A dramatic contraction of eucalyptus forests during the Late Pleistocene drove a previously unknown species of koala to extinction in Western Australia 28,000 years ago, with new research from the Western Australian Museum, Curtin University and Murdoch University identifying long-overlooked fossils in the museum’s collection as a distinct species named Phascolarctos sulcomaxilliaris, drawn from the Latin for “grooved maxilla” after the deep cheekbone channel that sets the species apart from every other known koala. That is according to a NEW paper published in Royal Society Open Science, which describes the WA-only species and dates its disappearance to a period when south-west eucalyptus forests shrank dramatically for nearly 10,000 years under colder, drier conditions.
The breakthrough was kick-started in 2024 when avid caver Lindsay Hatcher’s family donated a koala skull collected from Moondyne Cave near Margaret River to the Western Australian Museum, with an unusual feature in the upper jaw flagging the specimen as something other than the modern Phascolarctos cinereus the WA fossils had been quietly catalogued as for more than a century.
“This timing aligns with a major Late-Pleistocene climate event during which eucalyptus forests contracted to around 5% of their current extent,” said Dr Kenny Travouillon, Curator of Mammals at the Western Australian Museum and a researcher at Curtin University. “With food and shelter dramatically reduced, koalas in the region likely faced severe habitat loss, leading to the extinction of this unique species.”
It comes as the modern koala (Phascolarctos cinereus), Australia’s largest extant arboreal folivore, continues to face mounting pressure from land clearing, disease, vehicle strike and bushfire across its remaining east coast range, with the species currently listed as Vulnerable by the IUCN.
Working from 98 fossil bones held in the Western Australian Museum collection, including two rare and more complete adult skulls recovered from south-west caves over the past 25 years, the Travouillon-led team measured skulls, teeth and postcranial material against modern koala skeletons sourced from east coast institutions, with detailed comparative anatomy and evolutionary analyses confirming that the WA fossils consistently fall outside the shape range of modern koalas.
“Koalas are regionally extinct in Western Australia, but their fossils have been known since 1910,” Dr Travouillon said, referencing the first specimens recovered from Mammoth Cave near Margaret River and noting that the dentition was so close to Phascolarctos cinereus the WA fossils had traditionally been assumed to belong to the same species.
The deep cheekbone groove that gave the species its name proved the most distinctive feature, with the channel once anchoring a large facial muscle, pointing to unusually mobile lips capable of stripping tougher leaves and shoots or enhancing nostril movement and smell. The skeleton also marked Phascolarctos sulcomaxilliaris as a more slender, less agile climber than the modern koala, with longer, thinner bones, a shorter, more robust skull, differences in the ear-bone region of the skull, and generally broader teeth than its eastern relative.
Travouillon, Helen Ryan and Kailah Thorn returned to the caves with the Western Australian Speleological Group to confirm the provenance of the fossils, working through Koala Cave at Yanchep along with Moondyne and Foundation Caves near Margaret River to anchor the bones to specific deposit layers, with uranium-thorium dating of the newly described fossils and radiocarbon dating of others producing a consistent extinction window of around 28,000 years ago against pollen records confirming the south-west turned colder and drier at the same time.
Fossils have now been recovered from more than a dozen cave deposits across southern Western Australia, including sites at Yanchep, Margaret River and the Roe Plain near Madura, confirming that Phascolarctos sulcomaxilliaris held a far wider footprint across the continent during the late Pleistocene than the fossil record had previously suggested.
The discovery lifts the count of known Phascolarctos species to four across the last few million years of Australian fossil history, sitting alongside the modern eastern koala and the giant Pleistocene Phascolarctos stirtoni, which grew to nearly double the size of the living animal, with the find reshaping Australian koala history by confirming that Western Australia hosted its own distinctive lineage rather than a stray population of the eastern animal.
The new species description ties Phascolarctos sulcomaxillaris’ 28,000-year-old extinction directly to a south-west eucalyptus forest collapse that lasted nearly 10,000 years, with the same forest dependency now framing the modern koala’s Vulnerable listing across Queensland, New South Wales and the ACT as climate pressure intensifies across the species’ remaining continental range.
