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Fossil Info

In the early Eocene Epoch, over 50 million years ago, drainage from the newly uplifted Rocky Mountains filled an intermountain basin to form what geologists call Fossil Lake.

The climate of Fossil Lake was subtropical, similar to the climate of Florida today. The lake's paleoenvironment persisted for about 2 million years, and was home to palm trees, crocodiles, turtles, and an abundance of fish. For number, variety, and detail of fossil fish, few places can equal ancient Fossil Lake. Altogether, more than 20 species of freshwater fish have been identified there and many are recognizable as ancestors of today’s species.

On numerous occasions, unique conditions came together to result in some of the best preserved fossils ever discovered. Unlike other fossil formations around the world where fragments of specimens are prevalent, Green River fish are most often perfectly preserved in entirety. Many of the fish, for example, retain not only their entire skeletons, but their teeth, delicate scales, and skin as well. The unusually excellent preservation of the Green River fish fossils is generally attributed to a combination of two main factors: 1) a cold period during the Eocene that would have caused dead fish to sink faster due to a less inflated swim bladder; and 2) the great depth of the lake and the consequent anoxic (devoid of oxygen) conditions that would have often prevented scavengers from disturbing the carcasses and inhibited decomposition. Another essential ingredient for preservation was the rapid burial in calcium carbonate, which precipitated out of the water and fell like a constant gentle rain to the bottom of Fossil Lake. Whatever sank to the bottom – dead fish, fallen leaves – was covered by this protective blanket. Year after year for hundreds of thousands of years, this reoccurred. Perhaps, however, most remarkable of all is the story the fossils tell of an ancient life and landscape.

While many of Fossil Lake’s animals and plants probably died natural deaths, on several occasions huge numbers of fish were killed suddenly. These die-offs are recorded on great slabs of the Green River Formation called mass mortality layers. Several of the laminated sedimentary layers contain so many fish fossils that scientists believe there were relatively frequent events such as algal blooms which resulted in large scale fish kills. Algae and other plant and animal life would die and fall to the bottom as in lakes and ponds today. Storms brought runoff from the mountains, covering the fish with mineral rich material that would ensure their preservation.