A new and extinct, toothed species of platypus, the largest of its kind, has been discovered in the Riversleigh world heritage area in Queensland, Australia,1 contradicting the previous presumption that the modern edentate platypus could be traced back to ancient fossil species by a single, direct lineage.

A morphologically distinctive lower molar was found embedded in limestone, revealing an unknown branch of the platypus's evolution: the Obdurodon tharalkooschild, a much larger species with a distinct dental formation, though featuring stunted roots, anteroposterior compression of the crown and column-like primary buccal cusps similar to other toothed platypuses from the Obdurodon genus. With unique lophid and cusp relations, food was transversely masticated between the upper and lower molars. Prey will have included soft-bodied aquatic animals, as well as small vertebrates such as frogs and baby turtles. Apical cusp wear on the teeth suggest the extinct Obdurodon tharalkooschild crushed rather than cut its hard prey. These functional teeth, so unlike the dentally degenerate set the modern platypus is initially born with, suggests this new species belonged to a divergent group.

The tooth, dating between 5 and 15 million years old, is younger than the most ancient members of the platypus family, which spans 61 million years, but is the first in a previously unknown lineage of extinct giant platypuses.