'' skeleton Fossils at Riversleigh are found in limestone by lime-rich freshwater pools, and in caves, when the
ecosystem was evolving from rich
rainforest to semiarid
grassland community. Some of the fossils at Riversleigh are 25 million years old. High concentrations of
calcium carbonate have meant the fossils are extremely well preserved. Fossils have been found in more than 200 individual locations. The fossil record here is significant because it provides evidence on evolution and the distribution of species across
Gondwana. The presence of Riversleigh fauna in the Oligo-Miocene has been classified by four "faunal zones", and used to denote the presence of fossil taxa 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 A common claim is that thirty-five fossil
bat species have been identified at the site. However, this is outdated and only half that number are actually described. Regardless, cave deposits have been particularly rich in bat species. A survey of species-level taxa described in the Riversleigh Fauna in the decades of research preceding 2006, resulted in a total greater than 290 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. == Research ==