In 2024, the Western Australian Museum received a donation. It was a koala skull collected from Moondyne Cave in Margaret River by Lindsay Hatcher, an avid caver. There was something a bit odd about this skull, and we were able to put our finger in it.
This koala had dimples.
Koalas are iconic on Australia’s east coast, but they are regionally extinct in Western Australia today. Fossils tell a different story: koalas once lived across parts of WA, from the Margaret River region to as far north as Yanchep and as far east as Madura.
In our new study, published today in Royal Society Open Science, we show these WA koalas were not simply stray populations of the modern koala. They represent a distinct species that has been hiding in plain sight for more than a century.
Not like the koalas we know
Koala fossils in WA were first discovered in Mammoth Cave near Margaret River in 1910. But for the better part of a century, most specimens consisted of isolated jaws and teeth.
Over the past 25 years, however, two rare, more complete adult skulls were found in caves in the state’s south-west. Together with additional jaw, tooth and limb bones from multiple cave sites, these specimens allowed us to test a long-standing assumption: that WA’s fossil koalas belonged to the same species as modern koalas found in other states of Australia.
That assumption now appears false. Using detailed skull and tooth measurements, comparative anatomy and evolutionary analyses, we found the WA fossils consistently fall outside the shape range of modern koalas.
The most striking feature is a deep, rounded sulcus (groove) in the cheek region of the upper jaw, below the eye socket. This feature is far deeper than anything seen in living koalas and inspired the new species name: Phascolarctos sulcomaxilliaris, meaning “grooved maxilla” (maxilla is the name of the cheek bone).
The WA species also has a shorter, more robust skull, differences in the ear-bone region of the skull, and generally broader teeth.
What was the groove for? In living koalas, lip and nose muscles attach in the same general area. The exaggerated sulcus in the fossil species likely made space for larger muscles, potentially giving it a more mobile upper lip for manipulating tougher leaves or shoots, or enhancing nostril movement and smell.
The bones of the skeleton were also more long and thin, suggesting the WA koala was a more slender species.
Helen Ryan, Western Australian Museum
A cave visit
While the donation of material by Lindsay Hatcher’s family kickstarted this project, three of us went to find out where exactly these fossils came from so that we could say how old they are.
Visiting the caves themselves was an adventure in its own right. With the help of the local cave researchers, Western Australian Speleological Group, we revisited Koala Cave in Yanchep, and Moondyne and Foundation Caves near Margaret River, to find out where these fossils came from.
Author provided
Uranium-thorium dating of the newly described fossils, and radiocarbon dates for others, suggest our koala went extinct roughly 28,000 years ago. Around that time, the climate became colder and drier according to pollen records, and the south-west eucalyptus forests shrank dramatically for almost 10,000 years.
Koalas have a habit of eating themselves out of house and home, so as the shelter and food in their habitat declined, the extinction of this species was likely inevitable.
Reshaping koala history
This discovery matters for two reasons.
First, it reshapes koala history: the modern koala was not the only koala species in the recent past, and WA hosted its own distinctive lineage.
Indeed, four species of koalas are now known to have lived in Australia over the last few million years, including the living Phascolarctos cinereus in eastern Australia. One of these four species was the giant Pleistocene koala Phascolarctos stirtoni, nearly double the size of living koala.
Second, it is a deep-time reminder that koalas are tightly bound to forests. When those forests shrink fast enough, even adaptable mammals can vanish from entire regions. In a warming, drying Australia, understanding how past climate shifts transformed habitats helps us anticipate the risks facing the koalas that remain today.
The story of the WA koala is a lesson learned to protect the last living koala species. Protecting the eastern eucalypt forests from climate change and deforestation is paramount for the survival of koalas in the future.
