The illustration shows a reconstruction of one of the early Miocene species of koala, which was similar to the modern species in many respects but smaller in size.
Credit: Dorothy Dunphy
SYDNEY: Ancient koalas may have enjoyed a much more diverse diet than today’s koalas, but were probably just as loud and lazy as their modern cousins.
A team led by Julien Louys, a palaeontologist at the University of New South Wales (UNSW) in Sydney, compared skull fragments of two fossil species – Litokoala kutjamarpensis and Nimiokoala greystanesi – to the one living species.
Both prehistoric marsupials lived about 20 million years ago. They have been known about for many years, but researchers have only recently collected enough cranial fossil material from the Riversleigh World Heritage Site in Queensland to make the study possible.
Different diet
The new research, published in the current Journal of Vertebrate Palaeontology, showed there were significant differences in the teeth, palate and jaws of prehistoric koalas compared to modern koalas (Phascolarctos cinereus), but the remainder of the skulls were similar.
The experts said that the unique shape of the modern koala skull is a result of accommodating changes to the way koalas eat without compromising the way they hear.
UNSW palaeontologist Mike Archer said there is an intimate relationship between eating and hearing in mammals, because the jaw is immediately adjacent the ear and because mammalian ear bones are descended from ancestral jaw bones.
Jaw and ear
“It’s not uncommon to see changes in jaw structure impacting on the ability to hear,” he said. “But in this case [the changes] don’t appear to have had that impact.”
Louys said that over time the koala underwent substantial changes to its facial bones. The changes allowed koalas to chew with greater force to cope with a new diet consisting entirely of tough eucalyptus leaves, which are low in nutritional content and have high levels of tannins.
Archer said that it is probable the prehistoric koalas either had a more diverse plant diet, possibly avoiding eucalyptus altogether, or lived on species of eucalypts whose leaves were not as hard to eat as the leaves of eucalypts today. Australia then drifted north, causing its rainforests to retreat to northern refuges and eucalypts to become the dominant tree in most Australian forests.
The paper also suggests that prehistoric koalas had already developed a sedentary lifestyle and the ability to communicate over large distances found in koalas today.
Similarities in the size and shape of auditory ‘bullae’ – large bony structures that enclose the middle and inner ear – means that prehistoric koalas probably shared the ability to produce the loud, low frequency, bellows of the modern day koala. These can be heard by potential mates or competitors nearly a kilometre away.
Archer said prehistoric koalas may have used the loud bellows rather than moving around to communicate – which could suggest a sedentary lifestyle – but they were also necessary because the animals were probably rare and population densities were low.
Not endangered
He disputes the claim made by some conservationists that koalas are endangered and said the fossil record shows that six million years ago, as the rainforests opened up and eucalypts became more common, the abundance of koalas increased “staggeringly” ( see "Is the koala really at risk?").
“Having picked gum trees, they lucked out,” he said. “Koalas are not one of the animals you would regard to be endangered.”
Federal Government studies have estimated the national koala population in the hundreds of thousands, but the Australian Koala Foundation has estimated that the population is between 40,000 and 80,000.
“The deep time record of koalas shows… they are better off than at any time during their history,” added Archer.
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