Microleo attenboroughi is a very small marsupial lion from the Early Miocene of Australia, about 18 million years ago. M. attenboroughi is currently known from a broken palate containing an incomplete row of cheekteeth diagnostic of Thylacoleonidae found in the Early Miocene-aged area of the Riversleigh World Heritage Site, in Queensland. The anatomy of Microleo suggests it is the basalmost of all thylacoleonids, and that the marsupial lions as a whole are basal vombatomorphs that form the sister taxon of all other Vombatomorphia