaims Yale University graduate student Stephen Chester, peering through a microscope at a tooth embedded in a jawbone fragment. "That might be a primate."
"It is! It's totally Teilhardina!" Doug Boyer, a Ph.D. candidate at New York's Stony Brook University, enthusiastically agrees. Teilhardina, he explains, is the Latin name for a group of primates that appear in Asia, Europe and North America at roughly the same time.
"That's a horse, Douggie," Chester says, examining another tooth. It belongs to Hyracotherium sandrae, the unusually small species first identified at Polecat Bench. The tooth is a shiny dark amber and it's very tiny. Why was this horse, Hyracotherium sandrae, so small?
The most straightforward explanation is that Hyracotherium sandrae was simply a small-sized species that migrated into the Big Horn Basin from somewhere else. But the University of Michigan's Gingerich champions a more intriguing possibility. He suggests that its diminutive stature could be the consequence of a decline in available nutrients, notably protein. Horticultural experiments have shown that some plants, when bathed in high concentrations of carbon dioxide, have less protein in their leaves.
The same phenomenon may also have caused insects to become more voracious, which is consistent with the leaf damage displayed by Wing's fossils. Here again, however, there are other possible explanations. For example, higher temperatures alone would have raised insect food requirements by quickening metabolic rates and encouraging year-round breeding. "During the PETM, many things are changing all at once, and it's hard to separate one from another," Wing observes.
At present, Wing is in the field camp's cooking tent, heating up his favorite utensil, a big black wok that can handle dinner for 16. Just behind him, seated at a long metal table, Mary Kraus, a wiry sedimentologist from the University of Colorado, is starting to peel a big pile of russet potatoes. She and her daughter, Christina, have had a great day, she says, digging a trench through the pastel paleosols of the badlands surrounding a perennially dry fork of Nowater Creek. "Look at this treasure," she exclaims, holding up a fossilized insect burrow shaped vaguely like a cowboy's boot.
Like tree rings and deep sea sediments, burrows are what scientists refer to as "proxies." Simply put, proxies are natural systems that record and preserve information about past climates, not unlike modern instruments. Crayfish burrows indicate soils that experience large fluctuations in wetness; earthworm and beetle burrows suggest drier conditions. The colors of ancient soils are also proxies. For example, the degree of redness — whether the color tends towards orange or towards purple — can be correlated with specific ranges of soil moisture.
There are many other types of proxies, including fossil leaves, teeth and the shells of marine organisms. Typically these proxies record shifts in the ratio between heavy and light elements. Oxygen shifts can be translated into temperature; hydrogen shifts into relative humidity. Changes in leaf size and shape can likewise be read as proxies for temperature and moisture, though as Wing admits, "We don't fully understand why."
Multiple proxies, Wing says, suggest that, during the PETM, this area of Wyoming was rather similar to South Florida, with a mean annual temperature of around 75 degrees F and annual precipitation between 30 and 55 inches. The precipitation may have followed a strongly seasonal pattern, especially towards the beginning, with part of the year being quite dry, Wing believes. But that's just the broad-brush picture. Wing, Smith and Kraus, along with University of Florida paleontologist Jonathan Bloch, who heads up the vertebrate fossil collection effort, are working on reconstructing the regional climate in much finer detail.
Kraus is using ancient soils to begin mapping what seems like a climatological progression. The initial phase of the PETM looks rather dry, she says, and the middle phase appears drier still, though there are signs of very rapid soil deposition from flooding along rivers and streams. Towards the end of the PETM, in the Big Red itself, she is finding hints that conditions may have become wetter. Among other things, the rocks of the Big Red contain a lot of purple, a color suggestive of higher water tables and more poorly drained soils.
The Big Horn Basin, Kraus says, is probably the ideal place to try to pull together a comprehensive picture of how climate changed on a regional scale over the course of the PETM. "Where else can you go and find 5,000 year intervals stacked one on top of another?" she asks. "Where else can you go and know that 40 meters of rock (about 130 feet) equals 150,000 years?" That's around how long it took the PETM to wind up and wind down, so it's not surprising that, over the course of so many millennia, both regional and global climate patterns underwent successive changes. The wind-up, of course, was the fast part; it was the wind-down that took a long time.