At the entrance to Naracoorte Caves National Park in South Australia is a life-size statue of the extinct Diprotodon optatum, the largest marsupial to ever live in Australia. Filling the ecological niche of elephants on other continents, D. optatum stood two meters high at the shoulder, three-and-a-half meters long, and weighed up to two tons. With the largest living marsupial in Australia, the red kangaroo (Macropus rufus), weighing in at less than two hundred pounds, D. optatum was a giant by any standards.
Naracoorte Caves National Park hosts twenty-six known caves. Over time, predators used the caves as dens, and concealed cave entrances acted as pitfall traps for animals in the area, so the skeletal remains and fossils of numerous species are well-preserved within the caves. With a rich fossil record dating back 500,000 years, paleontologists have heaps to study, and 120 species of vertebrates have been recorded so far, including D. optatum.
The Wonambi Fossil Center within the national park hosts a sampling of Australia’s extinct animals in the form of life-size, animatronic models. With a theme-park vibe, the herbivorous D. optatum grazes on the long grasses at its feet and the giant leaf-eating kangaroo, Procoptodon goliah, moves robotically to better sense what other creatures might be in the surrounding area. But as I wandered through the Fossil Center, my interest was easily peaked as I learned about the Australian megafauna that once inhabited the continent.
In The Future Eaters: An Ecological History of the Australasian Lands and People, author and scientist Tim Flannery describes Australia’s extinct mega marsupials. But before D. optatum had a chance to grow so large, Flannery questions the origins of marsupials. Since marsupials are found both in South America and Australia, where did they originate? Did they cross from Australia to South America via an Antarctic land bridge, or vice versa?
In 1979, Professor Frederick Szalay came to Australia to study the foot bones of marsupials and discovered the answer. He found that all Australian marsupials shared a common foot structure that indicated they had all derived from an arboreally adapted ancestor. The only South American marsupial, alive or dead, with a similar foot structure was Dromiciops australis, also known as the “small monkey of the mountains.” This tiny marsupial inhabits ancient forests in Chile that are representative of the forests that once linked Australia and South America. Because Australian marsupials all derived from a common ancestor and form a specialized subset of marsupials, it makes the most sense that they have evolved from an animal like Dromiciops and crossed over into Australia from South America about 60 million years ago.
Among the giant marsupials that evolved from this common ancestor were several carnivores, including the marsupial lion (Thylacoleo carnifex), and the thylacine, or Tasmanian tiger (Thylacinus cynocephalus). Still alive are the smaller Tasmanian devil (Sarcophilus harrisii), which lived on mainland Australia until dingoes outcompeted it and is now threatened by Devil Facial Tumor Disease, and four species of quoll. The marsupial lion, not nearly as large as a lion, filled the ecological niche of medium-sized cats on other continents and went extinct 18,000 years ago. The thylacine, which filled the ecological niche of a wolf, has been extinct for less than 100 years. The last known individual died in captivity in 1936, though some speculate that others may still live on in uncharted areas of Tasmania and New Guinea.
Several herbivores also joined Australia’s megafauna, including D. optatum and P. goliah, mentioned above. There were giant echidnas (which are monotremes, or egg-laying mammals, like the platypus), at least twenty species of kangaroos in addition to today’s kangaroos, a wombat weighing more than 400 pounds, and a giant koala, about one third larger than today’s koala. There was also the marsupial tapir (Palorchestes azeal), a ground-dwelling marsupial with extremely sharp claws used for ripping through tough vegetation, and a marsupial hippo (Zygomaturus trilobus) that lived in small herds around the wet, coastal margins of Australia.
Each of these unique and very large animals was extinct by about 18,000 years ago. But why did they go extinct? Dr. Stephen Wroe, paleontologist and Associate Professor of Biological, Earth, and Environmental Science at the University of New South Wales in Sydney, believes that changes in climate caused the extinction of these large creatures. Even though the arrival of humans in Australia overlapped with the decline of these animals, Dr. Wroe believes the populations of Australia’s megafauna were already decreasing as a result of an erratic changing climate, and there is little evidence that humans drove these animals to extinction. As water became locked up as ice during the last ice age, Australia’s landscape became increasingly more arid, and the vast inland lakes on the continent began to decrease in size. The dry climate and lack of fresh water in Australia created an environment in which survival of such large animals was increasingly difficult.