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Bat fossil solves evolution poser


Fossil Records

In 2003, southwest Wyoming USA, a research team uncovered what could be the ‘missing link’ in bat evolution. The nearly complete fossilised remains of a 52-million-year-old bat species, Onychonycteris finneyi, has forelimb anatomy that indicated the species was capable of flight, however it lacks the ear morphology for echolocation. This supports the theory that flight in bats came first and echolocation evolved later.

Other features of Onychonycteris finneyi include:

Their wing shape suggests that an undulating gliding or fluttering flight style may be primitive for bats

A long calcar indicates that a broad tail membrane evolved early in Chiroptera, probably functioning as an additional aerofoil rather than as a prey-capture device.

Limb proportions and retention of claws on all digits, as opposed to two or three in all other known species, indicate that the new bat may have been an agile climber that employed quadrupedal locomotion and under-branch hanging behaviour.

(Simmons, NB, Seymour, KL Habersetzer, J and Gunnell, GF. 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature, 451, 818-821.)


A fossil found in Wyoming has resolved a long-standing question about when bats gained their sonar-like ability to navigate and locate food.

They found that flight came first, and only then did bats develop echolocation to track and trap their prey.

A large number of experts had previously thought this happened the other way around.

Details of the work by an international team of researchers is published in the prestigious journal Nature.

Echolocation - the ability to emit high-pitched squeaks and hear, for example, the echo bouncing off flying insects as small as a mosquito - is one of the defining features of bats as a group.

There are over 1,000 species of bats in the world today, and a great many of them can echolocate to navigate and find food.

But some, especially larger fruit bats, depend on their sense of smell and sight to find food, showing that the winged mammals can survive without their capacity to gauge the location, direction and speed of flying creatures in the dark.

The new fossil, named Onychonycteris finneyi, was found in the 52-million-year-old Green River Formation in Wyoming, US, in 2003. It is in a category all on its own, giving rise to a new genus and family.

Its large claws, primitive wings, broad tail and especially its underdeveloped cochlea - the part of the inner ear that makes echolocation possible - all set it apart from existing species. It is also drastically different from another bat fossil unearthed in 1960, Icaronycteris index, that lived during the same Early Eocene epoch.

Many experts had favored an "echolocation first" theory because this earlier find, also from the Green River geological formation in Wyoming, was so close in its anatomy to modern species.

But the new fossil suggests this wasn't the case.

"Its teeth seem to show that it was an insect eater," said co-author Kevin Seymour, a palaeontologist at the Royal Ontario Museum in Canada.

"And if it wasn't echolocating then it had to be using other methods to find food."

The next big question to be answered, he added, was when and how bats made the transition from being terrestrial to flying animals.


Oldest in Australia

Australonycteris clarkae, from the Eocene of Queensland, is the oldest bat from the Southern Hemisphere and one of the oldest in the world. It is similar to other archaic Eocene bats from the Northern Hemisphere, and could probably navigate using echolocation, like most bats do today. Until its discovery, palaeontologists thought that bats colonised Australia much later, perhaps during the Oligocene.
Australonycteris clarkae, from the Eocene of Queensland, is the oldest bat from the Southern Hemisphere and one of the oldest in the world. It is similar to other archaic Eocene bats from the Northern Hemisphere, and could probably navigate using echolocation, like most bats do today. Until its discovery, palaeontologists thought that bats colonised Australia much later, perhaps during the Oligocene.


Standard Common Name
Australonycteris clarkae

Identification
Australonycteris clarkae was an archaic Eocene microchiropteran bat, the only one known to date from Australasia. Other early microchiropterans from the Eocene of Europe and North America have been preserved in fine detail, giving us a good picture of ancient bat anatomy. In even the earliest bats, forelimbs were modified into wings, formed by a thin flight membrane supported by elongated finger bones (the second through fifth digits). The claw of the first finger (digit one, or the 'thumb') was free for grasping, and hind limbs were reduced. At least one early bat from Wyoming had claws on all digits. Most early bats could use echolocation to navigate. However, although one of the most primitive known bats (the Eocene bat from Wyoming) could fly, its ear morphology suggests that it could not use echolocation. A long calcar is preserved in the Wyoming bat, a tail membrane therefore evolving early in bat evolution. This bat had an undulating- gliding flight, probably the primitive flight pattern for bats and perhaps the mode of flight for Australonycteris.

Ear bones of Australonycteris show that it could navigate using echolocation. The teeth of Australonycteris are unusually worn, perhaps through regular contact with hard food like beetle carapaces. Postcranial material of Australonycteris has not been described.

