Reconstruction of a mother and baby Nimbadon Peter Schouten, Author provided

Although long dead, fossil skeletons provide an incredible window into the lifestyle and environment of an extinct animal.

By analysing the various features of fossil bones we can reveal not only the overall size and shape of the animal, but also what kind of movement 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?

TextAnusuya Chinsamy-TuranKaren BlackMike ArcherSue Hand

54 | Bushwalk August 2023

Our initial research showed that Nimbadon was not only a “tree-hugger”, but also a “tree-hanger”, spending some of its time suspended from tree branches like a sloth.

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

Sectioning the bonesDespite the wealth of information we have gleaned from Nimbadon skeletons, until now we hadn’t fully understood the growth patterns of these ancient marsupials.

Were they affected by seasonality? How long did they take to grow to adult body size in the canopies of the ancient forest? Clues to these questions lay in the bones’ microscopic structure.

To look inside the fossil bones, we needed to select the right material. Long bones, such as the bones of the leg, are known to preserve a good record of growth, so we analysed ten long bones of several different-sized individuals.

Fossil skeleton of a mature adult Nimbadon.Karen Black, Author provided

56 | Bushwalk August 2023

We began by removing a section from the shaft of the bone, and embedded it in resin. Using a diamond-edged blade, we cut our samples into thin sections and polished them further until light could pass through them. These thinned sections were mounted on glass microscope slides to be studied.

Remarkably, even after millions of years of fossilisation, the microscopic structure of the fossil bones had remained intact. We were amazed to discover that Nimbadon grew in periodic spurts. Individuals had fast growth periods, each followed by a slow growth period, often associated with a band of arrested growth.

Seasonal growersCyclical growth patterns have previously been documented for marsupials such as in the living western grey kangaroo. However, our results indicate that, overall, the limbs of Nimbadon had a much slower, more extenuated growth than kangaroo limbs.

One individual recorded at least seven to eight growth cycles, which suggests this arboreal giant needed at least this amount of time – and probably more – to become a fully-grown, sexually mature adult.

Based on these alternating cycles of fast and slow growth, Nimbadon may have been affected by seasonal conditions such as food availability. However, exactly how long it took for eight growth cycles to develop remains a mystery. If indeed they represent annual cycles, it would be at least eight years

until sexual maturity, which is unusual in the modern marsupial world.

For example, kangaroos are sexually mature at one to two years. That being said, Nimbadon is an unusual beast and a very large one at that, so an extended developmental period (and lifespan) is not unlikely.

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

While moving in herds through the rainforest canopy, both young and adult Nimbadon would have occasionally lost their grip before dropping down from the treetops. Sometimes they would end up in forest floor caves, which is where we have been finding their still-articulated skeletons.

Given the constant surprises that research into this extraordinary, extinct Riversleigh mammal has already produced, we are eager and prepared for still more.

Currently we are looking into wear in the enamel microstructure of Nimbadon’s teeth to determine this legendary drop bear’s diet. We expect that what we find down the track will continue to upend our naïve first presumptions about the lifestyles of this and many of the other strange inhabitants of the ancient inland rainforests of Riversleigh.