Fossils

Plastomenus thomasii turtle fossil

NPS Photo

The fossils found in the top layers of Fossil Butte and the surrounding ridges are the remains of plants and animals that lived in and around a large freshwater lake 52-million years ago in the early Eocene Epoch. They are world-renowned for their diversity, abundance, and preservation all of which is due to Fossil Lake’s unique conditions.

While Fossil Lake was a freshwater lake, there was a layer of low-oxygen saltwater at the bottom. The lack of oxygen and saltwater kept the freshwater fishes, crocodilians, and turtles from scavenging upon animals that sank to the bottom of the lake after death. A microbial mat, that included photosynthesizing cyanobacteria, grew on the lake bottom. When dead animals drifted down onto the mat, it would quickly grow over the organism to reach sunlight once again. Doing so would hold the animals in place, preventing them from breaking apart and floating as gases developed during decomposition.

The freshwater was rich in calcium and carbonate ions. When storms stirred the water, carbon dioxide was released causing the ions to combine into calcite particles. This precipitate sank to the lake bottom, creating a thin layer of sediment that would eventually become limestone. The microbial mat would grow through the sediment to reach the sun. This layering of alternating microbial mat and limestone sediment repeated thousands of times creating the beds containing the fossils.

Over the years, these layers slowly compacted, flattening the organisms. The soluble organic material was carried away by groundwater, leaving the less-soluble carbon which stained the bones and scales the variety of browns we see in the fossils today.

Due to these environmental conditions, animals that were once living in and around Fossil Lake were often preserved fully articulated (complete). Although most of the fossils collected from the Green River Formation (GRF) are fishes, many other organisms have been found including crocodilians, lizards, snakes, birds, bats, small horses, amphibians, gastropods, crustaceans, and insects. The astounding biodiversity preserved in the rock record makes Fossil Lake one of the better understood prehistoric ecosystems.

Find out more information about the various fossil species of the Fossil Butte Member (FBM) and check out the fossil photo galleries below. Learn more about the geology of the GRF.

 

 

 

If you want to view data from the fossils collected in the park's research quarry, visit our quarry data page. If you do not see recent data, check back as the page is updated periodically.