The Ice That Carved a Canyon of Bone: Wellington Caves' Fossil Mammal Deposits

Deep in central New South Wales, a limestone ridge riddled with caves has yielded the bones of more extinct giant marsupials than anywhere else on the continent. The Wellington Caves, first dug by European settlers in 1830, contain a 4-million-year record of Australia's vanished megafauna—preserved not in lake mud or river sand, but in the sediment floors of ancient underground chambers.

A Bone Quarry Born of Curiosity

The first Europeans to explore the caves in 1830 found the floor covered in bones. Local settlers dug out tonnes of fossil-rich sediment, shipping specimens to London where they puzzled naturalists who had never seen such creatures: a wombat the size of a rhinoceros, a kangaroo that stood three metres tall, a marsupial lion with slicing premolars.

Major Thomas Mitchell, the Surveyor-General of New South Wales, excavated the caves systematically in the 1830s. He sent crates of bones to the British Museum, where Richard Owen—the same anatomist who coined the word "dinosaur"—named the giant wombat Diprotodon and the marsupial lion Thylacoleo. The Wellington Caves thus became the type locality for Australia's Pleistocene megafauna.

How a Cave Becomes a Fossil Trap

Limestone dissolves slowly in slightly acidic rainwater, creating cavities over hundreds of thousands of years. At Wellington, the cave-bearing limestone is Devonian in age, roughly 370 million years old—a reef deposit from a shallow sea that covered eastern Australia long before any land animal existed.

The same rock that once held coral and crinoid now holds the bones of giant marsupials—a 370-million-year-old tomb for animals that walked the earth only tens of thousands of years ago.

When the cave roof collapsed at several points, it created natural pitfall traps. Animals fell in and could not escape. Their bones accumulated on the cave floor, mixed with sediment washed in during floods, and were slowly cemented into a fossil-rich breccia. The caves acted as natural sediment traps, concentrating bones across multiple glacial-interglacial cycles.

What the Bones Reveal

The Wellington Caves have produced fossils from at least 40 species of extinct vertebrates. The most famous is Diprotodon optatum, the largest marsupial that ever lived—a wombat-like herbivore weighing over two tonnes, with a skull the size of a cow's. Thylacoleo carnifex, the marsupial lion, was a stocky predator with bolt-cutter teeth and a retractable thumb claw, capable of taking prey larger than itself.

Other finds include:

• Procoptodon goliah, a short-faced kangaroo that stood three metres tall and could not hop—it walked on two legs like a human
• Zygomaturus, a diprotodontid with flaring cheekbones that lived in wet forests
• Varanus priscus (Megalania), a monitor lizard six metres long that was Australia's largest terrestrial predator

The mix of species suggests that the Wellington region was a mosaic of woodland and grassland during the Pleistocene. Climate shifts drove faunal turnover: some species appear only in glacial periods, others in interglacials.

The Extinction Question

Diprotodon and Thylacoleo disappeared roughly 45,000 years ago, soon after humans arrived in Australia. The Wellington Caves preserve both pre-human and post-human layers, allowing scientists to test competing extinction hypotheses. The youngest megafauna fossils at Wellington overlap with the earliest evidence of human presence in the region, supporting the view that hunting and landscape burning contributed to their loss.

But the caves also show that some species had already declined during the last glacial maximum, when Australia became drier and colder. The extinction was likely a double blow: climate stress made populations vulnerable, and human arrival delivered the final push.

Today the Wellington Caves are a tourist attraction, but the bone-bearing sediments remain largely unexcavated in the deeper chambers. The ridge holds perhaps the longest continuous record of mammal evolution in Australia—and much of it still lies underground.