Fossils at Riversleigh are encased in limestone deposited by lime-rich freshwater pools, and in caves, at a time when the ecosystem was evolving from a rich rainforest to semi-arid grassland community. Thirty-five fossil bat species have been identified at the site, which is the richest in the world. The skull and nearly complete dentition of a fifteen million-year-old monotreme, Obdurodon dicksoni, provide a window into the evolution of this characteristically Australian group. Fossil ancestors of the recently extinct Thylacinus cynocephalus, the marsupial "Tasmanian Tiger", have also been identified among Riversleigh's fauna.
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