Paljara tirarense

Introduction

Paljara tirarense was a small ringtail possum (family Pseudocheiridae) from the early Miocene of South Australia and northwestern Queensland. Ringtail possums were once much more diverse than they are today, distributed across many now-dry parts of Australia that were forested during the Cainozoic.

Identification

The possum family Pseudocheiridae includes ringtail possums and greater gliders, which are superficially quite different (ringtails are generalized arboreal possums with long, prehensile tails, while greater gliders have short gliding membranes formed by lateral extensions of skin). Ringtails and greater gliders are united by characters of the complex, selenodont molars, including off-set postprotocrista and premetaconulecrista (?).

Most fossil pseudocheirids are known from their dentitions. Pseudocheirids have narrow molars with selenodont cusps, distinct entostylid and metastylid, the cristid oblique on M/1 connected to the metastylid (rather than either the protoconid or protostylid), and a lingually displaced protoconid on M/1 (from Roberts et al. 2007). The dental formula of adult pseudocheirids is generally I1-3/1-2, C1/0, P1-3/1-3, M1-4/1-4/ (there is occasional variability in the number of non-molar teeth, also seen in other Phalangerida).

The low-crowned molar teeth of Paljara tirarense have a large, crested mesostyle, chevron-shaped paracristid and protocristid; a simple, subconical P/3; an anterobuccal cingulum below the trigonid on M/1; a distinct (possibly convergent) paraconid on M/1 at the end of the paracristid (FIX).

Paljara tirarense is most closely related to two other species of Paljara from Riversleigh in northwestern Queensland: P. nancyhawardae and P. maxbourkei.

Habitat

In the late Oligocene-early Miocene Tirari Desert region, Paljara lived alongside at least one other species of ringtail, Marlu kutjamarpensis. The Riversleigh area from the early to middle Miocene was mainly forested, with open areas near forest edges and freshwater streams or lakes in a karst (limestone) environment. At Riversleigh, Paljara tirarense co-existed with several other species of ringtail.

Distribution

Paljara tirarense has been found at Lake Ngapakaldi in the Tirari Desert, South Australia; and at the Riversleigh World Heritage Fossil Site in northwestern Queensland.

Feeding and diet

Ringtail possums are arboreal folivores, feeding mainly on the leaves of trees as well as on fruits, flowers, and (in smaller species) mosses and lichens. Leaves do not provide an energy-rich diet, and pseudocheirids have efficient digestive systems to cope with the large quantities of leaves ingested.

Life history cycle

Like all marsupials, Paljara would have had tiny, hairless young that developed to maturity in a pouch after birth. Like other ringtail possums, Paljara would have spent its life in the trees, feeding at night and probably building a nest of leaves to sleep in by day. Ringtail possums are often taken by predators, many of which are known from both the Tirari Desert and Riversleigh fossil sites (marsupial 'lions', thylacines and at least one species of large raptor).

Fossils description

The holotype of Paljara tirarense is a lower jaw from the Leaf Locality, Lake Ngapakaldi (Kutjumarpu Local Fauna), South Australia. Additional material from this site includes other isolated lower jaws and teeth. The Lake Ngapakaldi material is held by the South Australian Museum in Adelaide. Additional material of P. tirarense was found at the Riversleigh World Heritage Fossil Site in northwestern Queensland, and is held by the Queensland Museum, Brisbane.

Evolutionary relationships

The family Pseudocheiridae includes the ringtail possums and greater gliders. Most members of the family are found in the rainforests or wet sclerophyll forests of northern Australia and New Guinea, although pseudocheirids can be found in a range of other environments and regions. Ringtail possums were once a much more diverse group than they are today. Fossil ringtails are known from central Australia (the Lake Eyre region) and from Riversleigh in Queensland, where there were at least 18 species in five genera.

When Paljara was first described, it was believed to be the most primitive of the ringtail possums (Pseudocheiridae). It is not, however, thought to be an ancestor of any later ringtail species.

References

• Bassarova M. and Archer, M. 1999. Living and extinct pseudocheirids (Marsupialia, Pseudocheiridae): phylogenetic relationships and changes in diversity through time. Australian Mammalogy 21, 25-27.
• Bassarova, M., Archer, M. and Hand, S. J. 2001. New Oligo-Miocene pseudocheirids (Marsupialia) of the genus Paljara from Riversleigh, northwestern Queensland. Memoirs of the Association of Australasian Palaeontologists 25, 61-75.
• Roberts, K. K., Archer, M., Hand, S. J. and Godthelp, H. 2007. New genus and species of extinct Miocene ringtail possums (Marsupialia: Pseudocheiridae). American Museum Novitates 3560, 1-16.
• Roberts, K. K., Bassarova, M. and Archer, M., 2008. Oligo-Miocene ringtail possums of the genus Paljara (Pseudocheiridae: Marsupialia) from Queensland, Australia. Geobios 41, 833-844.
• Woodburne, M. O., Tedford, R. H. and Archer, M. 1987b. New Miocene ringtail possums (Marsupialia: Pseudocheiridae) from South Australia. Pp. 639-679 in Archer, M. (ed) Possums and Opossums: Studies in Evolution. Surrey Beatty & Sons, Chipping Norton.

Further reading

• 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.