My Uncle Frank always gave us kids the best Christmas gifts, like kites and gyroscopes and one year, a fossilized fish. That unidentified ichthyous slab still sits on a shelf at my parents’ house in Pennsylvania and I’ll bet any money that it’s from the Green River Formation in Wyoming. Southwestern Wyoming is the source of zillions of fossils from the Eocene epoch, back when mammals were first evolving hooves and placentas and the planet was much warmer and wetter and all the landmasses from pole to pole were covered by trees.
Around 50 million years ago, the region around Kemmerer, Wyoming was covered by a lake known today as Fossil Lake. The quiet water, fine-grained sediments and water chemistry were ideal for preserving dead organisms as they sank to the bottom of the basin and the many layers of shale that were formed over several million years of deposition contain one of the most complete and most detailed fossilized records found anywhere on Earth. We’re not just talking fish, but also alligators, bats, snakes, turtles, early horses, insects; a whole ecosystem of plants and animals. The record is so complete that paleontologists can piece together the lake’s food web: fish are found in the act of eating other fish and bite marks on fossilized leaves match up with the mouth parts of fossilized insects.
The Green River Formation is massive and less than 1.5 percent is protected within the bounds of Fossil Butte National Monument. Commercial digging outside the monument yields hundreds of thousands of fossilized fish and other specimens each year. Fish from Green River are the most common fossilized vertebrates offered for sale and the small herring-like fish Knightia eocaena is the most abundant vertebrate fossil in the world.
I visited Fossil Butte for the first time in April 2009, on a road trip from New Mexico to Montana, but it was freezing cold and raining and I didn’t take a hike. This time, it was a blue bird day so the dogs and I hiked a three mile loop up to the historic quarry at the base of Fish Cliff.
Love fossils? Check out my previous post Wonderful Life about my geo-pilgrimage to the Burgess Shale in British Columbia. I also wrote a feature about that trek for EARTH’s Travels in Geology column. Wait a minute, wasn’t I heading West before my SLC detour? Yep, but now Wyoming is calling me… stay tuned!🙂