Eocene Period, 56-33 million years BP. A fossil fish Knightia alta on a rectangular matrix with parts of other fish to both sides. 366 grams, 17cm (6 1/2"). From the Pradi collection, Boston, USA; acquired during the 1980s; from the Green River Formation, Wyoming, USA. Seasonally some 58 million years ago, in the Green River region of Wyoming, torrential rains would occur in the mountains. The rain would flow down the mountain sides forming wide shallow lakes in the valley below. When the dry season would occur, these lakes would dry up due to evaporation (similar to certain parts of Africa today"). Millions of fish would die in this short period of time. This climatic cycle happened over a 20-thousand-year period. Layer upon layer of mud was deposited over these years entombing all these fish. At a much later date, volcanic activity uplifted this area to expose all those layers of fossil fish. [No Reserve]