Middle Miocene origins for tough-browse dietary specialisations in the koala (Marsupialia, Phascolarctidae) evolutionary tree: description of a new genus and species from the Riversleigh World Heritage Area

Black, K.H. 2016. Middle Miocene origins for tough-browse dietary specialisations in the koala (Marsupialia, Phascolarctidae) evolutionary tree: description of a new genus and species from the Riversleigh World Heritage Area. Memoirs of Museum Victoria 74: 255–262. Stelakoala riversleighensis gen. et sp. nov. is described on the basis of an isolated M1 from the middle Miocene Jim’s Jaw Site of the Riversleigh World Heritage Area, northwestern Queensland. The new genus shares affinities, particularly in its possession of a metastylid fold, with a specialised clade of phascolarctids that includes species of Litokoala and the modern genus Phascolarctos. Dental specialisations of Stelakoala riversleighensis include a high molar crown, the presence of large accessory blades and cuspids, and the development of lingual ribs on the entoconid and metaconid. These features effectively increase both the number and available surface area of the molar cutting blades. Of particular note is the reinforcement of the primary cutting blade through the development of ribs on the lingual cuspids. These structures foreshadow the condition found in Phascolarctos, and may represent an early stage in the transition to a more fibrous, highly specialised diet in the evolutionary history of koalas.