Complete fossil bats are extremely rare. Bats share many physical characteristics with insectivores such as hedgehogs, shrews and moles and it is possible that mammal teeth that have been dated to the Paleocene Epoch (65 million years ago) are in fact those of bats. However, because no bones were found with the teeth it is impossible to know precisely what animal they belonged to.
The fossil pictured here is one of the oldest and most well preserved fossil bats known and dates from the Eocene Epoch. It is thought to approximately 50 million years old. Icaronycteris index represents the holotype of the species, the specimen for which the species name was given, and illustrates the delicate bone structure of the tail and ribs of the bat as well as the elongated finger bones which supported the bat's flexible wing membrane.
Icaronycteris index was discovered in the Green River Formation in Lincoln County, Wyoming in the United States. The area is a formation of sedimentary layers from a series of interconnected lakes and is named for the Green River, a tributary of the Colorado River. It is regarded as one of the most important sites in the world for understanding the Eocene Epoch. Fossils found at Green River include a diverse selection of plants, fish, birds, reptiles and mammals. The types of plants, such as ferns and palms, and types of reptiles, such as crocodiles, found in this area also indicate that the climate during the Eocene was subtropical, warm and moist, and fairly constant at between 15 - 20 degrees celsius.
One of the most critical aspects of the Icarnonycteris fossil is the structure and shape of it's skeleton suggests that bats had, by this time, evolved the ability to fly and the ability to echolocate.
The other highly important fossil site that has revealed intact bat fossils from the Eocene is the Messel Oil Shale Site near Frankfurt in Germany. Like Green River the site has provided an abundance of evidence of plant and animal life. The Messel fossils are perhaps even more remarkable in that they not only include the hard skeletons but also soft tissues and even stomach contents of a wide variety of creatures ranging from fish to early horses.