Molecular clock
Molecular clock data suggest echidnas split from platypuses between 19 and 48 million years ago, and that platypus-like fossils dating back to over 112.5 million years ago, therefore, represent basal forms, rather than close relatives of the modern platypus. This would imply that echidnas evolved from water-foraging ancestors that returned to living completely on the land, even though this put them in competition with marsupials. Though both existing monotremes such as the platypus and echidna have no teeth, the ancestor of monotremes once had adult teeth. Therefore, four out of the eight genes for tooth development were lost from those common ancestors.