Riversleigh Rainforest Koala

Riversleigh Rainforest Koala, Nimiokoala greystanesi was a small koala from the early Miocene of northern Australia. It had a longer snout than the living koala but was only about a third of its size. Nimiokoala is represented by a well preserved skull, a significant discovery since koalas are rare in the fossil record. Nimiokoala would have fed on the leaves of forest trees although it is not known whether it specialized on the leaves of eucalypts, as the living koala does.

Identification

Koalas are stocky, arboreal marsupials with long arms, short legs with syndactylous (joined) second and third toes, and a backwardly-facing pouch (an inheritance from burrowing ancestors). They have two forwardly directed lower incisors, and are thus included in the order Diprotodontia. The molar teeth of koalas are complex and selenodont, with W-shaped lophs or blades on each tooth that cut leafy vegetation like a pair of pinking shears. Fossil koalas are distinguished mainly by differences in cusps and crests on molars. All koalas lack first and second premolar teeth although they have retained a third premolar. They also have canine teeth on the upper jaw although the lower jaw has lost its canine.

Nimiokoala is exceptionally rare in being represented by a well-preserved skull with dentition. Lower jaws and isolated teeth have also been found, and the entire dentition has now been recovered. Nimiokoala had a longer snout than the living koala, Phascolarctos cinereus, which has an exceptionally short face. Nimiokoala is distinguished from other koalas by the following 'short list' of dental features: the paraconule and metaconule on M1-4/ are well-developed and crescentic; the neometaconule is double-cusped on more posterior molars; and the M4/ has comparatively reduced metacone and metaconule, with a more rounded posterior margin (see Black and Archer 1997 for further description).

Habitat

Riversleigh from the early to middle Miocene was mainly forested, with more open areas near the forest edges and freshwater streams or lakes in a karst (limestone) environment.

Distribution

Nimiokoala greystanesi is known only from the Riversleigh World Heritage Fossil Site in northwestern Queensland.

Related to: Nimiokoala is most closely related to a possible second species of Nimiokoala from the late Oligocene Frome Basin, South Australia (still undescribed). Nimiokoala is also closely related to the fossil koalas Litokoala and to the genus Phascolarctos, to which the living koala Phascolarctos cinereus belongs.

Feeding and diet

The diet of Nimiokoala is unknown, other than it browsed on the leaves of forest trees. Perhaps, like the living koala, it ate the leaves of gum trees (relatively rare in Miocene forests). Alternatively, as an early member of the koala family and therefore less specialised, Nimiokoala may have fed on a variety of leaves.

Life history cycle

As a marsupial, Nimiokoala would have had tiny young at birth that continued development in a pouch until ready for weaning. It probably had a backward-opening pouch, as in living koalas and wombats (an unlikely development for an arboreal animal, retained from the koala's burrowing ancestors).

The living koala has a very specialized lifestyle and is restricted to feeding on the leaves of eucalypts. Nimiokoala was almost certainly arboreal, but its diet is unknown other than the leaves of trees. Nimiokoala lived alongside at least one other species of koala, Litokoala kutjamarpensis, during the early Miocene. Since we know little else about Nimiokoala and have not yet recovered skeletal material, we can make few other assumptions about its life history.

Fossils description

Nimiokoala greystanesi is known from a partial skull with teeth (I1/ to M2/), several lower jaws and a number of isolated teeth. The entire dentition has now been recovered. Nimiokoala was found at the Riversleigh World Heritage Fossil Site (System B; Neville's Garden). Material is held by the Queensland Museum, Brisbane.

Evolutionary relationships

Koalas are most closely related to wombats, having shared a common ancestor in the Oligocene or earlier. Both are members of the suborder Vombatiformes (order Diprotodontia). Diprotodontoids (large, herbivorous marsupials whose best known member is the massive Diprotodon) are also included in Vombatiformes. Most scientists believe that vombatiforms are at the base of the diprotodontian radiation (vombatiforms, possums and kangaroos). One study of marsupial relationships based on molecular evidence places Vombatiformes at the base of the Diprotodontia, 'up the tree' from microbiotheres (South American marsupials closely related to Australian marsupials) and other non-diprotodontian, generally carnivorous marsupials (Kirsch et al. 1997).

The first arboreal koalas probably evolved from a terrestrial wombat-like ancestor, perhaps to take advantage of a food resource not being utilized by others. Koalas were once much more widespread across Australia: fossil koala species are known from southern, western and central Australia when rainforest was more widespread across these parts of the continent. There are now six genera and at least 18 species of fossil koalas (some species are not yet described). All are from either South Australia or Queensland (Riversleigh).

The late Oligocene fossil koala Madokoala and the late Oligocene-early Miocene Perikoala are closer to the stem of the koala family tree than is Nimiokoala. Nimiokoala is presently considered the primitive sister taxon to a group that includes the fossil koalas Litokoala and Phascolarctos (the genus to which the living koala Phascolarctos cinereus belongs). The genus Phascolarctos first appears in the late Miocene/early Pliocene, with at least two other species known (the huge P. yorkensisfrom the late Miocene to Pleistocene of South Australia and New South Wales); and P. maris and/or P. stirtoni from the early Pliocene of South Australia.

References

• Black, K. and Archer, M. 1997a. Nimiokoala gen. nov. (Marsupialia, Phascolarctidae) from Riversleigh, northwestern Queensland, with a revision of Litokoala. Memoirs of the Queensland Museum 41, 209-228.
• Louys, J., Black, K., Archer, M., Hand, S. J. and Godthelp, H. 2007. Descriptions of koala fossils from the Miocene of Riversleigh, northwestern Queensland and implications for Litokoala (Marsupialia, Phascolarctidae). Alcheringa 31, 99-110.
• Stirton, R. 1957b. A new koala from the Pliocene Palankarinna fauna of South Australia. Records of the South Australian Museum 13, 71-81.
• Stirton, R. A., Tedford, R. H. and Woodburne, M. O. 1967. A new Tertiary formation and fauna from the Tirari Desert, South Australia. Records of the South Australian Museum 15, 427-461.
• Woodburne, M. O., Tedford, R. H., Archer, M. and Pledge, N. S. 1987a. Madokoala, a new genus and two species of Miocene koalas (Marsupialia: Phascolarctidae) from South Australia, and a new species of Perikoala. Pp. 293-17 in Archer, M. (ed) Possums and Opossums: Studies in Evolution. Surrey Beatty & Sons, Chipping Norton.

Further reading

• Archer, M., Hand, S. J. and Godthelp, H. 1994. Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Chatswood.
• Jackson, S, 2007. Koala: Origins of an Icon. Allen and Unwin, Sydney.
• 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.