Fossil Butte National Monument Geologic Resources Inventory Report

Approximately 50 million years ago, at the beginning of the Eocene Epoch, frigatebirds swooped above the waves of Fossil Lake, a vast, expansive lake that once occupied today’s semi-arid, sagebrush and greasewood region of Wyoming’s Fossil Basin. At least 15 genera of fish swam in the lake, including paddlefish, stingrays, gar, pike, trout-perch and the abundant herring Knightia, Wyoming’s state fossil.