Green River Formation

Treasures from a Greenhouse World

Fossils from the Green River Formation


Fifty million years ago, our world was a different place. An atmosphere rich in greenhouse gases meant that global temperatures averaged around 82 degrees Fahrenheit (28 degrees Celsius), compared with just 57º Fahrenheit (14º Celsius) today. There were no polar ice caps. Forests inhabited by frost-sensitive plants and animals grew far north of the Arctic Circle. Much of what is now the United States was covered in dense tropical and subtropical rain forests.


What was this ancient environment like?


The area of the Green River, a tributary of the Colorado River, is today cold sagebrush desert. Precipitation averages between 9 and 12 inches (23 to 30 centimeters) per year; most of it falling as snow. Winters can be extreme, with temperatures sometimes falling to –30 °F (–34 °C) or less. The mean frost-free period is only 59 days.

The fossils preserved in the ancient lake sediments show that 50 million years ago this environment was very different. Evidence of palm trees, tropical vines, alligators and crocodiles, and fish that today are found only in tropical regions suggest a much warmer and wetter climate, with rainfall from 30 to 40 inches (76 to 102 centimeters) per year and an overall average temperature of 60 to 70 °F (16 to 21 °C).


Why was it so warm and wet?


During the early Eocene, Earth’s atmosphere was much warmer and damper than it is today. There are two main reasons why this was so. First, the concentration of “greenhouse gases” in the atmosphere was much higher than today. Greenhouse gases absorb and emit infrared radiation, which tends to have a warming effect on our planet’s atmosphere. In the Eocene, the most important of these gases was methane, also called swamp gas or natural gas.

Second, the positions of Earth’s continents at the time allowed warm water to circulate into high latitudes, which prevented the formation of polar ice caps. Ice caps are important to the maintenance of Earth’s cooler climate today because they reflect sunlight that might otherwise warm the planet’s surface. The ocean-cooling effect of the ice caps also helps to keep methane trapped in ocean sediments and ice caps trap water that might otherwise enter the atmosphere and fall as rain. Around 60% of Earth’s freshwater is held in the Antarctic ice cap. This water was circulating freely through rivers, oceans and the atmosphere 50 million years ago.

50 million years ago, the Green River region was a lush, subtropical environment inhabited by tapirs, crocodiles, palm trees, and other organisms that are restricted to equatorial regions today.

© National Park Service, mural by Robert Hynes.

YPM PU 18150
Icaronycteris index
Kemmerer, Lincoln County, Wyoming, USA
Collector: C. Cushman


Complete bat fossils are very rare—this is one of only four intact specimens of Icaronycteris known, all of which are from the Green River Formation. At first sight, Icaronycteris looks very like one of today’s bats, but there are some crucial differences: the tail is much longer and was not connected to the hind legs by membranes; there is a claw on the first wing finger; and its teeth are much less specialized than those of a modern bat. Until recently, Icaronycteris was the most morpho- logically primitive bat known.  Although that is no longer the case, it is still the earliest known bat to have hunted insect prey using echolocation.

YPM PU 23662
Chisternon sp.       
Lincoln County, Wyoming, USA
Collector: W. Ulrich

Chisternon is a member of an extinct group of bottom-dwelling, freshwater turtles called baenids. Turtles are one of the groups of animals that prospered in the aftermath of the extinction event at the end of the Cretaceous 65 million years ago. Baenids were common turtles in North America from the Early Cretaceous until the Late Eocene, when they became extinct.

Aves, species indeterminate
Sweetwater County, Wyoming, USA. Collector: unknown


The Green River Formation has produced more specimens of fossil birds than any other site in North America earlier than the Pleistocene, including complete skeletons, disarticulated bones and skulls, and even a nest with eggs. This isolated feather is an example of the exquisite preservation of detail in fossils from these fine-grained lake sediments.

YPM 7261
Heliobatis radians   
Lincoln County, Wyoming, USA
Collector: unknown


Heliobatis is a bottom-dwelling, freshwater stingray.  Male rays, like their close relatives the sharks, have a pair of specialized reproductive appendages called “claspers” behind their pelvic fins; this specimen has no claspers, so it was a female. The name Heliobatis means “sun ray” and refers to the animal’s circular disc-like body. Like modern stingrays, Heliobatis probably fed on crustaceans, clams, snails, and small fish.