Temporal range: Pleistocene
The giant koala (Phascolarctos stirtoni) is an extinct arboreal marsupial which existed in Australia during the Pleistocene epoch. Phascolarctos stirtoni was about one third larger than the contemporary koala, P. cinereus, and has had an estimated weight of 13 kg (29 lb), which is the same weight as a large contemporary male koala. Although considered a part of the Australian megafauna, its body mass excludes it from most formal definitions of megafauna. It is better described as a more robust koala, rather than a "giant"; in contrast, a number of Australian megafauna, such as Diprotodon and Procoptodon goliah, were unambiguously giants. The two koala species co-existed during the Pleistocene, occupying the same arboreal niche. The reason for the extinction of the larger of the two about 50,000 years ago is unknown, although there are various hypotheses for the extinction.
The giant koala was an arboreal marsupial weighing about 13 kg, a little more than modern koalas. It is the largest known tree dwelling marsupial ever to have lived. Scientists say there is a clear similarity between the physical appearance of P. stirtoni and the modern koala. It is assumed that the giant koala was a folivore which was specialised to feed on eucalyptus leaves, like its extant relative.
Fossil remains of P. stirtoni have been discovered in Lake Eyre and Tarkarooloo basins in South Australia. It was once thought that the modern-day koala was descended from the giant koala, but this has now been determined to be incorrect. Gilbert Price, of the University of Queensland, used improved dating techniques to analyze fossils of both types of koala to find that the two species were living together in the same arboreal niche. It is unclear from what the koalas descend and why one species survived while the other became extinct. It is hypothesized that there was a change in climate and a restriction of food supply. Another possible reason is the hunting by humans for larger animals. The "dwarfing" hypothesis, based on the similarities of the two koalas has been used to support both ideas.
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