Murgon Bat

Murgon Bat, 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.

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.

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.

Distribution

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

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 history 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 description

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

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.

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.

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.