AUSTRALIA'S second Mesozoic mammal, Kollikodon ritchiei (Monotremata, Kollikodontidae, new family) has an extreme bunodont molar morphology. The existence of two distinctively different families of monotremes in the Early Cretaceous suggests that the order originated long before the Cretaceous and was very diverse in at least the Australian portion of eastern Gondwana. With four families now known from Australia, it is probable that monotremes originated and diversified in the Australian/Antarctic sector of Gondwana, followed by a single dispersal (ornithorhyn-chid) to the South American sector before or during the early Paleocene. It is probable that kollikodontids are the sister-group of a steropodontid/ornithorhynchid/tachyglossid clade. K. ritchiei and Steropodon galmani are among the largest Mesozoic mammals known.