Hot Times in the Bighorn Basin

Natural History, April, 2001 by Scott L. Wing

A modern desert provides dues to an ancient period of global warming.

To most people who drive through, hurrying west to Yellowstone National Park, Wyoming's Bighorn Basin is nothing but a vast sagebrush plain crossed by two lanes of blacktop stretching away toward distant snowcapped peaks. Those with their eyes on the horizon may never notice badland hills striped with red and purple, and only those who leave the road will smell pungent, fresh-crushed sage, hear the total silence of the desert at midday, and feel dust as fine as powder between their fingers. Even those who experience the desert may not realize it is a country haunted by time travelers--paleontologists--who pull from the rocks not only fossils but an understanding of how the earth's climate has changed in the past and how plants and animals have responded to those changes. This part of Wyoming contains the world's best record of a period 55 million years ago, when the earth experienced an episode of global warming more rapid than any before, perhaps as rapid as the one we humans are about to cause and experience.

The Bighorn Basin is roughly 4,000 square miles (about 10,000 square kilometers) of badlands, sagebrush flats, and irrigated fields. Except for a narrow opening to the northwest, it is surrounded by mountains. Like other basins in the Rocky Mountains, the Bighorn Basin formed 60-50 million ?ears ago, during the late Paleocene and early Eocene, as mountains were pushed up on all sides. Fast-moving streams eroded mud and sand from the rising mountains and, slowing as they reached flatter land, spread sediment across the bottom of the basin. Year after year, flood after flood, layers of sediment accumulated until in some areas the pile was more than six miles deep, burying--and preserving--the remains of countless organisms. In the past few million years, this part of the North American continent experienced renewed uplift, the climate became colder and drier, and the vast deposits began to erode rapidly, dissecting the soft rocks into strange and intricate shapes and littering the slopes and flats with the fossils once contained inside.

Fossil riches first attracted paleontologists to the Bighorn Basin more than a century ago. Early scientists worked from horseback; photographs of their expeditions show U.S. Army cavalrymen, brought along to ensure safety. In photographs from the early twentieth century, field crews cluster around buckboard wagons or Model T Fords, replaced in more recent photos by weathered pickups and four-wheel-drive vehicles. Traveling in the badlands is still difficult and sometimes dangerous--anyone who has worked long in the basin has spent time digging a stuck vehicle out of a dry creek bed or walked miles to the nearest road to seek help. Stories of such experiences, retold around campfires, remind us of some constants in fieldwork--even if we now locate fossil sites with global positioning systems rather than with cairns and enter data into computers at night as well as into notebooks during the day.

The generations of effort have paid off handsomely. Hundreds of thousands of fossils--mammal bones, leaves, shells--fill the cabinets of museums around the country and even the world. There are fossil pollen grains by the millions. Each fossil reveals something about a once-living organism: a leaf may contain the fossilized trail of an insect larva that tunneled within it for food; the cusps and crests of a mammal tooth bear evidence of the food it was suited to chew; the bones of large land tortoises, soft-shelled aquatic turtles, and alligators show that the ancient climate was warm and that the rivers teemed with life.

Evidence of past conditions also comes from the Bighorn Basin rocks themselves. The varicolored bands running across the hills are fossilized soils. The bands' colors indicate such things as the wetness of the original soil, and their thickness provides evidence of the length of time over which they developed. The depth and sinuosity of sandstone deposits reveal the original dimensions and course of ancient river channels. Coal-dark deposits show where the floodplain was especially wet, inhibiting the decay of plant remains by the fungi, bacteria, and arthropods living in the soil. Veterans of fieldwork in the basin are familiar with the colors and shapes that indicate the likely presence of fossils. Bands of red, orange, purple, and light gray (evidence of well-drained soil) often contain fossil bones. Plant fossils occur in several types of rocks, including brown and dark gray layers--all that is left of ancient swamps--and also coarse silt and fine sand layers deposited millions of years ago by overflowing rivers. Sediments containing fossil plants also typically contain the mineral gypsum, which forms large crystals as it weathers out of the rocks. The flashing of gypsum crystals in the desert sunlight can beckon a fossil hunter from miles away.

Taken together, all the evidence yields a picture of the environment and life of the basin during the late Paleocene and early Eocene. The streams--mostly small, slow moving, and gently meandering--were lined with low, natural levees on which grew a variety of trees, including relatives of sycamore, poplar, walnut, and hazelnut. The most common streamside tree was a relative of the katsura tree (Cercidiphyllum), a genus restricted today to two species in eastern Asia but once common across the mid and high latitudes of the northern continents. In the understory of these streamside woodlands lived several types of ferns still found in temperate forests, but the areas of open grassland that are so common today were absent. (Grasses had evolved by this time but had not yet become important forms of vegetation.) A variety of turtles lived in the rivers, along with gar, freshwater clams and snails, crayfish, and alligators. More than a hundred species of mammals roamed the forested floodplains, including some of the earliest primates, dawn horses, the earliest even-toed ungulates (the group that includes deer, pigs, and antelope), early true carnivores, and other mammals that have no extant close relatives (see "Wyoming's Garden of Eden," page 55). Among the birds was Diatryma, a six-foot-tall relative of today's cranes.