Australia’s ancient past was home to diverse and unique creatures, including marsupials that dwarfed their modern counterparts. While today the koala (Phascolarctos cinereus) is known for its moderate size, a distinct lineage of larger koalas once roamed the continent. These prehistoric koalas, particularly Phascolarctos stirtoni, represent a chapter in Australia’s natural history. They were part of Australia’s ancient megafauna, a collection of large animals that shaped the ecosystems of a bygone era.
Giants of the Ancient Australian Forests
Phascolarctos stirtoni, known as the “giant koala,” was an arboreal marsupial significantly larger than its modern counterpart. While Phascolarctos cinereus weighs 4 to 15 kilograms, P. stirtoni had an estimated weight of around 13 kilograms, making it about one-third larger. This extinct species is considered the largest known tree-dwelling marsupial.
Fossil evidence reveals a clear similarity in physical appearance between P. stirtoni and the modern koala, despite the size difference. Alan Bartholomai first described the species in 1968, based on tooth remains found in a Pleistocene deposit near Gore, Queensland. Additional fossil remains have been unearthed in the Lake Eyre and Lake Tarkarooloo basins in South Australia, and in Victoria. These discoveries help scientists reconstruct the anatomy of these larger koalas.
Diet and Environment of Prehistoric Koalas
Prehistoric koalas, including Phascolarctos stirtoni, were likely folivores, specializing in leaves, similar to modern koalas. Given the abundance of eucalyptus in ancient Australian environments, the giant koala primarily consumed eucalyptus leaves. However, details about whether they consumed a broader range of plant species or specific types of ancient eucalypts are less defined.
The ancient Australian environments inhabited by these prehistoric koalas likely included more diverse woodlands than the drier eucalypt forests often associated with modern koalas. Fossil records of Phascolarctos species have been found across southern and eastern Australia, in South Australia, Queensland, and Victoria. This suggests a wide distribution across varied arboreal niches. The presence of other large browsing kangaroos and arboreal tree kangaroos in similar fossil sites indicates a mixed habitat with both forested and more open areas.
The Prehistoric Koala’s Place in History
Phascolarctos stirtoni fossil records date from the early Pliocene to the late Pleistocene in Australia. The earliest confirmed record of Phascolarctos cinereus, the modern koala, dates back approximately 350,000 years ago. While it was once hypothesized that the modern koala descended directly from the giant koala, this theory has been determined to be incorrect.
Current research suggests that both P. stirtoni and P. cinereus co-existed in Australia during the Pleistocene, occupying the same arboreal niche. The reasons for the extinction of the larger P. stirtoni about 50,000 years ago are not fully known, but hypotheses include environmental shifts such as changes in climate and limitations in food supply. Some theories also consider the impact of human hunting, particularly on larger animals like P. stirtoni.