Chunia illuminata

Introduction

Chunia was a primitive ektopodontid, a distinctive group of Cainozoic Australian possums that may have been specialized seed-eaters. Ektopodontids, first thought to be monotremes, had short faces, large, forward-facing eyes and the most unusual and complex teeth of any marsupial. Chunia, the most primitive of the ektopodontids, had molar teeth that were simpler than those of other ektopodontids and much like those of phalangeroid possums.

Identification

There are three genera and five species of ektopodontids, known only from partial skulls, lower jaws and teeth. They had very short snouts and large, forward-facing eyes (a possible adaptation for nocturnal vision). The subtriangular to rectangular, low-crowned molar teeth of ektopodontids are highly distinctive, with transverse ridges broken up into radially or longitudinally arranged patterns of ribs or struts. Upper and lower molars are similar structurally, and occlusal relations are complex. Like other diprotodontians, ektopodontids had two forward-facing lower incisors. Upper molars have multiple roots (3-4) and lower molars are double-rooted. A complete upper/lower dentition of an ektopodontid is still unknown.

In Chunia, the network of radiating ribs and struts on the longitudinal cusps is unique, upper molars have five to six widely spaced cusps, and the transverse valley separating crests is narrow and shallow. The teeth of Chunia illuminata differ from a second, slightly younger species of Chunia, Chunia omega, in having less numerous ribs and struts. The sublophodont teeth of Chunia species resemble those of cuscuses from the rainforests of northern Australia and New Guinea.

Habitat

During the late Oligocene the Lake Eyre region was covered by a large lake surrounded by humid woodland.

Distribution

Chunia illuminata is known from Lake Palankarinna in the Lake Eyre Basin of South Australia. Other fossil material similar to Chunia illuminata (Chunia cf. illuminata) was found at nearby Lake Pinpa, South Australia.

Feeding and diet

Several suggestions for the diet of ektopodontids have been proposed, including seeds, nuts, fruits and leaves as well as aquatic invertebrates (presuming an aquatic lifestyle for ektopodontids) or insects (caterpillars or other soft bodied types). Judging by their rodent-like molars, ektopodontids were most likely gramnivores (seed-eaters).

Life history cycle

Little is known of the life history of ektopodontids other than that they were most likely arboreal, as are other possums. No postcranial (all or part of the skeleton apart from the skull) fossils of ektopodontids have been found, making further assessment of their lifestyle difficult.

Fossils description

Chunia illuminata, from the late Oligocene Lake Palankarinna area (Etadunna Formation, Lake Eyre Basin, South Australia) is represented by isolated teeth and a maxillary fragment. There is a closely related or conspecific ektopodontid from Lake Pinpa (Namba Formation, Tarkarooloo Basin, South Australia) of about the same age.

Evolutionary relationships

Ektopodontids, known from the late Oligocene of central Australia to the Pleistocene of Victoria, are almost certainly a specialized group of phalangeroid possums. Monotreme affinities, first proposed, were soon discounted, and a proposed relationship between ektopodontids and phascolarctids (koalas) is debatable. The ancestors of ektopodontids have not yet been identified, but may have been phalangerids (cuscuses and brush-tailed possums).

Relationships between ektopodontid species are uncertain (although there may be an ancestor-descendant relationship between Ektopodon and Chunia). The evolutionary trend within ektopodontids was towards increasing complexity in the number of cusps, crests, ribs and struts has been useful in biostratigraphy and biochronology.

References

• Pledge, N. 1982. Enigmatic Ektopodon: A case history of palaeontological interpretation. Pp. 477-488 in Rich, P. V. and Thompson, E. M. (eds) The Fossil Vertebrate Record of Australasia. Monash University Offset Printing Unit, Clayton, Victoria.
• Rich, T. H. 1982. Monotremes, placentals, and marsupials: their record in Australia and its biases. pp. 386-477 in Rich, P.V. and Thompson, E. M. (eds) The Fossil Vertebrate Record of Australasia. Monash University Offset Printing Unit, Clayton, Victoria.
• Rich, T. H., Piper, K. J., Pickering, D. and Wright, S. 2006. Further Ektopodontidae (Phalangeroidea, Mammalia) from southwestern Victoria. Alcheringa 30, 133-140.
• Stirton, R. A., Tedford, R. H. and Woodburne, M. O. 1967. A new Tertiary Formation and vertebrate fauna from the Tirari Desert, South Australia. Records of the South Australian Museum 15, 427-462.
• Woodburne, M. O., 1987. The Ektopodontidae, an unusual family of Neogene phalangeroid marsupials. Pp. 603-606 in Archer, M. (ed) Possums and Opossums: Studies in Evolution. Surrey Beatty & Sons/The Royal Zoological Society of New South Wales, Chipping Norton.
• Woodburne, M. O. and Clemens, W. A. 1986a. Revision of the Ektopodontidae (Mammalia; Marsupialia; Phalangeroidea) of the Australian Neogene. Woodburne, M. O. and Clemens, W. A. (eds), University of California Publications in Geological Sciences Vol. 131.
• Woodburne, M. O. and Clemens, W. A. 1986b. A new genus of Ektopodontidae and additional comments on Ektopodon serratus. pp. 10-42 in Woodburne, M. O. and Clemens, W. A. (eds), Revision of the Ektopodontidae (Mammalia; Marsupialia; Phalangeroidea) of the Australian Neogene. University of California Publications in Geological Sciences Vol. 131.

Further reading

• Archer, M. 1987. A Possum of a Very Different Kind. Pp. 67-69 in The Antipodean Ark edited by S. Hand and M. Archer, and illustrated by P. Schouten. Angus and Robertson Publishers, North Ryde.
• 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.