''Onychonycteris'' is the more primitive of the two oldest known

monospecific In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genera of

bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...

, having lived in the area that is current day

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

during the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

period, 52.5 million years ago.

Taxonomy

Two specimens of ''Onychonycteris'' were found in the

Green River Formation The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine ...

in 2003, and placed in a new family when the discovery was published in ''Nature'', in February 2008. ''Onychonycteris'' means "clawed bat", in reference to the fact that this animal had claws on all five of its digits, whereas modern bats only have claws on the thumb (in most species) or thumb and index finger (in pteropodids). The

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

is a tribute to the fossil prospector and preparator who discovered it, Bonnie Finney.

Description

''Onychonycteris'' was unique among bats in that it had claws on all five fingers, as opposed to two or three in all other known species. Its unique limb proportions represent an intermediate between bats and non-flying mammals, with longer hind-limbs and proportionally shorter wings. The aerofoil of ''O. finneyi'' lacks any aerodynamic equivalent among known bats, and this aerofoil likely contributed to an undulating flight style that alternated fluttering and gliding. This flight style may represent both a functional and evolutionary intermediate between gliding and powered flight.

Paleobiology

Flight vs. echolocation

''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 The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

of all

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

echolocating bats, closely resembling the

old world fruit bats Megabats constitute the family Pteropodidae of the order Chiroptera (bats). They are also called fruit bats, Old World fruit bats, or—especially the genera ''Acerodon'' and ''Pteropus''—flying foxes. They are the only member of the sup ...

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 whether ''Onychonycteris'' had the large eyes of most

nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...

animals as specimens with intact eye sockets have yet to be found. A lack of enlarged eyes would indicate that this species may have been diurnal, solving the problem of how primitive bats evolved flight without the ability to navigate at night using echolocation. ''Onychonycteris'' occurs alongside '' Icaronycteris index'', previously thought to be the most primitive known bat species.

References

{{Taxonbar, from=Q138826 Eocene bats Prehistoric bat genera Prehistoric monotypic mammal genera Eocene mammals of North America Transitional fossils Fossil taxa described in 2008