Fossil Lake quarries, Kemmerer

This is where the line falls. Commercial fossil collecting is not allowed inside Fossil Butte National Monument, a few miles west, but the quarries on private land around Kemmerer have never stopped working, and several of them sell dig sessions to the public. American Fossil and Ulrich's Fossil Gallery both run them on the Green River Formation, both with guides on site, and both let you take home what you split out. Ulrich's works the Eighteen-Inch Layer, one of the most productive fossil-bearing beds in the formation. Same lake bed, same fish, opposite rules, and the boundary between them is worth knowing before you start swinging a hammer.

The rock is the Green River Formation, and it is one of the most important windows on the Eocene anywhere. Berkeley's Museum of Paleontology puts the reason simply: the fossils here are complete and articulated, which tells you the lakes were deep enough for the bottom water to go anoxic. Nothing scavenged the bodies and no current scattered them, so a fish that sank fifty million years ago is still laid out as a fish. Around sixty vertebrate taxa have come out of the formation, fish, reptiles, birds and mammals among them.

The one nearly everybody finds is Knightia, a small herring-like fish common enough that it is the specimen most Green River slabs carry. The gar Lepisosteus is the other regular, a large predator and a far better wall piece. The first fossil fish described from these beds was collected by Dr John Evans in 1856 and named Clupea humilis, later renamed Knightia eocaena. Edward Drinker Cope collected here extensively and published on the fish from 1870 onward.

The commercial trade is older than the science is tidy. Coal mining for the railroad brought people to the area, miners noticed what was in the shale and dug it to sell, and Lee Craig ran fossils out of these beds from 1897 until 1937. The quarries feeding museums today are the direct descendants of that, and that is how a Wyoming fish ends up on so many walls.