Stirtodon

Stirtodon is an extinct genus of monotreme mammal from the Late Cretaceous (Cenomanian) Griman Creek Formation of Australia. The genus contains a single species, S. elizabethae, known from a large isolated premolar. Stirtodon may be the largest toothed monotreme discovered. Several other monotremes are known from the Griman Creek Formation, including Dharragarra, Kollikodon, Opalios, Parvopalus, and Steropodon.

It may have been pig sized and lived in an area with mainly monotremes. It helps demonstrate that within 26 million years of when monotremes were thought to have arisen, the group was rapidly evolving. By 100mya the group had diversified to pig-sized species, rat-sized species, in-between sized species, some terrestrial, some aquatic.