Australia was historically considered a poor prospect for subterranean fauna but, in reality, the continent holds a great variety of subterranean habitats, with associated faunas, found both in karst and non-karst environments. This paper critically examines the diversity of subterranean fauna in several key regions for the mostly arid western half of Australia. Weaimed to document levels of species richness for major taxon groups and examine the degree of uniqueness of the fauna. We also wanted to compare the composition of these ecosystems, and their origins, with other regions of subterranean diversity world-wide.Usinginformationonthenumberof describedandknown invertebrate species (recognisedbasedon morphological and/or molecular data),wepredict that the total subterranean fauna for the western half of the continent is4140 species, of which ~10% is described and 9% is known but not yet described. The stygofauna, water beetles, ostracods and copepods have the largest number of described species, while arachnids dominate the described troglofauna. Conversely, copepods, water beetles and isopods are the poorest known groups with less than 20% described species, while hexapods (comprising mostly Collembola, Coleoptera, Blattodea and Hemiptera) are the least known of the troglofauna. Compared with other regions of the world,weconsider the Australian subterranean fauna to be unique in its diversity compared with the northern hemisphere for three key reasons: the range and diversity of subterranean habitats is both extensive and novel; direct faunal links to ancient Pangaea and Gondwana are evident, emphasising their early biogeographic history; and Miocene aridification, rather than Pleistocene post-ice agedriven diversification events (as is predicted in the northern hemisphere), are likely to have dominated Australias subterranean speciation explosion. Finally, we predict that the geologically younger, althoughmorepoorly studied, eastern half of the Australian continent is unlikely tobeas diverse as the western half, except for stygofauna in porous media. Furthermore, based on similar geology, palaeogeography and tectonic history to that seen in the western parts of Australia, southern Africa, parts of South America and Indiamayalso yield similar subterranean biodiversity to that described here.
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