The Green River Formation of Utah (Colorado and Wyoming) is one of the best locations on the planet to find fossilized fish. The rocks of the Green River Formation contain a story of the ancient past. During this time, the formation of the Rocky Mountains was almost completed. This area featured tall, rugged mountain peaks and deep basins. Preserved in these mountain lake basins, Eocene fossilized fish can be found in large numbers. An unusually cold period paired with the extreme depths of these lake basins, prepped a perfect recipe for this unique preservation. Pristine samples such as pictured below are harder to find.
The Green River Formation has been dated to approximately 50 million years old by analyzing volcanic mineral grains. Volcanic eruptions from volcanoes in Yellowstone and San Juan fields would produce ash clouds that settled in the basin lakes. These clouds form layers of crystallized mineral grains scientists can use to determine the age.
Cockerellites (Priscacara) Liops (pictured above) that dates from the Eocene is about 7 ¾ inches in length and is on a slab that is about 11 inches by 10 inches. Green River is well known for the highly preserved specimens and has been a source of this material dating back to the early to mid 19th Century.
The Green River Formation is also known for the oldest known flying mammal fossil. This fossil is a 5.5 inch bat. This extremely unique and rare specimen was found completely reserved with its skeleton intact. The specimen was so well preserved that is even featured food in its stomach! Scientists were able to study the remains as well as sediment found with the fossil. This gave some great insight into the pollens, algae and anthropods of the time. The Green River formation has been a very important factor in determining the climate of the Eocene. The amount of well preserved specimens helps to reveal a great deal about this period.