Eocene Knightia Fossil Fish

Between 34 to 56 million years ago, during the Eocene Epoch, the subtropical climate in southwestern Wyoming supported life for numerous aquatic species in Lake Gosiute. As a thriving ecosystem with abundant marine plankton, the freshwater lake sustained herring shoals and varieties of predatory fish, stingrays, and crustaceans. Palm trees, crocodiles, and lizards also inhabited the area.

One species of herring, the Knightia, is so plentiful that it later becomes the most common vertebrate fossil discovered anywhere the world. The factors were optimal for perfectly preserving mass amounts of Knightia in a large muddy lake bed with high concentrations of lime. Lake Gosiute, covering an area of fifteen thousand square miles, fluctuated in depth frequently and often exposed its lake bed to dry heat.

Over time, sediment layers flattened under pressure and temperature swings to create large shale and soda ash deposits known collectively as the Green River Formation. Countless, well-preserved fossils have been unearthed from the Green River Formation, including the oldest known flying mammal – an ancient twenty-clawed bat that flew without echolocation.

Almost fifty years ago, the government recognized the historical importance of the vast Fossil Lake and acted to preserve the area itself by declaring it a national monument. The act has helped protect the ancient lake, but opportunistic collectors still seek to exploit the land.

Knightia fossils, though abundant, are a detailed reminder of a very different time in Wyoming’s history when a varying subtropical climate actively preserved life in shale. Today we are able to understand more about the ancient life of Lake Gosiute with thanks to national protection and recognition. It is imperative, for the Green River Formation and fossil beds like it, to keep these areas as fossils themselves and preserved for many generations of people to learn from.