Size range
20 cm wingspan

Distribution
Australonycteris is only known from a single fossil site near the town of Murgon in southeastern Queensland.

Habitat
The Murgon area during the early Eocene was a shallow swamp or lake. The vegetation and climate of the period have not yet been determined.

Feeding and Diet

The tips of the molar teeth of Australonycteris are worn, perhaps through abrasion by hard food, and Australonycteris may have carried prey with hard exoskeletons or even bones over stretches of water. Suggestions include hard-shelled beetles or perhaps small fish (given the mid-range size of Australonycteris).

Life cycle
Little is known of Australonycteris or of its relationships to other bats. It is not known where it roosted, how it raised its young or how it fed.

Fossils
To date, teeth, a dentary bone (lower jaw), isolated teeth, periotics (ear bones) and postcranial bones of Australonycteris have been found.

Era / Period
Eocene Epoch

Evolutionary Relationships
Higher-level relationships of bats are the subject of much debate. The closest relatives of bats among mammals may be tree shrews (tupaiids), primates, colugos/flying lemurs (dermopterans) (together forming the grandorder Archonta). However, other groups have been proposed as ancestors, including carnivores, insectivores and ungulates among others. Almost all scientists think that both fruit bats (megachiropterans) and 'insectivorous' (echolocating) bats (microchiropterans) are a natural group, descended from a common ancestor, although there are some that think the megabats are the descendants of a primate group. Due to the enormity of the task of sorting bat relationships (these generally tiny mammals make up about one-quarter of all living mammal species), a clear idea of the interrelationships of bats is probably well in the future.

Archaeonycterids are the oldest known bats, but there are no known 'protobats'; when archaeonycterids first appeared, they had most of the basic characteristics of other bats - they were small, winged mammals fully capable of powered flight - albeit with certain primitive features. One of the most primitive known bats, from the early Eocene of Wyoming, could not echolocate, suggesting that powered flight evolved in bats before echolocation did. There is therefore debate over bat origins. The place of origin of modern bats is also debated. One view, presented in the paper describing Australonycteris, suggests that modern bats may have originated during the early Eocene from an isolated group of archaic bats in the Southern Hemisphere (Hand et al. 1994). Although this view can't be discounted, the fossil record for archaic bats is extremely fragmentary and new discoveries will undoubtedly change the story. For example, a recently discovered early Eocene site in India has a high diversity of archaic bats, suggesting early contact between India and Eurasia and India as an unexpected centre of bat diversity.

ClassificationSpecies:clarkaeGenus:AustralonycterisFamily:ArchaeonycteridaeSuborder:MicrochiropteraOrder:ChiropteraMagnorder:BoreoeutheriaCohort:PlacentaliaInfralegion:TheriaSublegion:BoreosphenidaSubdivision:TheriimorphaDivision:TheriiformesInfraclass:HolotheriaSubclass:MammaliaformesClass:MammaliaSuperclass:TetrapodaSubphylum:VertebrataPhylum:ChordataKingdom:Animalia

Further Reading

Hill, J. E. and Smith, J. D. 1984. Bats: A Natural History. British Museum of Natural History, London.

Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore, 240 pp.

Nowak, R.M. 1994. Walker's Bats of the World. Johns Hopkins University Press, Baltimore.

References

Hand, S, J. 2006. Bat beginnings and biogeography. Pp. 673-705 in Merrick, J. R., Archer, M., Hickey, G. M. and Lee, M. S. Y. (eds) Evolution and Biogeography of Australasian Vertebrates. Hand, S. J., Novacek, M. J., Godthelp, H. and Archer, M. 1994. First Eocene bat from Australia. Journal of Vertebrate Paleontology 14, 375-381.

McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York.

Simmons, N. B. and Geisler, J. H. 1998. Phylogenetic relationships of Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments on the evolution of echolocation and foraging strategies in Microchiroptera. Bulletin of the American Museum of Natural History, No. 235. American Museum of Natural History, New York.

Simmons, N. B., Seymour, K. L., Habersetzer, J. and Gunnell, G. F. 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451, 818-821.

Smith, T., Rana, R. S., Missiaen, P., Rose, K. D., Sahni, A., Sing, H. and Singh, L. 2007. High bat (Chiroptera) diversity in the Early Eocene of India. Naturwissenschaften 94, 1003-1009.

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BATS. Megabats, Flying-foxes, Fruit bats and Microbats: Bat fossil solves evolution poser
Bat fossil solves evolution poser
Bat fossil solves evolution poser
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