Onychonycteris is the more primitive of the two oldest known monospecific genera of bat. It lived in Wyoming area in the Early Eocene period, 52.5 million years ago.
Two specimens were found in the Green River Formation in 2003, and placed in a new family when the discovery was published in Nature, in February 2008 (1). Onychonycteris occurs alongside Icaronycteris index, previously thought to be the most primitive known bat species (2). Onychonycteris was the only bat to have claws on all five fingers, as opposed to two or three in all other known species (3); Onychonycteris means "clawed bat". The specific epithet is a tribute to the fossil prospector and preparator who discovered it, Bonnie Finney.
Onychonycteris finneyi was the strongest evidence so far in the debate on whether bats developed echolocation before or after they evolved the ability to fly. O. finneyi had well-developed wings, and could clearly fly, but lacked the enlarged cochlea of all extant echolocating bats, closer resembling the old world fruit bats which do not echolocate. This indicates that early bats could fly before they could echolocate.
However, an independent evaluation of the Onychonycteris reference fossil in 2010 provided some evidence for other bone structures indicative of laryngeal echolocation, raising the possibility that Onychonycteris finneyi possessed the ability to echolocate after all. They did acknowledge that the fossil itself has been flattened by the fossilization process (a 'pancake fossil'), and thus it was difficult to ascertain the exact bone structure and configuration, a fact that still casts a degree of uncertainty on the results of both studies.
It is unknown if Onychonycteris had the large eyes of most nocturnal animals as no specimens have been found with intact eye sockets (1). If it lacked enlarged eyes, it may have been diurnal, solving the problem of how primitive bats evolved flight without the ability to navigate at night using echolocation.