ABSTRACT-A new, toothed ornithorhynchid monotreme from Two Trees Site in the Riversleigh World Heritage Area, northwestern Queensland, Australia, is described. This species is the largest known ornithorhynchid, fossil or extant, the fourth extinct platypus described, and the second species discovered at Riversleigh. It exhibits a unique molar morphology that significantly broadens understanding about disparity within this group of monotremes and challenges a previous presumption that fossil species of Obdurodon form an anagenetic lineage leading directly to the living dentally degenerate Platypus Ornithorhynchus anatinus. Apical cusp wear on its teeth suggests that it was crushing rather than cutting hard prey items. Its relatively derived features also adds to mounting evidence that the Two Trees deposit that contains several unique taxa may be younger than the surrounding middle Miocene fossil deposits, possibly late Miocene or even Pliocene in age, intervals of time previously unrepresented by ornithorhynchids.
Mesozoic mammals from Australia are rare, so far only known from the Early Cretaceous, and most are poorly represented in terms of dentitions much less cranial material. No upper molars of any have been described. Kollikodon ritchiei is perhaps the most bizarre of these, originally described on the basis of a dentary fragment with three molars. Here we describe a second specimen of this extremely rare taxon, one that retains extraordinarily specialised upper cheekteeth (last premolar and all four molars). Each molar supports rows of bladeless, rounded cuspules many of which exhibit apical pits that may be the result of masticating hard items such as shells or chitin. Reanalysis of the phylogenetic position of this taxon suggests, based on a limited number of apparent synapomorphies, that it is an australosphenidan mammal and probably the sister group to Monotremata. This reanalysis also supports the view that within Monotremata, tachyglossids and ornithorhynchids diverged in the early to middle Cenozoic.
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