Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years ago - Lake Gosiute, Lake Uinta, and Fossil Lake, the smallest. All are gone today, but they left behind a wealth of fossils in lake sediments that turned into the rock layers known as the Green River Formation, made up of limestone, mudstone, and laminated volcanic ash. The fossils are among the most perfectly preserved remains of ancient life in the world. Some of the most extraordinary of these fossils came from Fossil Lake, represented today in flat-topped hills where Fossil Lake once was. Fossil Butte National Monument preserves a butte at the lake's center and its invaluable, fascinating record of the past.

Priscacara, up to 16 inches.

The fossils of Fossil Lake are remarkable for their abundance and the broad spectrum of species found here - more than 25 kinds of fish, many varieties of insects, plants, reptiles, birds, and mammals. Paleontologists, the scientists who study fossils, and private collectors have unearthed literally millions of specimens during the past 100 years. Many billions more still lie buried in the hills.

The fossils are remarkable for their detail. Many fish, for example, retain not only their entire skeletons, but their teeth, delicate scales, and skin as well. And perhaps most remarkable of all is the story the fossils tell of an ancient life and landscape.

The lush Eocene swamps around Fossil Lake contrast with present day Wyoming.

The scene 50 million years ago, during the Eocene Epoch of the Cenozoic Era, was quite different from today. Fossil Lake, 50 miles long and 20 miles wide at its maximum, nestled among mountains in a lush green forest of palms, figs, cypress and other subtropical trees and shrubs. Willows, beeches, oaks, maples, and ferns grew on the lower slopes, and on the cool mountain sides was a spruce and fir forest. In and around the warm waters of the lake animal life was diverse and abundant. A broad range of fish

A large palm frond at the visitor center

inhabited the tributaries, shallows, and deep water of Fossil Lake during its unusually long life of more than 2 million years. Gars, paddlefish, bowfins, and stingrays, though they may appear primitive to some, still survive today, as do herring, perch, and mooneyes. The lakeshore was alive with crocodiles and turtles; insects, dog-sized horses, and early primates inhabited the land; birds and bats mastered the air.

Ideal Conditions for Fossil-Making

What events led to the preservation of so much of Fossil Lake's life as fossils? No one knows for sure, but after careful study scientists have developed hypotheses to explain the process. One essential ingredient for preservation, they believe, was burial in calcium carbonate, which precipitated out of the water and fell like a constant gentle snow on the bottom of Fossil Lake. Whatever sank to the bottom - dead fish, fallen leaves - was covered by this protective blanket. Year after year for

A well preserved stingray.

hundreds of thousands of years, this reoccurred. Some of the most perfectly preserved fossils come from the deep-water sediment layers of whitish to buff-colored limestone alternating with brown oil shale commonly called the 18-inch layer. The fossils are generally adult fish. An equally important fossil-bearing layer comes from nearer the lake shallows and is composed of lighter colored limestone with weak laminations. The rock tends to split right through the fossils, which is how it got its name, the "split-fish" layer, averaging 6.5 feet thick. Here one finds younger fish and species that would have survived better in near shore shallows - crayfish and stingrays, for example.

Part of a Knightia mass mortality.

Unsolved Mysteries

While many of Fossil Lake's animals and plants probably died natural deaths, on several occasions huge numbers of fish were killed suddenly. These die-offs are recorded on great slabs of the Green River Formation called mass mortality layers. What killed these fish? A superbloom of blue-green algae that used up oxygen in the water? A sudden change in water temperature or salinity? All of these? Ongoing research may solve the mystery.