Natural History - British Sigaloceras Fossil Ammonite Multi Display

Diplomystus sp. on a large rectangular matrix. 724 grams, 21.1 cm



From The Green River Formation, Wyoming, USA. Ex Lincolnshire, UK, collection.

In southwest Wyoming (and parts of Colorado and Utah) in the Green River Formation are found some of the world's most outstanding specimens of fossil fish. The Green River system was composed of three lakes: Lake Ulinta, Lake Gosiute and Fossil Lake. These Eocene lakes lay in a series of intermountain basins formed by geological events that uplifted the Rocky Mountains during the early Tertiary time. The climate was much different from the desert-like climate of this area today. Both the fauna (crocodiles, alligators, boa constrictors and some subtropical fish families) and the flora (such as large palm trees) indicate a climate much like that found along the Gulf Coast today. Large amounts of ash found in the sediments indicate that volcanoes were particularly active at this time.

Cleoniceras sp. with much of the original shell material remaining, showing red, green, and gold iridescence. 293 grams, 10.5 cm



Ex Lincolnshire, UK, collection.

A Diplomystus sp. fossil fish on a rectangular matrix. 439 grams, 20.7 cm



From Wyoming (Green River Formation), USA. From an old Bristol, UK, collection.

Seasonally some 58 million years ago, in the Green River region of Wyoming, torrential rains fell in the mountains. They flowed down the mountain sides, forming wide shallow lakes in the valley below. During the dry season, these lakes would dry up due to evaporation (as happens in certain parts of Africa today). Millions of fish died in this short period of time. This climatic cycle happened over a 20-thousand-year period. Layer upon layer of mud entombed all these fish. At a much later date, volcanic activity uplifted this area to expose the fossils.