Melbourne:
Pitch black darkness. Crushing bottlenecks, muddy passages, icy waterfalls. Bats and spiders. Rappelling over ledges into the unknown. How far would you go for a fossil? During a two-year retrieval mission of almost 60 hours in an underground cave, we reached our limits – and even went beyond them.
The limestone escarpment of Potholes Cave Reserve is located in Gunaikurnai Country, north of the Buchan township in eastern Victoria.
Here the river valley is peppered with shady entrances to underground caves. Portals barely large enough to allow a willing caver to access miles of subterranean passageways, encrusted with delicate crystals that sparkle in the light of torches.
In one, Nightshade Cave, the Museums Victoria Research Institute led a team of recreational cavers and park rangers from Parks Victoria to unearth an extraordinary fossil: a nearly complete skeleton of the extinct short-beaked kangaroo. Simosthenurus occidentalis. It will go on display at the Melbourne Museum in June this year.
It started with an unusual skull
As is often the case in paleontology, the discovery started with involved citizens in nature. In 2011, a local caving group entered Nightshade Cave for the first time through an opening previously blocked by earth. One of the group, Joshua Van Dyk, saw an unusual animal skull.
Recognizing its potential significance, he reported the find to the Melbourne Museum. However, Van Dyk thought it was irreparable and appeared to have been crushed under boulders in a narrow vertical collapse. The cave was closed with a fence to protect its contents, and a decade passed quietly.
In 2021, I became interested in the intriguing find. Members of the Victorian Speleological Society were only too happy to assist in a return to the cave.
Rigging a rope line, we descended along a tight thirty-foot gorge, emptying our lungs to pass tight points in the air. We corkscrewed into a narrow passage and squeezed single file through low-domed rooms hung with dripping stalactites and plastered with popcorn-like calcite formations.
The cave descended deeper and turned into high, narrow gorges with tight walls, full of dark recesses. Hours passed as we walked through the corridors, until a scream rang out: found again! We climbed up to a chimney-like trough filled with lodged boulders, to come face to face with an ancient one.
When I reached it, I felt a sudden sadness: the beautiful fossilized skull had begun to collapse in the intervening years. It seemed that despite its long survival, the fossil was once again vulnerable – to little more than the altered air currents and changing humidity caused by the cave's new entrance.
We reinforced the exposed bones with protective resins, but left the cave after leaving them in place: more time would be needed to plan their retrieval.
A difficult recovery
On our return journey I carefully brushed away fine layers of mud and we photographed and packed the newly released fossils. The skull had a deep snout, with robust jaws and teeth that marked it as a short-faced kangaroo.
Behind it were more bones. It was a wonder to see vertebrae, shoulders and hips, limbs and a narrow rib cage: many of the bones were completely unaffected and still in their original positions. This was a single animal, not a random scattering of bones. It felt like a fossil holy grail.
A detailed comparison with fossils in the Museum's Victoria State Collection identified our skeleton as Simothenurus occidentalis. It consists of 150 preserved bones and is the most complete fossil skeleton yet found in a Victorian cave.
That it is a juvenile rather than an adult kangaroo further distinguishes it from other examples of the species. The teeth show little wear, the skull bones are not yet fused, and the extremities of the limbs were not yet joined, indicating that he was still young at the time of death.
Based on the size of his limbs, we estimate that he weighed about 80 kilograms – as much as an average person – but might have grown to half that size if he had reached adulthood.
Australia's extinct megafauna
Short-billed kangaroos appear in Australia's fossil record 10 to 15 million years ago, when widespread rainforests began to give way to drier habitats. They became particularly diverse during the shift to our current arid climate in the latter part of the Pleistocene, from about 500,000 years ago.
But in a wave of extinctions about 45,000 years ago, they disappeared across the continent, along with up to 85% of Australia's megafauna. Radiocarbon dating by the Australian Nuclear Science & Technology Organization dated the skeleton's burial to 49,400 years ago. This means that our S. occidentalis was among the very last of its kind.
Today the hills of eastern Gippsland are home to a precious population of the rock wallaby, a vulnerable species. They once shared the land with larger relatives.
An important idea being explored is whether sthenurine kangaroos walked with strides instead of jumps. The skeleton we found has a uniquely complete spinal column, providing new insights that we could not gain from isolated bones. Using detailed 3D models, this almost complete skeleton can also be studied anywhere in the world.
This fossil, along with others from Nightshade Cave, is now forever housed and cared for at the Melbourne Museum. Through the Museums Victoria Research Institute we can maintain the link with our former homeland of East Gippsland, while opening the door to global research.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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