Eocene Period, 58-36 million years B.P. A Knightia alta fossil fish in a matrix. 313 grams, 17 cm (6 1/2 in.). Green River region, Wyoming, USA. Acquired 1980s. Pradi collection, Boston, USA. Seasonally, some 58 million years ago, in the Green River region of Wyoming, torrential rains occurred in the mountains. The rain would flow down the mountain sides, forming wide, shallow lakes in the valley below. During the dry season, these lakes would dry up due to evaporation (similar to certain parts of Africa today). Millions of fish died in this short period of time. This climatic cycle recurred over a 20-thousand-year period. Layer upon layer of mud was deposited over these years, entombing huge numbers of fish. At a much later date volcanic activity uplifted this area to expose the fossil layers. [No Reserve]