These giant ‘drop bears’ with opposable thumbs once scaled trees in Australia. But how did they grow so huge?

Though long dead, fossilized skeletons provide an incredible picture of an extinct animal’s lifestyle and environment.

By analyzing the various characteristics of fossil bones, we can reveal not only the overall size and shape of the animal, but also what movements the animal was capable of, its lifestyle and the environment in which it lived.

But what if we looked inside fossil bones? What secrets would it reveal about the growth and development of an extinct animal? In a newly published article in the Journal of Paleontologywe’ve done just that, using 15-million-year-old skeletons of a giant bear-like marsupial from the world-renowned Riversleigh World Heritage Area (Boodjamulla) in northwestern Queensland’s Waanyi Country.

Read more: Fossils reveal Australia’s heavyweight herbivore

Tree-dwelling wombat relatives

The huge tree-dwelling herbivorous marsupials known as Nimbadonweighed about 70 kg, making them the largest arboreal (arboreal) mammals known from Australia.

Nimbadon belongs to a diverse group of long-extinct large-bodied marsupials known as diprotodontoids, including the largest marsupial that ever lived, the 2.5-ton megafaunal Diprotodonand bizarre marsupials with trunks reminiscent of modern tapirs.

Among living animals, Nimbadon is most closely related to wombats. But surprisingly, in terms of body size and lifestyle, they are more similar to sun bearswhich can be found today in the rainforests of Southeast Asia.

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

But like many of the species we’ve unearthed at Riversleigh, the closer we look at these animals, the more bizarre and fascinating they become.

Nimbadon is now known from its complete skeleton, including material representing developmental ages ranging from tiny pouches to mature adults. It had strong arms with very flexible shoulder and elbow joints. Its hands and feet had specially adapted opposable thumbs with huge curved claws for climbing, penetrating bark and grasping branches.

These animals were highly specialized climbers and lived vastly different lifestyles compared to their closest living relatives – the land-dwelling, burrowing wombats.

Our initial research showed that Nimbadon was not only a “tree hugger”, but also a “tree hanger”, spending part of his time hanging on tree branches as a sloth.

Nimbadon lived 15 million years ago in the canopy of Australian lowland rainforests. These biodiverse, lush forests were home to some equally strange animals: carnivorous kangaroos, tree-climbing crocodiles, ancestral thylacines, cat-to-leopard-sized marsupial lions, huge anaconda-like snakes, giant toothed platypuses, and mysterious marsupials so strange that they have been called “Thingodonta”. It was a very different Australia than the country we see today.

Read more: Meet the giant wombat relative that made a living in Australia 25 million years ago

Cutting the bones

Despite the wealth of information we have collected Nimbadon skeletons, until now we had not fully understood the growth patterns of these ancient marsupials.

Were they influenced by seasonality? How long did it take them to mature in the canopy of the ancient forest? Clues to these questions lie in the microscopic structure of the bones.

To look inside the fossil bones, we had to select the right material. Long bones, such as the bones of the leg, are known to maintain good growth, so we analyzed ten long bones from different individuals of different sizes.

We started by removing part of the shaft from the bone, and embedded it in resin. Using a diamond-cut blade, we thinly sliced ​​our samples and further polished them until light could pass through. These thinned sections were mounted on glass microscope slides for study.

Remarkably, even after millions of years of fossilization, the microscopic structure of the fossil bones remained intact. We were amazed to find that out Nimbadon grew in periodic spurts. Individuals had periods of rapid growth, each followed by a period of slow growth, often associated with a series of stunted growth.

Seasonal growers

Cyclic growth patterns have previously been documented for marsupials such as in the living western gray kangaroo. However, our results indicate that, overall, the limbs of Nimbadon had much slower, more attenuated growth than kangaroo limbs.

One person recorded at least seven to eight growth cycles, suggesting that this arboreal giant took at least that much time — and probably more — to become a full-grown, sexually mature adult.

Based on these alternating cycles of fast and slow growth, Nimbadon may have been influenced by seasonal conditions such as food availability. However, exactly how long it took eight growth cycles to develop remains a mystery. If they do indeed represent annual cycles, it would take them at least eight years to reach sexual maturity, which is unusual in the modern marsupial world.

Kangaroos, for example, are sexually mature after one to two years. That being said, Nimbadon is an unusual beast and also a very large one, so a longer development period (and lifespan) is not unlikely.

Real drop bears

We’ve come to think of these strange arboreal marsupials as real-life versions of the legendary “drop bears” of Australian folklore – mysterious tree-dwelling creatures that would drop down on unsuspecting animals below.

As they move in herds through the rainforest canopy, both young and adult Nimbadon would have occasionally lost their grip before falling down from the treetops. Sometimes they ended up in caves in the forest floor, where we found their still articulated skeletons.

Given the constant surprises that research on this extraordinary, extinct Riversleigh mammal has already produced, we’re excited and ready for even more.

We are currently investigating wear in the enamel microstructure of Nimbadonteeth to determine the diet of this legendary licorice bear. We expect that what we find along the way will continue to shake our naive initial suspicions about the lifestyles of these and many other strange inhabitants of the ancient rainforests of Riversleigh’s interior.