In Australia, these enormous ‘drop bears’ with opposable thumbs used to ascend trees. But how could they become so massive?

Albeit long dead, fossil skeletons give an amazing window into the way of life and climate of a wiped-out creature.

We can learn a lot about an animal’s overall size and shape, as well as its ability to move, its way of life, and the environment in which it lived, by examining the various features of fossil bones.

However, what if we examined the inside of fossil bones? What privileged insights could it uncover about the development and advancement of a terminated creature? Using 15 million-year-old skeletons of a giant bear-like marsupial from the world-renowned Riversleigh World Heritage Area (Boodjamulla) in Waanyi country in northwest Queensland, we have done just that in a new paper in the Journal of Paleontology.

The massive tree-dwelling herbivorous marsupials known as Nimbadon weighed approximately 70 kilograms, making them the largest arboreal (tree-dwelling) mammals from Australia that have been identified.

Nimbadon has a place with a different gathering of long-terminated, enormous-bodied marsupials known as diprotodontoids, any semblance of which incorporates the biggest marsupial to have at any point resided, the 2.5-ton megafaunal Diprotodon, and odd-trunked marsupials suggestive of cutting-edge ungulates.

Nimbadon is most closely related to wombats among living things. However, shockingly, as far as body size and way of life go, they are more equivalent to sun bears, which today can be found scaling the rainforest coverings of Southeast Asia.

Similar to sloths, modern sun bears climb trees and lounge there. Credit: Shutterstock

When the jawbones of Nimbadon were first discovered at Riversleigh in 1993, we thought we were looking at very large leaf-eating marsupials that foraged on the forest floor for food.

Be that as it may, in the same way as other of the species we’ve uncovered from Riversleigh, the closer we take a gander at these creatures, the more odd and entrancing they become.

Nimbadon is now known from its entire skeleton, which contains parts that represent a variety of developmental ages, from pouch-young larvae to mature adults. It had solid arms with exceptionally versatile shoulder and elbow joints. Its hands and feet had exceptionally adjusted opposable thumbs with immense bended paws for climbing, infiltrating bark, and getting a handle on branches.

Compared to their closest living relatives, the land-dwelling, burrowing wombats, these animals were highly specialized climbers who led very different lives.

Our underlying exploration showed that Nimbadon was an “environmentalist,” yet additionally a “tree-holder,” investing a portion of its energy suspended from tree limbs like a sloth.

Fossil skeleton of an experienced grown-up Nimbadon Credit: Karen Dark, Creator

Nimbadon lived quite a while back in the overhang of marshy Australian rainforests. Others that were just as bizarre lived in these lush, biodiverse forests: kangaroos that eat flesh, tree-climbing crocodiles, ancestral thylacines, cat-to-leopard-sized marsupial lions, enormous snakes that look like anacondas, giant toothed platypuses, and mysterious marsupials that have been dubbed “Thingodonta” for their bizarre behavior. Australia then was very different from the one we see today.

Segment the bones.
Regardless of the abundance of data we have gathered from Nimbadon skeletons, as of not long ago, we hadn’t completely perceived the development examples of these old marsupials.

Did the change in seasons affect them? In the ancient forest’s canopy, how long did it take them to reach adult size? Signs of these inquiries lay in the bones’ minuscule design.

We needed the right material to look inside the fossil bones. Since long bones, like those in the leg, are known to keep good records of growth, we looked at ten long bones from people of varying sizes.

A large slab of limestone containing articulated fossilized Nimbadon skeletons was found in a 15 million-year-old fossil cave deposit in the Riversleigh World Heritage Area, northwestern Queensland. Credit: Anna Gillespie, Author provided

We began by removing a section from the shaft of the bone and embedding it in resin, according to Anna Gillespie, the author. We cut our samples into thin sections with a blade with a diamond edge and polished them further until light could pass through them. These diminished segments were mounted on glass magnifying instrument slides to be contemplated.

Surprisingly, even after many long periods of fossilization, the minuscule construction of the fossil bones stayed in one piece. We were surprised to learn that Nimbadon grew in short bursts. People went through periods of rapid growth, which were followed by slower growth periods, which were frequently accompanied by a band of stunted growth.

Occasional cultivators
Repeating development designs have recently been recorded for marsupials, like the living western dim kangaroo. Nimbadon’s limbs, on the other hand, grew at a much slower and more uneven rate than kangaroo limbs, as shown by our findings.

One person noted at least seven to eight growth cycles, indicating that this arboreal giant required at least that much time—and probably more—to mature sexually.

In view of these patterns of quick and slow development, Nimbadon might have been impacted by occasional circumstances like food accessibility. However, it is still unknown how long it took for eight growth cycles to develop. In the modern marsupial world, sexual maturity would take at least eight years if it truly represented annual cycles.

Kangaroos, for instance, reach sexual maturity between one and two years old. Nimbadon, on the other hand, is a rare beast of a very large size, so it is not out of the question that it would have a long lifespan.

The mythical “drop bears” of Australian folklore—mysterious tree-dwelling creatures that would drop down on unsuspecting animals below—have come to be compared to these strange arboreal marsupials.

Both young and mature Nimbadon would occasionally lose their grip as they moved in herds through the canopy of the rainforest before plummeting from the trees. Their still-articulated skeletons have been discovered in forest floor caves, where they occasionally ended up.

We are eager and prepared for additional discoveries made through research into this extraordinary, extinct Riversleigh mammal.

In order to determine the diet of this legendary drop bear, we are currently examining wear in the enamel microstructure of Nimbadon’s teeth. We anticipate that what we discover in the future will continue to challenge our naive initial assumptions regarding the bizarre lifestyles of this and many other residents of Riversleigh’s ancient inland rainforests.

More information: Anusuya Chinsamy et al, Paleobiological implications of the bone histology of the extinct Australian marsupial Nimbadon lavarackorum, Journal of Paleontology (2023). DOI: 10.1017/jpa.2023.22

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