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South-western Australia holds an exceptional number of endemic taxa and has been recognized as a biodiversity hotspot at a global scale. We report a much higher diversity in the genus Bennelongia (Ostracoda) in Western than in eastern Australia. Using mitochondrial COI sequence data for phylogenies, relative age estimates, lineage-through-time plots, and reconstructions of ancestral distributions, we test four hypotheses that might explain the higher diversity and endemicity in Western Australia. (1) We find no evidence for ancient relictualism as most Bennelongia species are probably of Miocene age. (2) There are also no apparent links to vicariant events: speciation has mostly taken place in Western Australia and has been ongoing through the evolutionary history of Bennelongia. (3) Dispersal has apparently not negatively affected Western Australian Bennelongia endemicity although these ostracods produce drought-resistant eggs. We report one case of recent long distance dispersal in B. dedeckkeri with genetically identical populations occurring more than 2,000 km apart. (4) Since speciation has been ongoing, there is no evidence of recent explosive speciation through genetic isolation. The underlying mechanisms of Bennelongia speciation thus remain elusive, although speciation has mostly occurred during a period of increasing aridification of Australia