There are few fossilized remains of bats, as they are terrestrial and light-boned. An Eocene bat, Onychonycteris finneyi, was found in the fifty-two-million-year-old Green River Formation in South Dakota, United States, in 2004 and was added as a new genus and placed in a new family when published inNature in 2008. It had characteristics indicating that it could fly, yet the well-preserved skeleton showed that the cochlea of the inner ear lacked development needed to support the greater hearing abilities of modern bats. This provided evidence that flight in bats developed well before echolocation.The team that found the remains of this species, named Onychonycteris finneyi, recognized that it lacked ear and throat features present not only in echolocating bats today, but also in other known prehistoric species.
Fossil remains of another Eocene bat, Icaronycteris, were found in 1960.
The appearance and flight movement of bats 52.5 million years ago were different from those of bats today. Onychonycteris had claws on all five of its fingers, whereas modern bats have at most two claws appearing on two digits of each hand. It also had longer hind legs and shorter forearms, similar to climbing mammals that hang under branches such as sloths and gibbons. This palm-sized bat had broad, short wings suggesting that it could not fly as fast or as far as later bat species. Instead of flapping its wings continuously while flying, Onychonycteris likely alternated between flaps and glides while in the air. Such physical characteristics suggest that this bat did not fly as much as modern bats do, rather flying from tree to tree and spending most of its waking day climbing or hanging on the branches of trees.