Know Your Terms



Fossil fish are ultimately a matter of preference (and price!). The following species are abundant and relatively inexpensive when compared to rarer fish and high-demand fossils, such as palm fronds and reptiles. Any or all would work well for your project.

Green River Basin fossil fish priscacara


This member of the perch family is perhaps the most popular of the Green River fish fossils. Mass mortality of this species suggests they schooled like modern bluegills or crappie.

Shape: Like a sunfish, with sturdy, protective dorsal and anal spines.

Length: From 6″ (Priscacara liops) to 17″ (Priscacara serrata)

Green River Basin fossil fish knightia


This member of the herring family is by far the most commonly excavated fossil fish in the Green River Formation — and is the official state fossil of Wyoming. Knightia were a small schooling fish, which made them an abundant food source for larger predators.

Shape: Like a sardine with heavy scales and small, conical teeth.

Length: 3″-4″ is average, but larger specimens can be found.

Green River Basin fossil fish diplo


“Diplos” were a larger order predator and can often be found with a Knightia in their jaws or stomach.

Shape: Diplos have the classic upturned jaw of a surface feeder, and are a large, substantial fish.

Length: Normal range is 5″-8″, but larger 16″-20″ specimens are also common.

Green River Basin fossil fish mioplosus


With its mouthful of sharply pointed teeth, this perch-like fish is thought to have been a voracious predator. Mioplosus fossils are never found in large groups, which suggest they were solitary hunters.

Shape: The long, strongly built body is distinguished by a fan shaped tail and symmetrical back dorsal and anal fins.

Length: The Mioplosus averages 6″ or more in length.


Articulated Fossils

Intact fossils in which all bones are in place, rather than scattered about. This is one of the things that makes fossils from the Green River Formation so desirable.

18-inch Layer

The best-preserved fish from the Fossil Lake area come from the so-called 18-inch Layer. Just as it sounds, this layer of sediment averages about 18 inches in thickness and is highly laminated, so the fish can often be removed nearly whole.

Split Fish Layer

By contrast, the Split Fish Layer is about six feet thick. This layer is unlaminated, making it far more difficult to extract and prepare the best fossil fish.

Mass Mortality Pieces

Some of the most stunning fossil slabs contain hundreds of individual fish and likely represent an instantaneous die-off, or mass mortality, of schooling fish.

Flora & Fauna

By far the rarest and most desirable Green River fossils are of turtles, bats, birds and snakes. Palm fronds, ferns and sycamore leaves are also highly desired.