The giant koala (''Phascolarctos stirtoni'') is an extinct

arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...

which existed in

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

epoch. ''Phascolarctos stirtoni'' was about one third larger than the contemporary

koala The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae and its closest living relatives are the w ...

, ''P. cinereus'', and has had an estimated weight of , which is the same weight as a large contemporary male koala. Although considered a part of the

Australian megafauna The term Australian megafauna refers to the megafauna in Australia during the Pleistocene Epoch. Most of these species became extinct during the latter half of the Pleistocene, and the roles of human and climatic factors in their extinction are ...

, its body mass excludes it from most formal definitions of

megafauna In terrestrial zoology, the megafauna (from Greek μέγας ''megas'' "large" and New Latin ''fauna'' "animal life") comprises the large or giant animals of an area, habitat, or geological period, extinct and/or extant. The most common threshold ...

. It is better described as a more robust koala, rather than a "giant"; in contrast, a number of Australian megafauna, such as ''

Diprotodon ''Diprotodon'' (Ancient Greek: "two protruding front teeth") is an extinct genus of marsupial from the Pleistocene of Australia, containing one species, ''D. optatum''. The earliest finds date to 1.77 million to 780,000 years ago, but most speci ...

'' 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.

Taxonomy

A description of the species was published by

Alan Bartholomai Alan Bartholomai AM (1938-2015) was a geologist and palaeontologist, and Director of the Queensland Museum from 1969-1999. Alan Bartholomai was born on 31 December 1938 in Boonah, Queensland. He attended Boonah State School and after his paren ...

in 1968, based on a partial maxilla with remains of teeth that was uncovered at the Cement Mills limestone quarry near

Gore, Queensland Gore is a rural town and Suburbs and localities (Australia), locality in the Goondiwindi Region, Queensland, Australia. In the , the locality of Gore had a population of 36 people. Geography The Cunningham Highway passes through the locality f ...

. The material was determined to have occurred at a Pleistocene horizon of the site. The author assigned the species to the genus of the extant koala as '' Phascolarctos stirtoni'', the specific epithet commemorates the American professor, R. A. Stirton, in recognition of his work on marsupial fossils.

Description

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 In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds.Jones, S., Martin, R., & Pilbeam, D. (1 ...

which was specialised to feed on

eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as euca ...

leaves, like its extant relative.

Extinction

Fossil remains of ''Phascolarctos stirtoni'' have been discovered in

Lake Eyre Lake Eyre ( ), officially known as Kati Thanda–Lake Eyre, is an endorheic lake in east-central Far North South Australia, some north of Adelaide. The shallow lake is the depocentre of the vast endorheic Lake Eyre basin, and contains the ...

and Lake Tarkarooloo basins in South Australia.http://www.artistwd.com/joyaustralia/articles/megafauna/phascolarctos_stirtoni.php#.VmWMU7grKUk 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 , mottoeng = By means of knowledge and hard work , established = , endowment = A$224.3 million , budget = A$2.1 billion , type = Public research university , chancellor = Peter Varghese , vice_chancellor = Deborah Terry , city = B ...

, used improved dating techniques to analyse 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.

References

* * * {{Taxonbar, from=Q2340546 Pleistocene mammals of Australia Prehistoric vombatiforms Pleistocene marsupials Koalas Fossil taxa described in 1968