>>26815498Not to be that cunt, but they're venomous, actually. I think some species of shrews are also venomous. It's rare for mammals, but it happens.
As for the egg-laying, it's not 100% agreed upon, but the most sound theory we have right now is that monotremes (which is just platypuses and echidnas) diverged from marsupials earlier than placental mammals. They retained characteristics of really early mammals and never updated their reproductive hardware to the point that marsupials and placental mammals did.
And the bill, is... not really a bill, but more of just a fleshy snout.