Green River Formation

Some of the world's most recognizable fossils come from the famous Green River Formation located in the western part of the United States of America. Luckily, this formation is also one of the world's most abundant producers of fossils. This means you have likely seen some of these specimens in person or even have one in your own personal collection! Get ready as we dive into the past of this fascinating part of the world's natural history as well as the many different species of life that thrived there, both in and out of the water.

Browse our Green River Formation fossils here!

The Formation

The Green River Formation spans three separate US states: Colorado, Utah, and Wyoming. This formation contains accurate sedimentation records within what used to be a series of lakes located in multiple basins sat between a range of mountains. While today it is near the present-day Green River, these ancient lakes were abundant with life over 50 million years ago, during what is known as the Eocene Epoch. The water flowing from the high surrounding altitudes fed into a lake that was 120 square miles, eventually contributing to the great fossilization found in this area.

Ancient Lakes of the Green River Formation


The Discovery

The first documented fossil records found in this area are the journals of various missionaries and explorers during the 1840s. While the first specimens ever found were invertebrates, the first ever fish fossil, Knightia eocaena, was excavated in 1856 by Geologist Dr. John Evans and originally described as Clupea humilis. The location of this specimen is not well documented. Dr. Evans traveled to this area of the US many times during his life.

Ferdinand Vandeveer Hayden, the head geologist at the United States Geological and Geographical Survey of the Territories, originally named the formation the "Green River Shales" beginning in 1869.

 Ferdinand V. Hayden (1870) (Left) & John C. Frémont (Un.) (Right)


Natural History

The Green River Formation is crucial for understanding the Eocene Epoch. This was the second epoch of the Paleogene Period during the Cenozoic Era. It lasted between 55.8 and 33.9 million years ago. While determining the age of rocks can be difficult, the Green River Formation has been dated by examining volcanic grains. The volcanoes currently located in Yellowstone National Park would occasionally spew ash into the air. This fallout would then settle in thin layers on the lakebeds. Due to the crystallization that took place during these eruptions, dates were able to be applied to the samples found, leading us to an approximate age of the fossils. The sediments contained in the Green River Formation include sandstone, mudstone, siltstone, oil shales, coal, limestones, and dolomites.

The corresponding area of the US was located roughly the same as it is now in regards to its latitude, but the climate was different. Many of the organisms found in the fossil record at these sites indicate the climate during that time was moist temperate to sub-tropical. One of these examples is the presence of crocodile specimens. From what we know about these species, they are only able to survive in areas of constant, warm temperatures. It is estimated the average temperature range from 15-20 degrees Celsius (59-68 degrees Fahrenheit). 

If we were to travel back in time, we would see various species of plants and animals seen in North America in addition to Eastern Asia. The lakes and swamps located in this area at the time allowed near-perfect conditions for fossilization. They were steady, rather calm ecosystems where the remains of wildlife were quickly covered by sediment, chemical molecules, and water provided by the mountain streams. Because of this, we are able to find magnificent specimens of fish, insects, and plants. In addition to the more common fossils, we have also found, birds, turtles, snakes, feathers, and various mammals including bats!


The Fossilization Process

The reason many of the fossils found in the Green River Formation are so well preserved is greatly due to a little molecule known as calcium carbonate (CaCO₃), the building block of limestone. If you swim in a freshwater lake today, you may notice little particles of this material floating around. Eventually, this will settle to the lake floor, continuously layering into the muck you feel between your toes. Downward water pressure of millions of years creates "sheets" of limestone. Anything that died and found its way into the lake waters had the possibility of being fossilized! However, everything has to line up near perfectly for this process to happen. Animals and plants are less likely to fossilize if they are picked apart by scavengers, and decompose too quickly before they are covered due to excess amounts of oxygen.


Species Found Here

There have been hundreds of different species found from all walks of life here: vertebrates, invertebrates, vegetation, you name it. So many that we could not possibly go through all of them. But, is that not part of the beauty of such a rich and vibrant fossil record? Below you will read about some of the commonly found specimens as well as some very unique finds!


Knightia sp.

These little fellas are arguably the most abundant fossil specimen found in the Green River Formation. This fish is related to modern-day herrings, and sardines. Being smaller fish, it is likely they fed on insects, plankton, and algae. Typically, these specimens measure 3-4 inches in length but have been found up to 10 inches long. Similar fish species are found in age-appropriate sediments in South America. It is very common to excavate these specimens in what is called "mass mortality" plates. These events can be caused by a wide variety of reasons including...

- A sudden change in water temperature. a Change in just a couple of degrees could drastically change the survival rate of a species.

- A significant increase in algae. This change could greatly impact the amount of oxygen in the water, causing fish to suffocate.

