Within the Ornithorhynchidae are the main Cenozoic genera Ornithorhynchus and Obdurodon, and several potential stem-genera dating back to the Late Cretaceous, of which the oldest is possibly Dharragarra.

[2] This decision was made based on differences in the dentary recovered from the Griman Creek Formation, Lightning Ridge, New South Wales, Australia.

This dentary is the holotype for the genus Steropodon, thus the lack of information led to the original misclassification.

Further research on Teinolophos has indeed shown it to be an animal much different from ornithochrynchids, lacking a beak, possessing a more complete mammalian dentition, and retaining primitive ears connected to the jaw as in more basal mammals.

It has been theorized that the loss of teeth in the platypus was a geologically recent event, occurring only in the Pleistocene (after over 95 million years of tooth presence in the ornithorhynchid lineage) after the migration of the rakali (Hydromys chrysogaster), a large semiaquatic rodent, to Australia.