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Ancient tree-wombat behaved like a koala

Tree wombat The rainforests of Australia were once inhabited by a 70-kilogram tree-dwelling wombat that hugged trees like a koala, say researchers.

The team of Australian researchers report their analysis of fossilised remains of the marsupial Nimbadon lavarackorum today in the journal PLoS ONE.

Nimbadon belongs to a group of large-bodied wombat-like herbivores, called diprotodontids.

"It's quite exciting really," says palaeontologist, Dr Karen Black of the University of New South Wales.

"It was always believed diprotodontids roamed in mobs on the ground but looking at the skeleton of nimbadon, we've actually found it was climbing trees and most probably lived in trees."

During the middle Miocene - approximately 15 million years ago - much of Australia was covered in temperate rainforest teeming with possums, marsupial lions and other tree life.

Since 1993, Black has been involved in gradually excavating Miocene nimbadon fossils from the famous Australian site of Riversleigh in north-west Queensland.

Previous analysis of the skull, teeth and jaw of nimbadon revealed the animal ate leaves, says Black.

But this is the first time the full skeleton has been described, revealing nimbadon was a 70-kilogram highly specialised climber.

"It was quite unusual to find such a big animal up in the tree tops," says Black.

Black and colleagues compared the nimbadon skeleton with a range of modern and extinct marsupials of known habitat and found its forearms were "strikingly similar to koalas'".

The animal's shoulder, elbow joint, and wrist was extremely mobile which would have made it easy for it to weave in and out of tree branches, says Black.

And like koalas it had really long fingers and toes, a semi-opposable thumb and first toe making it easy for it to grasp branches.

Unlike the koala, however, the nimbadon had relatively short hind limbs and its limb proportions were similar to today's sloths and orang-utans that spend a lot of their time suspended from branches.

"The animal has the shortest hindlegs of any known marsupial," says Black.

"This also indicates that this guy wasn't only hugging trees like koalas but was also hanging from trees."

Black thinks it would have been "wonderful" to bump into nimbadon while out for a forest walk.

"Its claws are absolutely massive ... so you wouldn't want to have been hugged by one of these guys," she says. "But they probably would have been quite docile creatures."

The researchers have taken CT scans of nimbadon's skull and now plan to investigate its brain and its likely ability to smell, hear and make noise.

Nimbadon had a bulbous snout so could have had a good sense of smell and may have also eaten fruit as well as leaves, says Black.

She says the animal would have died out as its habitat shrunk due to climate change that led to a much drier Australia.