Karen Black from the University of New South Wales said the creatures, knows as Nimbadons, were climbers living in rainforest canopy.
"Nimbadon was a highly capable, habitual climber and its specialized anatomy suggests it may have adopted a trunk-hugging method of climbing similar to the living koala," Black said.
The climbing ability would have helped Nimbadons compete for food with other herbivores, such as kangaroos, and escape from predators, such as marsupial lions, Black said.
The researchers have discovered a number of Nimbadon skulls in a cave at Riversleigh in northwestern Queensland.
"The Nimbadon fossil material is an incredibly rare and significant resource, not only because it is so exceptionally well-preserved, but because it represents individuals from a range of ages," Black said.
The skulls reveal almost the entire life cycle of the Nimbadon, from suckling young in the pouch still cutting their milk teeth to elderly adults, she said.
"The Nimbadon material has allowed the first detailed study of skull development in a fossil marsupial as well as brain development and behavior."