The monotremes branch off before mammals invented live birth. This has to do with the separation of Australia from the rest of Gondwonaland . . .
Well, why am I telling this?
The Natural History of the Monotremes.
Your main point is that a lack of record is a lack of history. Wrong. Even if you don't know who your Daddy is, you had one.
Did they get a patent on that?! < GRIN! >
Seriously, how does Evolution explain the poison of the duck-billed platipus better than Intelligent Design explains it?