Fossil lake

Natural History, July-August, 1998 by Lance Grande

In the high mountain desert of southwestern Wyoming, about 7,500 feet above sea level, rain is scarce and winters are long and cold. Starting about September, temperatures frequently go below freezing, and winter snows often persist through late May. When summer finally comes to the area, I arrive with a crew from Chicago's Field Museum of Natural History to explore one or more of the half dozen fossil quarries that lie about ten miles west of the town of Kemmerer. Even after twenty years of fieldwork here, I'm still thrilled to lift up a limestone slab, peer beneath it, and find a plant or animal that has no seen the light of day for millions of years. Entombed in the fossil-rich layers (geologically known as the Fossil Butte Member of the larger Green River Formation) are the remains of an extinct lake community. Beautifully preserved organisms--from pollen to palm fronds, stingrays, and thirteen-foot crocodiles--allow us to glimpse a time when this now arid expanse was warm, watery, and lush.

Paleontologists refer to mother lodes of fossilized flora and fauna as Lagerstatten, and the Fossil Butte deposits represent one of the richest Lagerstatten in the world. My field crew and I do most of our excavating in the limestone derived from the 50-million-year-old sediments of Fossil Lake. This lake was by far the smallest of three extinct bodies of water than once formed a huge lake complex. Fossil Lake, Lake Uinta, and Lake Goshuite covered tens of thousands of square miles of what are now parts of Wyoming, Utah, and Colorado. One of the world's longest-lived great-lake systems, it originated about 60 million years ago and lasted for nearly 20 million years before disappearing toward the end of the Eocene. (In contrast, the present-day Great Lakes system of North America has been around only about 15 thousand years, and the Great Rift Valley lakes of Africa, thought to be the oldest existing lakes, are about 10 million years old.)

Literally millions of fossils have been mined from the Fossil Butte deposits since they became known in the mid-1800s; more than half a million fossil fish skeletons have been excavated during the last three decades alone, and the sediments are still phenomenally productive. The Fossil Butte National Monument helps protect part of the Lagerstatten and displays fossils in its Visitor Center. My field crew and I work on private land--a horse ranch near monument lands, and I owe much to the Lewis family, who own the ranch; the Tynsky family, who allow me and my crew to excavate in their leased quarry; and the National Park Service personnel, who sometimes lend a hand with the excavations.

The fossilized plants and animals of the once-thriving aquatic community have given us clues to the ecology of the region 50 million years ago. The presence of lily pads, cattails, insect larvae, certain snails and clams, particular kinds of fishes and turtles, a salamander, and a frog point to freshwater conditions, and the pure calcium carbonate/organic composition of the roc indicates that the water was alkaline. Only a tropical, or at least subtropical, climate could have supported the palms, banana plants, balloon vines, tsetse flies, alligators, crocodiles, four-foot lizards, and boa constrictor-like snakes that are preserved in the rocks. As the climate changed drastically some 40 million years ago, all of these species vanished from western North America, although close relatives of some of them still live in tropical and subtropical regions of the world today.

Fossil fishes--plentiful, diverse, and often preserved down to the finest details of bones, scales, and teeth--point to a subdivision of Fossil Lake into near-shore and midlake communities. The near-shore areas were the habitat of most of the stingrays, paddlefishes, mooneyes, trout-perches, and the larval and juvenile forms of many fish species, as well as shrimps and crayfishes. The deeper, midlake waters were home to the largest individuals of most species and to the long-bodied Notogoneus, a fish curiously absent from near-shore localities. Gars, bowfins, herringlike fishes, bony tongues, and spiny-rayed fishes occupied both parts of the lake. Although absent from Fossil Lake, catfishes and suckers inhabited Eocene Lake Goshuite to the east. The Green River Formation contains early members of many extant North American fish families, and this fauna is key to better understanding the origin and development of modern freshwater fauna.

In the midlake quarry where my field crew and I often work, most of the fossils come from a fourteen-to-eighteen-inch layer of limestone, still nearly horizontal, that lies beneath twenty feet of dirt and broken rock. This limestone layer is naturally protected from groundwater and other weathering elements by a thin cap of oil shale on its upper and lower surfaces. In a few short weeks of work, we can expect to fill the Field Museum's trucks with a ton or more of fine fossils neatly trimmed from limestone slabs. Fossil Butte specimens from previous excavations are on view not only at the Field Museum and Fossil Butte National Monument but also at many other institutions, including the American Museum of Natural History in New York and the British Museum in London.