New fossil koala (Marsupialia: Phascolarctidae) from the Pleistocene of Western Australia

Abstract Koalas are regionally extinct in Western Australia (WA), but their fossils have been known since 1910. Bones were previously reported from several cave deposits in the south-west of WA, Koala Cave in Yanchep and from Madura Cave on the Roe Plain. Due to the similarity of the dentition with the east coast koala (Phascolarctos cinereus), they were traditionally assumed to be the same species. Two complete additional adult skulls were collected in the past 25 years. These skulls, interpreted to belong to a male and female koala, are similar in body size to koalas from Victoria but differ markedly in being relatively much shorter in length and having obvious deeper concavities on the maxilla, below the zygomatic arch. Differences from the eastern species are also apparent in the postcranial skeleton, having less robust joints. Analysis of measurements on the skulls and teeth demonstrates quantitatively that the Western Australian koala is morphologically distinct from its east coast relative and warrants consideration as its own species. It probably went extinct in WA as a result of climate change during the late Pleistocene, which reduced eucalyptus forests to around 5% of their current cover, greatly limiting resources for food and shelter.