Plastomenus thomasii turtle fossil with slightly disarticulated head and legs. From the Green River Formation.
Plastomenus thomasii fossil

NPS Photo

The fossils found here once 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 which can be attributed 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 fell like snow, covering the lake bottom in a thin layer that would eventually become limestone. The microbial mat would grow through this sediment creating another paper-thin layer. This process 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 (intact). 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.
A fossil paddlefish Crossopholis magnicaudatus with dark nose, tail, and fins. Green River Formation
Fossil Fish Species

27 fish species have been identified from the Fossil Butte Member of the Green River Formation.

A fossil bat with right wing outstretched and left wing down and cut off by the edge of the rock.
Fossil Mammal Species

10 mammal species have been identified from the Fossil Butte Member of the Green River Formation.

A coiled snake fossil Boavus idelmani with its head jutting to the right. From Green River Formation
Fossil Reptile Species

The 15 species of reptiles identified from the Fossil Butte Member include turtles, lizards, and crocodilians.

A fossil frog facing left with legs stretched out. From Green River Formation.
Fossil Amphibian Species

2 amphibian species have been identified from the Fossil Butte Member.

A fossil beetle with zigzag dark and light pattern on the wings. From Green River Formation.
Fossil Arthropod Species

Several arthropod specimens have been identified from the Fossil Butte Member.

Several fossil leaves coming off a stem. From Green River Formation.
Fossil Plant Species

Over 400 plant specimens have been identified from the Fossil Butte Member.

Fossil bird Pseudocrypturus cercanaxius with wings disarticulated above spine and legs spread apart.
Fossil Bird Species

Over 30 species of birds have been identified from the Fossil Butte Member.

A pinkish, spiral fossil poop

Coprolites, fossilized poops, come in a variety of shapes and sizes. Discover what we can learn from coprolites.


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.

Last updated: March 13, 2024

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