Science

Treetop marsupial was a real swinger

Nimbadon - artist reconstruction by Peter Schouten (Creative Commons)
Thursday, 22 November, 2012
Bob Beale

It used massive sharp claws to haul its hefty body up trees, hugging the trunk like a bear, and its huge hands and long arms let it hang from branches like an orang-utan – weighing in at 70 kg, Nimbadon must have been an impressive sight in Australia’s prehistoric rainforests.

The long-extinct Nimbadon was the largest arboreal marsupial herbivore ever to have lived and it was well suited to life in the treetops, according to a new study published in the journal PLOS ONE.

Mobs of these large bear-like animals – giant marsupial members of a group of known as diprotodontoids  - inhabited northern Australia’s lush forests 15 million years ago and the new study suggests they may have filled the role of marsupial bears or sloths.

“Because of their large size and abundance in the fossil record, it has generally been accepted that most diprotodontoids were terrestrial and roamed in herds or mobs like modern-day kangaroos,” says the lead author of the study, Dr Karen Black, who worked with colleagues Michael Archer and Suzanne Hand, of the UNSW School of Biological, Earth and Environmental Sciences, along with Aaron Camens of the University of Adelaide. “Our study has turned many of these long-held preconceptions about the lifestyles of these marsupials literally on their heads.

“We compared some exceptionally well-preserved Nimbadon skeletons from a fossil cave in the Riversleigh World Heritage Area with those of a range of modern and extinct mammals of varying lifestyles. Strikingly, we found Nimbadon's skeleton - in particular its limbs, hands and feet - to be most similar to that of the living koala.”

Like the koala, Nimbadon possessed powerful forelimbs with highly mobile shoulder and elbow joints that would have allowed significant extension and rotation of the arms, an essential trait for climbing through a tree canopy and for balancing on tree branches.

Nimbadon's hands and feet were extremely large, with long, flexible fingers and toes and semi-opposable first digits. Combined with the deep carpal tunnel of the wrist (to house the tendons of muscles that flex the fingers), it probably possessed an exceptionally powerful grasp. 

Nimbadon’s massive, sharp, recurved claws on its hands and feet were identical to those of the koala - but far larger - and could deeply penetrate a tree trunk during climbing.

“Our study showed that claws of this kind are only found in species that live in or regularly climb trees,” says Dr Black. “The numerous similarities between koala and Nimbadon skeletons suggests they functioned in much the same way and that Nimbadon used a trunk-hugging climbing method.”

Nimbadon had the shortest hind-limbs relative to its forelimbs of any known marsupial.  Those same limb proportions are found today in animals such as sloths and some apes that regularly hang by their forelimbs from tree branches . It is a behaviour no longer regularly used by any marsupial.

Like some tree-climbing bears, it's possible that Nimbadon may have supplemented its diet with fruit and possibly played a role as a large seed disperser in Australia's ancient forests.  It's climbing ability would have allowed it access to multiple layers of the rainforest canopy, perhaps to reduce competition for resources with terrestrial kangaroos and also for protection from predators.

“Our study indicates that modern Australian ecosystems have suffered an even greater loss in ecological diversity than previously expected,” says Dr Black.

Links:

Remarkable fossil cave shows how ancient marsupials grew

Fossil site and fossil images

Media contact:

Karen Black 02 9385 2113  k.black@unsw.edu.au