Sexually Dimorphic Bandicoots (Marsupialia: Peramelemorphia) From the Oligo-Miocene of Australia, First Cranial Ontogeny for Fossil Bandicoots and New Species Descriptions

Peramelemorphians (bandicoots and bilbies) are unique among mammals in having the shortest gestation period. Very little is known about their evolutionary history primarily because until recently their fossil record was scarce. Here we describe two new species, Madju variae, gen. et sp. nov., from late Oligocene to middle Miocene deposits from the Riversleigh World Heritage Area, Queensland, and the Kutjamarpu Local Fauna, South Australia, and Madju encorensis, gen. et sp. nov., also from Riversleigh WHA but from the late middle to early late Miocene. Phylogenetic analysis suggests that they are best regarded as basal members of the Superfamily Perameloidea. Species of Madju are unusual in showing a distinct reduction in size through time, possibly reflecting environmental change from the early to late Miocene. Madju variae is the first-known sexually dimorphic fossil peramelemorphian. The preservation and representation of specimens of M. variae is exceptional, enabling documentation of ontogenetic development from juvenile to old adult stage suggesting that juveniles of M. variae developed more slowly than their modern counterparts and that lactation lasted for a longer time. If so, the short gestation of modern peramelemorphians would appear to be a specialisation that might have evolved sometime after the middle Miocene.