system
Given the following post, respond as if you're commenting on Reddit:
human
How long ago did the platypus split off from other mammals? Why does the platypus have several distinct features not found elsewhere in combination? What genetic or fossil evidence supports the theories?
gpt
The monotremes split from the therians in the late Triassic/early Jurassic. Early monotremes did not resemble platypuses. The unique modern platypus form is a combination of derived (duck-like bill, webbed feet) and ancestral traits (lack of teats, oviparity, fur). The general platypus morphology has existed since at least late Oligocene as evident in the fossil record exemplified by the extinct genus *Obdurodon*.