Finding Fossil Fish in Wyoming

September 22, 2026

A 52-Million-Year-Old Adventure

What was Wyoming like 52 million years ago?

Today, much of southwestern Wyoming is dry and rugged, but during the Eocene Epoch, the landscape looked dramatically different. Large freshwater lakes surrounded by lush vegetation supported an incredible variety of life, including fish, turtles, crocodilians, birds, plants, and more.

We traveled to the Green River Formation in Wyoming to experience this ancient ecosystem firsthand and search for fossils preserved in the limestone.

The Ancient Lakes of the Green River Formation

During the Eocene, three enormous lakes occupied parts of what are now Wyoming, Colorado, and Utah. One of these was Fossil Lake, where the fossils in our adventure were discovered.

When a fish died, it could sink to the lake bottom, where microbial mats helped cover and preserve it. Over millions of years, layers of sediment accumulated above the remains. The weight of those layers compressed many of the fish into the remarkably detailed, flattened fossils we uncover today.

Splitting Limestone to Find Fossils

Finding these fossils requires carefully splitting layers of laminated limestone.

Instead of smashing the rock, the limestone is treated almost like a book. A hammer and chisel are used to separate the thin layers or “pages,” hopefully revealing a fossil that hasn't seen daylight in approximately 52 million years.

And sometimes you never know what you're going to uncover!

During our dig, we discovered several different fossils, including fossil fish, a freshwater gastropod, and even coprolites, or fossilized poop.

Fossil Fish of the Green River Formation

One of our finds was Diplomystus, a freshwater relative of modern herring. Its upward-turned mouth was well suited for feeding near the surface, where it could prey on smaller fish and possibly insects. Larger Diplomystus could reach more than 20 inches in length.

We also uncovered Knightia, one of the most common vertebrate fossils in the Green River Formation and the state fossil of Wyoming. These small schooling fish were incredibly abundant in Fossil Lake and are sometimes found together in large groups on a single slab.

Another impressive find was Priscacara, a relative of modern sunfish. Its prominent spines and well-developed vertebrae hint at a powerful swimmer capable of quick bursts of speed, useful for both catching prey and escaping larger predators.

More Than Just Fish

The Green River Formation preserves much more than fish.

During our visit, we found a fossil freshwater gastropod whose shell offered another glimpse into the ecosystem. Mollusks were an important part of the ancient lake environment and provided food for shell-crushing fish.

Coprolites were another surprisingly common discovery. Unlike many of the flattened fish fossils, some coprolites retain a three-dimensional shape. They may not be the most glamorous fossils, but they can provide valuable evidence about ancient animals and ecosystems.

A Very Different Wyoming

Standing at the quarry today, it can be difficult to imagine just how different this area once looked.

The ancient lake was surrounded by vegetation including sycamores, palms, and lily pads. Crocodilians lived in the region, along with turtles, tapirs, birds, and many other animals we wouldn't normally associate with modern Wyoming.

The Green River Formation has also produced an extraordinary diversity of bird fossils, including small songbirds, parrots, wading birds, frigatebird-like birds, and falcon-like predators. Together with the fish, reptiles, plants, insects, and other organisms, these fossils reveal a rich Eocene ecosystem.

Bringing the Past Back to Tyrannostorus

At the end of the dig, we brought some of our discoveries back to Tyrannostorus, where several specimens will receive additional preparation.

Every layer of limestone offers another opportunity to uncover a small piece of an ecosystem that existed more than 50 million years ago. That's what makes fossil hunting so exciting. You never know what the next split will reveal.