Long-overlooked fossils in the Western Australian Museum collection have been identified as a new species of koala. Named Phascolarctos sulcomaxilliaris, the ancient animal disappeared from Western Australia as drought and cold reshaped its habitat at the end of the Pleistocene.
Reconstruction of Phascolarctos sulcomaxilliaris. Image credit: Nellie Pease.
The modern koala (Phascolarctos cinereus) is Australia’s largest extant arboreal folivore and one of the most iconic Australian marsupials.
Koalas occur in a variety of forest types on the east coast of Australia and have been translocated to several areas of South Australia near Adelaide and on Kangaroo Island.
The species is listed as Vulnerable by the IUCN and has suffered population declines historically through land clearing, disease, vehicle strike, dogs and hunting for the fur trade.
“Koalas are regionally extinct in Western Australia (WA), but their fossils have been known since 1910,” said Dr. Kenny Travouillon, a researcher at the Western Australian Museum and Curtin University, and colleagues.
“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 Phascolarctos cinereus, they were traditionally assumed to be the same species.”
In their study, the paleontologists examined 98 bones from fossil specimens in the Western Australian Museum collection.
They compared skulls, teeth and postcranial bones with modern koala skeletons from museum collections on Australia’s east coast.
The results showed clear and quantifiable differences between the WA fossils and their eastern relatives, confirming the presence of a new species, named Phascolarctos sulcomaxilliaris.
“The species differs from modern koalas in several striking ways,” Dr. Travouillon said.
“Deep grooves in the cheekbone housed a large facial muscle, suggesting the animal may have had unusually large, mobile lips, possibly for manipulating eucalyptus leaves, or maybe to flare its nostrils to enhance its sense of smell and detect food from greater distances.”
“Its skeleton was likely less agile than modern koalas and may have spent less time moving between trees.”
Phascolarctos sulcomaxilliaris’ fossils have now been identified from more than a dozen cave deposits across southern Western Australia, including sites in Yanchep, Margaret River and the Roe Plain near Madura.
This widespread distribution shows that these koalas once occupied a far broader range than previously recognised.
Radiometric dating indicates that Phascolarctos sulcomaxilliaris disappeared around 28,000 years ago.
“This timing aligns with a major Late‑Pleistocene climate event during which eucalyptus forests contracted to around 5% of their current extent,” Dr. Travouillon said.
“With food and shelter dramatically reduced, koalas in the region likely faced severe habitat loss, leading to the extinction of this unique species.”
A paper describing Phascolarctos sulcomaxilliaris was published today in the journal Royal Society Open Science.
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Kenny Travouillon et al. 2026. New fossil koala (Marsupialia: Phascolarctidae) from the Pleistocene of Western Australia. R Soc Open Sci 13 (5): 251572; doi: 10.1098/rsos.251572
