New wombats from Riversleigh, northwestern Queensland, include pouch young

Despite an estimated molecular divergence date of 40 million years (middle Eocene) between wombats and their closest living relative, the koala, the fossil record of wombats is poor. The two oldest described wombats, Nimbavombatus boodjamullensis and Rhizophascolonus crowcrofti, are both early Miocene in age. The former is known from maxillae, upper cheek teeth and isolated lower molars from Riversleigh in northwestern Queensland. The latter is known only from the isolated teeth from central Australia. Here we describe additional specimens of Rhizophascolonus from Riversleigh that are late Oligocene to middle Miocene in age as well as at least one additional species. Excitingly, the new specimens show an ontogenetic range from pouch young with occlusal cusp details intact to fully adult specimens with heavily worn teeth.

The unworn teeth from Riversleigh are compared with those pouch young from two extant wombat taxa and the extant koala. In addition, comparisons of quantitative and qualitative characters of all known Rhizophascolonus specimens with other koala and wombat taxa reveal a large degree of variation in the sample of Rhizophascolonus teeth from Riversleigh. However, the number of species of Rhizophascolonus present is difficult to ascertain and it is not inconceivable that there is only one or two highly variable species present.

Despite uncertainty at the species level, these new specimens provide an unprecedented insight into this enigmatic group, and we can now start to reconstruct intrafamilial relationships with a greater degree of confidence. Furthermore, occlusal morphology can now be used to help clarify interfamilial relationships in the suborder Vombatiformes, which in addition to wombats and koalas, includes five additional extinct families.