Travels in Geology September 2006, posted September 19, 2006
Summer roadtrip: A fossil aquarium in Wyoming
Callan Bentley, Geotimes contributing writer, has been traveling across North America in his Volkswagen Westfalia camper van this summer. From May to September, he traveled from Washington, D.C., up to central Alaska, and then back. When he came across areas with interesting geology, he used the van's solar power to plug into a laptop and fire off a dispatch for Geotimes. The following is Part 3 of his three-part roadtrip series.
years, I've been seeing Green River Formation fossils everywhere. Chances
are, you've seen them too. The ubiquitous, iconic and beautiful fossil
fish are found in nature stores, museums, private collections and geology
departments throughout America. Their abundance in nature makes the fish
among the least expensive vertebrate fossils.
The size of a chocolate bar, a typical Green River Formation fossil fish looks like a dark brown etching on a tan background. The contrast in color is one of their most recognizable characteristics to the amateur collector, and professional paleontologists venerate them for their incredible detail. In most fish fossils, only bones are present, often strewn about in a disarticulated mess. But not so for Knightia (a herring, which is the most common Green River species) and its kin: They display scales, teeth and fins, all exactly where they would be in a living fish.
On my return trip across the United States, I wanted to visit the source of all these exquisite fossils. It turns out that they come from the southwestern corner of Wyoming, near the Utah and Colorado state borders.
Fossil Butte National Monument
is located in southwestern Wyoming, near the town of Kemmerer. It is the
best place in the world to see freshwater lake fossils from 50 million
The National Park Service maintains a free museum at the site, where visitors may view some of the finest examples of local fossils. The museum features some amazing examples of the diversity of life preserved at Fossil Butte. Although fossil fish are the foot-soldiers of the Green River, the formation also yields reptiles such as turtle and crocodiles, and at least one snake. Other fossils show insects, crayfish and freshwater shrimp. It's remarkable to see the fine details of these small, delicate organisms: antennae stretched out for several inches, or the delicate lace of veins in a dragonfly's wings. Leaves of shore-bound plants are also prevalent, especially sycamore trees and fan-palms. Even flying creatures like birds and bats fell into Fossil Lake and were preserved.
Palm trees surrounded Fossil Lake during the Eocene, and dropped in their leaves, where they became fossilized.
The amazing thing about Fossil Butte is this diversity: The Green River Formation preserves an entire ecosystem here. The fish fauna includes 25 species, ranging from small minnows to massive gars, which were 6 feet long and armored in giant diamond-shaped scales. There are even stingrays preserved in the sediments of Fossil Lake, though modern freshwater stingrays are restricted to the Amazon basin in South America. Many of the fish (and other organisms) are concentrated in particularly rich layers that show massive die-offs in the lake community. What caused these die-offs is not known, but is the subject of a short film detailing the work of several paleontologists who do field work at Fossil Butte.
Wandering through the visitor center, I was struck by how complete a community was displayed in the rocks. From the foundations of the food chain to the highest predators, Fossil Butte gives us a unique snapshot of life in the Eocene.
The only snake known from the Green
River Formation is on display in the Fossil Butte National Monument Visitors
"We started it nine years ago, and it's increasing in popularity," says Arvid Aase, a specialist at Fossil Butte. "It's a great experience for them, and it's free." Because it is a national park, visitors cannot keep the fossils they quarry, but the rangers do issue a certificate bearing each 