in these time periods. These may be summarised as Faunal Zone A (FZA): Late or Upper Oligocene, 28.4 to 23.03 million years ago Faunal Zone B (FZB): Early or Lower Miocene, 23.03 to 15.97 Ma Faunal Zone C (FZC): Middle Miocene, 15.97 to 11.608 Ma Faunal Zone D (FZD): Late or Upper Miocene, 11.608 to 5.332 Ma Thirty-five fossil bat species have been identified at the site, which is the richest in the world. Cave deposits have been particularly rich in bat species. The skull and nearly complete dentition of a 15-million-year-old monotreme, Obdurodon dicksoni, provide a window into the evolution of this characteristically Australian group. Fossil ancestors of the recently extinct thylacine, Thylacinus cynocephalus, have also been identified among Riversleigh's fauna. In 1993, skulls of the koala-like Nimbadon were unearthed in a previously unknown cave in the region. Researchers estimate that this marsupial first appeared about 15 million years ago and died out about 12 million years ago, perhaps from climate change-induced losses in habitat. A well-preserved skull of the ancient Nimbacinus dicksoni, an extinct relative of the thylacine, has been used to determine the hunting behavior of the species. Other fossils have provided evidence of how the koala has evolved in response to Australia's change from predominant rainforest vegetation to drier eucalypt forests. The fossil bird fauna at Riversleigh includes an artamid Kurrartapu johnnguyeni, a fossil sittella, and representatives of various other families of modern birds. Some fossil insects and plants have also been discovered. The fossil species identified at the sites are collectively known as the Riversleigh fauna. Research Scientific studies are mostly conducted by a group of palaeontologists from the University of New South Wales. Mike Archer is a paleontologist who has been working at Riversleigh since 1983. He and his co-workers discovered that diluted acetic acid was the most effective method of extracting fossils. Karen Black, a palaeontologist from UNSW, discovered a new species of extinct koala, at Riversleigh, which was then named after Dick Smith. See also List of fossil sites Agate Fossil Beds National Monument Santa Rosa local fauna Messel pit Laguna del Hunco Formation Posidonia Shale References Further reading Archer, M. et al. 1991. Riversleigh: the Story of Australia's Inland Rainforests, (Sydney: Reed Books) External links World heritage listing for Riversleigh UNESCO site with information on Riversleigh, Australia Information about fossils from Riversleigh, Australian Museum The Riversleigh Society supports scientific research at