分子時計
The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around themfor example, that the monotremes were "inferior" or quasireptilianstill endure. In 1947, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect. In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups. Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48million years ago.