Fossils at Riversleigh are found in limestone by lime-rich freshwater pools, and in caves. They are from when the ecosystem was evolving from rich rainforest to semi-arid grassland. Thirty-five fossil bat species have been found at the site, which is the richest in the world. A skull, complete with all its teeth, of a 15 million-year-old monotreme, "Obdurodon dicksoni", shows how this Australia group of animals evolved. Fossil ancestors of the extinct thylacine, "Thylacinus cynocephalus", have also been found at Riversleigh. In 1993, "Nimbadon" skulls were found in newly discovered cave. Scientists think this prehistoric marsupial first appeared about 15 million years ago and died out about 12 million years ago, perhaps from the effects of climate change. Other fossils have shown how the koala has changed in response to Australia's change from rainforest to drier eucalypt forests.