A Miocene Graveyard
In the Riversleigh World Heritage Area of northwestern Queensland, a limestone cave deposit known as AL90 holds an entire community of skeletons. This site, dating to the Middle Miocene Epoch about 15 million years ago, acted as a natural pitfall trap. For millennia, animals moving through the lush rainforest canopy would occasionally fall through the vegetation-obscured opening, creating a perfectly preserved collection of Australia's ancient fauna. The most numerous resident of this prehistoric tomb is Nimbadon lavarackorum, a sheep-sized marsupial distantly related to modern wombats and koalas.
Paleontologists have recovered more than 30 skulls and numerous complete, articulated skeletons from the AL90 site. The collection represents individuals of all ages, from suckling pouch-young to old adults, showing a single population. Before the discovery of these complete skeletons, researchers assumed most diprotodontoids—the family to which Nimbadon belongs—were ground-dwelling animals, with some growing to the size of a rhinoceros. The pristine, three-dimensional preservation of the Nimbadon fossils, uncrushed in the soft freshwater limestone, completely changed this understanding.