A new species of Mukupirna (Diprotodontia, Mukupirnidae) from the Oligocene of Central Australia sheds light on basal vombatoid interrelationships

The late Oligocene taxa Marada arcanum and Mukupirna nambensis (Diprotodontia, Vombatiformes) are the only known representatives of the families Maradidae and Mukupirnidae, respectively. Mukupirna nambensis was described from a partial skeleton, including a cranium but no dentary, and reconstructed as the sister taxon to Vombatidae (wombats). By contrast, Ma. arcanum is known only from a single dentary, preventing direct comparison between the two. Here, we describe a new species, Mu. fortidentata sp. nov., based on craniodental and postcranial specimens from the Oligocene Pwerte Marnte Marnte Local Fauna, Northern Territory, Australia. Phylogenetic analysis of Vombatiformes, using 124 craniodental and 20 postcranial characters, places these three species within Vombatoidea, wherein Marada arcanum is sister to species of Mukupirna + Vombatidae. Mukupirna fortidentata sp. nov. does not share any robust synapomorphies of the dentary with Ma. arcanum that would support placing them together in a clade to the exclusion of Vombatidae. We therefore maintain separation of the families Mukupirnidae and Maradidae. From a functional perspective, the craniodental specimens of Mu. fortidentata sp. nov. reveal a suite of morphological traits that are unusual among vombatiforms, which we interpret as adaptations for acquiring and processing hard plant material. These include: a short, broad rostrum; large, robust, steeply upturned incisors; and a steep, anteroposteriorly decreasing gradient in cheek tooth size. The dental specimens of Mu. fortidentata sp. nov. also assist in the identification of two further allied taxa: an early vombatid from the younger late Oligocene Tarkarooloo Local Fauna, South Australia; and a possible vombatoid from the earliest Miocene Geilston Bay Local Fauna, Tasmania. The Tarkarooloo Local Fauna taxon indicates that vombatids diverged from other vombatoids prior to 24 million years ago.