- A sharp change in the water pH. The spews of volcanic ash could have changed the acid-base balance in the water.

- Increased evaporation could cause water levels to drop, creating small pools of water where trapped wildlife would starve.

Knightia sp. - Mineralogy Specimen 

Mass Mortality Fossil Plate - Multiple Species

Diplomystus sp.

This is the second most commonly found fish fossil in the Green River Formation. It is also related to herrings but can be easily distinguished from Knightia by its wider abdomen and sharper angles surrounding its jaw. Specimens of this fish have been found at lengths up to 25 inches long. It has also been found in areas of Brazil, Syria, and West Africa. Modern-day relatives can be found swimming in the waters of Peru and Australia.

Diplomystus sp. - Mineralogy Specimen (18 In.)

Cockerellites sp.

This fish closely resembles the modern-day sunfish. It is a genus of extinct bass that has been found up to 6 inches long. These specimens have also been found in mass mortality plates, suggesting they were schooling fish.

 Cockerellites sp. - Mineralogy Specimen (4.5 In.)

Xiphotrygon sp.

While this is not actually a fish, this genus of stingrays is closely related to sharks and can be found in the layer of sediment at the Green River Formation. Like sharks, its body is heavily composed of cartilage, which does not normally fossilize well. However, this is a prime example of just how well the preservation record is at these fossil sites. Its mouth was on its underside, suggesting it fed on the bottom of lake beds. It possessed flatted teeth, suggesting it feasted on various shelled invertebrates. Relatives can be found in coastal waters.

Xiphotrygon sp. - Collection of University of Wyoming


Apatemys chardini

There has only been one specimen of this species discovered to this day. From the fossil, we can see that it had both feet and claws that were likely used to grasp at tree branches. Because there has only been one example found, many scientists believe this species was not necessarily native to the area, making it a very special case. 


Apatemys chardini with fish (Cast) - National Park Service

Icaronycteris sp.

The Green River Formation has produced the oldest specimens of a fossil bat known at Icaronycteris index, along with three others. What makes this fossil so extraordinary is just how well-preserved it truly is. You are able to see the full skeleton, cartilage, and membranes of the wings. This specimen was so well preserved it still had recently eaten contents in its stomach. Because of the preciseness, scientists determined that bats had fully developed flight by the Eocene Epoch.

Icaronycteris index - Berkeley

Palaeosinopa didelphoides

This otter-like mammal was found with many similarities to what we see in the wild today, even the contents of its stomach. However, it was not related to otters at all. This is a prime example of convergent evolution when different species develop similar traits independent of one another due to environmental factors. 


Palaeosinopa didelphoides (Cast) - National Park Service

Tapiromorph Discovered

In 2016 the largest mammalian fossil ever found in the Green River Formation was excavated. As with most fossils discovered, it was found in several different parts with associated damage. It has been identified as a member of the tapiromorph family, meaning it is related to modern-day tapirs. Not only does this specimen further conclude the type of environment this formation used to be in, but it was dated older than any other discovered tapiromorph fossil. This means it could be the oldest member of the family ever found!

Tapiromorph Fossil Preparation - Wyoming Geological Survey ALL RIGHTS RESERVED


Many of the world's absolute best-fossilized bird specimens have come from the Green River Formation. There have been dozens of bird species found to this day, many of which have yet to be accurately identified. Just east of this area, scientists have discovered fossilized nests of flamingos! These are known scientifically as rookeries.

Cyrilavis colburnorum

Parrots today have very strong beaks used for breaking into a wide variety of hard tree nuts. This species appears to lack that feature, leading scientists to believe they did not have as specialized of a diet. 

Cyrilavis colburnorum - The Field Museum

Salmilidae Family

This species is a relatively new discovery. As you can see from the specimen, it did have feathers but this rail-like bird lacked the ability to fly. Because of this they often developed strong, long legs to run across the ground. You can find living relatives in wetlands as they are adapted to the saturated ground and can even walk on lily pads.

Salmilidae Family - The Field Museum


There have been many, different reptile specimens found in the area, with turtles being the most abundant. Unfortunately, many of these specimens are found in smaller pieces, making it difficult to gather as much information as we would like to. However, there are a few cases of full fossilized shells.

Chisternon undatum Aquatic turtle recognized by a longer tail and inability to retract its head into its shell. This species is often considered a poor swimmer, suggesting it stuck to the lake floors.

Oliveremys uintaensis, Axestemys sp., & Plastomenus thomasii - All three of these species are soft-shelled turtles that fed exclusively on other creatures such as fish, amphibians, mammals, and even birds. Their light, flexible shells allowed them to move rapidly through the water with their webbed feet.

