First known extinct feathertail possums (Acrobatidae, Marsupialia): palaeobiodiversity, phylogenetics, palaeoecology and palaeogeography

Four new fossil feathertail possum species (Marsupialia, Diprotodontia, Phalangerida, Petauroidea, Acrobatidae) are described from late Oligocene to middle Miocene fossil deposits in the Riversleigh World Heritage Area, northwestern Queensland. They are the first pre-Pleistocene fossil representatives of this family to be described. Two species are referred to the modern genus Acrobates and two to the modern genus Distoechurus. These species are distinguished from each other and from the living Distoechurus pennatus and Acrobates pygmaeus on the basis of qualitative and quantitative characters of the first lower molar (m1), which is the only tooth known for all four fossil species. Fortunately, m1 is morphologically the most variable tooth in the cheektooth row of acrobatids, and it exhibits numerous genus-and species-specific features. Phylogenetic analyses based on dental characters strongly support monophyly of Acrobatidae relative to other petauroids, as well as providing relatively strong support for reciprocal monophyly of Acrobates and Distoechurus, including the newly described fossil members of these genera. Recognition of species of Acrobates and Distoechurus in these fossil deposits is broadly congruent with recent estimates for the time of divergence of the two modern genera based on molecular data, and also provides an additional fossil calibration point for future studies of marsupial divergence times. These fossil species provide new insights into the biogeographical and ecological history of this enigmatic family of small possums, specifically that the oldest known species of Acrobates occurred in closed forest environments (in contrast to the living species, A. pygmaeus and Acrobates frontalis, which today inhabit open sclerophyll forests and woodlands) and that Distoechurus appears to have originated in Australia, only subsequently dispersing to New Guinea before becoming extinct in its Australian homeland.