Fish Fossil – Knightia altaIn South West Wyoming, Colorado and Utah in the Green River Formation are found exquisite fossil fish. The Green River system was composed of three lakes. These Eocene lakes were in a series of basins formed during the uplift of the Rocky Mountains. Green River Formation fossils are between 60 and 40 million years old. They are in an excellent state of preservation. The most common fish is Knightia, a herring like fish. They are found singly or in large groups. These groups are called ‘mortality plates’. The cause is believed to be volcanic eruptions producing ash which poisoned the waters. Green River Formation fossils are some of the finest known. They are much sought after throughout the world.