Ancient koalas had the pick of not-so-tough nosh

30 Dec, 2009 10:22 AM
Canopy capers ... two examples of one type of ancient koala. Below, the skull of another type of ancient koala.
Canopy capers ... two examples of one type of ancient koala. Below, the skull of another type of ancient koala.

SCIENTISTS have gained a glimpse into how the koala, one of the nation's most loved creatures, may have acted tens of millions of years ago.

We may never know exactly how an ancient koala, living 24 million years ago in a since vanished rainforest, differed from its modern counterparts.

But a scientific paper, published in the Journal of Vertebrate Paleontology, suggests some old habits die hard. The team of scientists included researchers from the CSIRO and the University of NSW.

One of the authors of the paper, Mike Archer, professor of biological, earth and environmental sciences at the University of NSW, says, for him, perhaps the most important finding to come out of the research is that never in their history have koalas had a period when they were so abundant as they are now.

The fossil remains of the extinct koalas, which were about a quarter to a third smaller than today's koalas, were collected over the past quarter of a century at Riversleigh fossil fields, a World Heritage-listed fossil site in Lawn Hill National Park in north-west Queensland.

But because ancient koala fossils are so rare, it has taken until now to have enough material to make the current analysis.

''From Riversleigh we have found over 40,000 museum-quality specimens,'' Professor Archer said. ''But we have only six koala specimens.

''They are probably among the rarest animals we see in the Riversleigh fossil deposits. In terms of abundance, they have never been better off than they are today."

Professor Archer's comments are controversial given many other researchers say koala numbers are falling alarmingly.

From the Riversleigh fossils the team has determined that, even 24 million years ago, koalas and their close relatives, wombats, had long diverged on their evolutionary tree. Koalas were already creatures living in the forest canopy and specialising in eating leaves.

Similarities between the jaw structure of the two species of ancient koalas and the modern animals also suggest that the Australian bush has long been home to the call of booming koalas.

''Modern koalas are extremely vocal animals that produce loud bellows, typically in the context of mating or agonistic behaviour,'' the journal says. ''Koala bellows are low-pitched, and approach the optimum for long-distance sound propagation. Ranges of up to 800 metres have been recorded, exceeding the home range limits of male koalas.''

The two ancient, extinct koala species also developed unique cranial structures used for vocalisation, though not quite as large as modern koalas.

''I doubt the call would have been much different to what we would hear today,'' Professor Archer said.

Perhaps the biggest difference between the ancient koalas and the modern variety is that it is clear from the fossil jaws and teeth that whatever the extinct creatures were eating it was nowhere near as tough as the leaves from present day gum trees.

The dominance of eucalypts in Australian forests is a relatively new thing - the result of the drying of the continent following a succession of ice ages. Koala teeth reflect this rarity of eucalyptus in ancient Australian forests.



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