Fossil Hunting in Wyoming

Wyoming is one of North America’s great fossil states. Its claim to fame is the Green River Formation — arguably the world’s finest source of Eocene fossil fish — but the state also boasts legendary dinosaur sites spanning the Jurassic and Cretaceous periods, all set against the dramatic scenery of the American West.

Why Wyoming?

Wyoming’s geology reads like a textbook of the last 500 million years. The state sits at the edge of the Western Interior Seaway (Cretaceous), contains some of the most productive Morrison Formation (Jurassic) exposures in the world, and preserves an extraordinary snapshot of Eocene lake life in the Green River Formation.

The Green River Formation is the star attraction for collectors. Approximately 52 million years ago, a series of large lakes covered parts of Wyoming, Utah, and Colorado. Fine-grained limestone accumulated on the lake bottoms, preserving fish, insects, plants, crocodilians, turtles, and even bats in exceptional detail. The Kemmerer area in southwestern Wyoming has been producing museum-quality fish fossils for over a century.

For dinosaur enthusiasts, Wyoming has an unmatched pedigree. Como Bluff near Medicine Bow was ground zero for the famous “Bone Wars” between paleontologists O.C. Marsh and E.D. Cope in the 1870s and 1880s. The Lance Formation in eastern Wyoming has yielded Triceratops, Tyrannosaurus rex, and Edmontosaurus. And new discoveries continue every year.

What You’ll Find

Eocene lake fossils (~52 million years ago):

• Knightia eocaena — the most common fossil fish in the world, a small herring found in abundance in the Green River Formation
• Diplomystus dentatus — a larger predatory herring, less common and more prized
• Priscacara serrata — a perch-like fish, one of the most sought-after Green River species
• Mioplosus labracoides — a large bass-like predator, rare and valuable
• Stingrays, gars, paddlefish, and bowfin (rare)
• Insects with wing venation preserved in extraordinary detail
• Palm fronds, ferns, and other subtropical plant fossils
• Turtles, crocodilians, and the occasional bird or bat (extremely rare)

Jurassic dinosaurs and marine life (~160-145 million years ago):

• Morrison Formation dinosaurs: Allosaurus, Apatosaurus, Diplodocus, Stegosaurus, Camarasaurus
• Sundance Formation marine invertebrates: ammonites, belemnites, bivalves, oysters

Late Cretaceous (~68-66 million years ago):

• Lance Formation: Triceratops, T. rex, Edmontosaurus, turtles, crocodilians
• Meeteetse Formation mammals and small vertebrates

Best Time to Visit

• Summer (June-August): The primary season for the Kemmerer fish quarries, which typically operate from Memorial Day through Labor Day. Long days and warm (but not extreme) temperatures make fieldwork pleasant at Wyoming’s elevation.
• Late spring (May): Some quarries open in late May. Good weather and fewer crowds than midsummer.
• Early fall (September): Some quarries remain open into September. Cooler temperatures and golden aspens make for a beautiful collecting trip.
• Avoid winter: Most quarries and remote sites are inaccessible due to snow. Wyoming winters are harsh.

Tips for Wyoming Collecting

1. Start at a commercial quarry in Kemmerer. The dig-your-own fish quarries near Fossil Butte are the quintessential Wyoming collecting experience. You will find fish — Knightia are extremely abundant, and with patience you may uncover larger, rarer species.
2. Split carefully and slowly. Green River limestone splits cleanly along bedding planes. Use a thin-bladed chisel or flat pry bar to separate layers. Fish are preserved between layers — rush the split and you may crack through a specimen.
3. Examine every surface. Small insects, plant fragments, and juvenile fish are easily overlooked. Turn every slab over and inspect both faces in good light.
4. Prepare for elevation and weather. Kemmerer sits at about 6,900 feet. The sun is intense, afternoon thunderstorms are common in summer, and temperatures can swing 40 degrees in a day. Bring layers, sunscreen, and rain gear.
5. Visit Fossil Butte National Monument. Even though you cannot collect there, the visitor center displays some of the finest Green River Formation specimens ever found, including complete crocodilians, turtles with soft tissue impressions, and mass mortality fish plates.
6. For dinosaurs, join an organized dig. The best Wyoming dinosaur sites are on private ranch land. Several outfitters and museums run guided excavation programs each summer where paying participants assist with real paleontological fieldwork. The Wyoming Dinosaur Center in Thermopolis runs a popular “Dig for a Day” program in Morrison Formation exposures.
7. Respect ranchers and private land. Much of Wyoming is private ranch land that may not be fenced or posted. Always confirm land ownership before collecting. The BLM has online maps and local field offices that can help clarify boundaries.
8. Pack out everything. Wyoming’s fossil sites are remote and beautiful. Leave no trash, fill any holes, and preserve the experience for future collectors.

Collecting Sites

Legal Rules for Wyoming

Key Geological Formations

• Green River Formation (Eocene)
• Morrison Formation (Late Jurassic)
• Lance Formation (Late Cretaceous)
• Sundance Formation (Late Jurassic)
• Phosphoria Formation (Permian)

Local Fossil Clubs & Museums

• Wyoming Geological Survey
• Tate Geological Museum
• Wyoming Dinosaur Center
• University of Wyoming Geological Museum