Within the laminated shales of the Green River Formation is preserved an abundant and diverse assemblage of fossil insects ([Fig. 13]).
These insects are important because they demonstrate that many modern families and even genera were in existence during the Eocene. In contrast to this, knowledge of insect evolution prior to the Eocene is rather poorly known. Scudder (1890) and Cockerell (1920) have described most of the Green River insects.
Beetles are the most common forms, followed by dragonflies. Maggots and larvae of flies are commonly preserved.
Except for these insects preserved as whole "mummies" in the oil shales, the majority of the insect fossils are preserved as distilled outlines. This distillation process resulted from the weight and heat of overlying sediments of driving off the volatile substances from the buried insects, leaving a hydrocarbon outline. The process is so precise that the fine hairs and wing veins and even body-color markings are preserved.
Fig. 13. An impression of a horse fly from the Fossil Lake deposits. Length of original, 12 mm. Collection of University of Wyoming.
Vertebrates
FISH.
The single factor that has made Fossil Butte world famous is the occurrence of literally millions of beautifully preserved fossil fish. In one layer approximately 14 inches thick, the fish are nearly black and preserved in a nearly white shale. Most of the fish are perfectly preserved, retaining every detail of the skeleton and even undisturbed scales. Fish from Fossil Butte have been collected and sold since the 1870s and may be found on sales counters, in museums, and on living room walls throughout the world.
The fossil fish from the Green River Formation have a history of discovery stretching back to 1856. At that time Leidy (1856:256) described a fish given to him by a Dr. John Evans. Leidy called the fish Clupea humilis, a type of herring. Exactly where Dr. Evans obtained the specimen is not known. He passed through the area of the Green River Formation several times and could have collected it during any one of those trips (Knight 1955:12).