A Graveyard in the Salt
In the arid expanse of South Australia lies Lake Callabonna, a dry salt lake that holds the grim record of a mass tragedy from the Pleistocene epoch. This is a collection of bones, but records final, desperate moments. Preserved in the baked mud are the trackways of Diprotodon optatum, the largest marsupial ever known. These trace fossils, the subject of the scientific discipline of ichnology, record the doomed march.
Diprotodon optatum was an immense herbivore, reaching up to 1.7 meters at the shoulder, over 3.8 meters long, and weighing as much as 2,800 kilograms. Its closest living relatives are wombats and koalas. During a period of intense drought approximately 50,000 to 100,000 years ago, the vast inland lakes of Australia began to vanish. The trackways found at Lake Callabonna show herds of these giant animals walking in a single direction. The accepted interpretation is that they were trudging toward the shimmering illusion of water, a mirage on the horizon of the drying lakebed. The animals became stuck in the thick, clinging mud, eventually succumbing to thirst and starvation. Hundreds of individuals perished together, their skeletons and footprints left as proof of their death.
The Lake of Bones
The fossil beds at Lake Callabonna were first reported by a station owner in 1892, leading to expeditions by the South Australian Museum. Excavations revealed many paleontological fossils. Hundreds of Diprotodon skeletons were found, many of them complete and articulated. This density of fossils in one location shows that these animals were social and moved in groups, perhaps smaller family units drawn together by the shrinking water sources.
The site is remarkable for its quality of preservation. Researchers have found bones and tracks; some specimens include preserved hair, foot impressions, and even stomach contents. Analysis of these gut contents revealed the animals' last meals consisted of saltbush, a tough plant they likely would not have eaten unless under severe duress. The mud that trapped the animals also preserved them, creating a record of this megafauna ecosystem. Alongside Diprotodon, the remains of the giant flightless bird Genyornis newtoni and extinct kangaroo species have also been unearthed at the site.