The Green River Formation, Wyoming: A Hotbed Of Fossilised Fish

If you're searching for the best fossil fish specimens in the world, all roads lead to one place, Wyoming’s Green River Formation. These ancient fish are more than just glimpses into the past, they’re proof of a vibrant ecosystem that existed over 50 million years ago during the Eocene epoch.

Notogoneus detail

What makes Wyoming fossil fish so special is the exceptional level of preservation. The Green River Formation, located in southwestern Wyoming, is made up of ancient lake beds that once formed a lush, subtropical environment. These calm, mineral-rich waters were the perfect conditions for fossilization. When fish died, they sank to the bottom of the lake where oxygen levels were low and sediment quickly covered their bodies, helping preserve even the most delicate features like scales, fins and bones! Unlike many fossil sites where you’re lucky to find a single bone or partial imprint, here you regularly find complete fossils that look as if the fish died only yesterday.

Knightia School

The most commonly found species in this formation is Knightia, a small herring-like fish that lived in large schools. These fish were a crucial part of the food pyramid, serving as prey for larger fish, birds and reptiles. Knightia fossils are often found in slabs containing multiple individuals and sometimes in what appear to be dramatic death scenes, evidence of sudden die-offs, perhaps from algal blooms or volcanic activity.

Priscacara

Slightly rarer but even more sought-after is Priscacara, a thicker-bodied, spiny-finned predator that resembles perch or sunfish. These fish had strong jaws and were likely hunters in their ecosystem. When you find a well-preserved Priscacara, it’s often a showstopper, with visible teeth, pronounced spikey fins, and sometimes even fossilized stomach contents!

Diplomystus

Diplomystus is another well-known fossil fish from the Green River Formation, this extinct genus is closely related to modern-day herrings but can be distinguished by its upturned mouth and more robust build. With its distinctive jaw structure and impressive size, often reaching over 20 inches in length, Diplomystus was a mid-level predator that likely fed on smaller schooling fish like Knightia, and it’s not uncommon to find fossils showing a Diplomystus in the act of swallowing its prey!

Mioplosus (larger fish)

Mioplosus was one of the top predators in the ancient lakes of the Green River Formation. Closely resembling modern-day bass, this fish had a streamlined body, sharp teeth and a fierce predatory nature. Unlike Knightia or Diplomystus, Mioplosus fossils are usually found alone, likely because they were solitary hunters rather than schooling fish. Their powerful jaws and muscular build suggest they fed on a wide range of smaller fish and aquatic animals.

Notogoneus (larger fish)

Notogoneus is one of the more unique and less commonly found fossil fish from Wyoming’s Green River Formation. Unlike the smaller, schooling fish like Knightia or the sharp-toothed predators like Mioplosus, Notogoneus was a larger, bottom-dwelling species that could grow up to three feet in length! It had a long, torpedo-shaped body and a distinctive downturned mouth, which suggests it fed by scavenging along the lakebed, likely consuming small invertebrates & plant material. Its structure hints at a slow-moving lifestyle and fossils of Notogoneus are far rarer than those of other species in the region.

	Knightia	Diplomystus	Mioplosus	Priscacara	Notogoneus
Size Range	3–10 inches	6–20 inches	6–20 inches	4–10 inches	Up to 3 feet
Diet	Plankton, algae	Small fish	Small fish	Small fish & invertebrates	Plant material & small fish/creatures
Predatory?	No prey	Yes mid-level predator	Yes top predator	Yes ambush predator	No bottom-feeder
Social Behaviour	Schooling fish	Likely solitary or in small groups	Solitary predator	Possibly solitary or loosely grouped	Likely solitary or slow-moving
Distinctive Traits	Streamlined body, found in groups	Upturned jaw	Torpedo shaped body, visible teeth	Rounded body, spiny fins	Long-bodied, downturned mouth

Knightia

Diplomystus

Mioplosus

Priscacara

Notogoneus

Fossil fish are just the beginning. The Green River Formation is incredibly diverse and has yielded fossils of turtles, crocodiles, stingrays, insects, bats, birds and plant material like palm fronds and sycamore leaves. While dinosaurs were long extinct by the time these fish lived, the Eocene epoch came about 15 million years after the last dinosaurs died out, but the world was still full of fascinating prehistoric creatures. Early mammals like small primates, large flightless birds and giant land crocodiles roamed the forests nearby.

It’s important to note that while these fossils are found as they are, naturally preserved in the rock exactly where they died, some may be enhanced by hand to bring out the details more clearly. This can include light painting, staining or surface preparation to emphasize the bones against the limestone background. These techniques do not alter the fossil itself but help make the anatomical features more visible, especially for display purposes.