Ngapakaldia bonythoni (Marsupialia, Diprotodontidae): new material from Riversleigh, northwestern Queensland, and a reassessment of the genus Bematherium

Ngapakaldia bonythoni has been recognized in several Oligo-Miocene deposits at Riversleigh, thus extending the known temporal and geographic range of the species. The excellent state of preservation of the Riversleigh material has enabled a description of diagnostic features of the palate, dentary and dentition previously not possible from the badly fractured holotype. In light of this new material, a reanalysis of the single known dentary of Bematherium angulum from Riversleigh's Site C (Tedford 1967), suggests B. angulum is a junior synonym of N. bonythoni, a relationship first proposed tentatively by Archer (1984). Few morphological differences (apart from size) distinguish N. bonythoni from its smaller congeneric N. tedfordi. Despite this similarity in form, the abundance of N. bonythoni, and the absence of any smaller forms such as N. tedfordi at Riversleigh, supports the recognition of two distinct species in Ngapakaldia. The shared presence of Ngapakaldia bonythoni in several Faunal Zone A local faunas at Riversleigh, and the Ngapakaldi Local Fauna (Zone C) of the Etadunna Formation, lends further support to a late Oligocene age for these deposits.