Baptemys wyomingenis - This species was an easy-going night crawler. This nocturnal, aquatic turtle fed primarily on vegetation and has only one living relative, Dermatemys mawii.

 Chisternon undatum (Left) & Axestemys sp. (Right) (Casts)  - National Park Service

Boavus idelmani

A complete skeletal specimen was found of this species. It is currently the most complete fossilized snake found in North America. Boavus is an extinct genus of boa that is considered small and primitive. This specimen was first described in 1938 by C.W. Gilmore. The original fossil has been lost, but there are many casts available to study. There was another snake fossil excavated in 2017 but its identification has not been determined yet.

 Boavus idelmani (Cast of 2007 Specimen)

Saniwa ensidens

This is the largest species of lizard found in the Green River Formation, with one discovered specimen measuring six feet long! It is related to modern-day monitor lizards as it is a member of the Varanidae family. This species of lizard was likely a climber, using its large tail to defend itself. 

 Saniwa ensidens - Field Museum of Natural History

Bahndwivici ammoskius

This species of lizard is a unique case. When comparing the fossilized skeleton of Bahndwivici ammoskius to the living Shinisaurus crocodilurus, they are indistinguishable. Interestingly enough, the living species today are only found in freshwater environments in present-day China. It is believed this lizard fed on fish, insects, and snails.

Afairiguana avius

While many fossilized lizards are known from jaw fragments, this species has been identified by a nearly complete and articulated skeleton uncovered in 2007! It likely fed on smaller creatures such as insects and spiders. Currently, this discovery is the oldest member of the Iguanidae family in the Western Hemisphere.

Afairiguana avius - Field Museum of Natural History


While insects are a very common occurrence in the Green River Formation, spiders are not. There have been a few different specimens found which have been narrowed down to the Thomisidae family (crab spider), Salticidae family (jumping spider), & the Lycosidae family (wolf spider). It is also worth noting none of these spiders use a web to catch their prey. The reasons behind the lack of spiders in the fossil record could include scavenging before their bodies could be covered by water and sediment, and the bodies generally decaying too quickly because of their soft nature. 

 Lycosidae Family (Wolf Spider) - National Park Service


Insect fossils are a favorite here in our galleries. Fossils that show insects are actually very thin layers of carbon left behind on the rock. These are known as, you guessed it, carbon fossils! These are found by splitting layers of rock. The vast majority of insect fossils found in the Green River Formation are flying or aquatic. Here is a list of some that have been found so far...

- Butterflies, Wasps, Beetles, Dragonflies, Crickets, Ants, Months, Bees

Fossil Dragonfly with Manganese Dendrites - Mineralogy Specimen


There have been over 400 different specimens of plants identified in the area. This is great because the vegetation is a HUGE indicator of what type of climate and ecosystem was thriving there in the past. However, because of the nature and variety of vegetation, is often very difficult to identify exactly what specimens are down to the species level.


Some of the most famous and recognizable plants from the Green River Formation are the palms! These are very common fossils from the area and are arguably one of the most beautiful too. There is A LOT of variation among the fossil specimens discovered, suggesting the area was home to many different species of palm. None of these have been officially described. There are thousands of level relatives found across the globe today.

Sabalites powelli with Fish - Field Museum of Natural History

Salvinia preauriculata

Also known as a "water fern", this species had leaves that would float along the water's surface, allowing its "roots" to hang below. There are just over a dozen living relatives today, spanning many different continents. These thrive in warm water climates.


Salvinia preauriculata - National Park Service


Fun Facts

There have been very, very few amphibians found in the Green River Formation. This included frogs, toads, and salamanders. Some form of their existence must have been present in the area during the Eocene Epoch as specimens have been found in much, much older rocks. It is believed the lack of discovery is likely due to coincidence.

While uncommon, there has been the jaw of a Triceratops sp. found near the area.

In the early 2010s, an almost 11-foot crocodile skeletal fossil was recovered from the Green River Formation.

A fossilized horse was found in the quarry during the spring of 2015. While this specimen was in amazing condition and articulated, it was less than two feet long! 



Berkeley - Localities of the Eocene: The Green River Formation

Geoscience News and Information - Green River Formation Fossils

National Park Service - Fossil Butte: Fossils

National Park Service - The Geological History of Fossil Butte National Monument and Fossil Basin

Smithsonian - Green River Fossil Collections

The Field Museum - Fossilized Birds from the Green River Formation

Wikipedia - Green River Formation

Wyoming Public Media - Researcher Prepares Largest Fossilized Mammal Found In Green River Formation For Science

Wyoming States Geological Survey - Tapiromorph Fossil