Problem:
It seems unlikely that an ovoviparous ancestor of mammals long ago could have had a viviparous offspring in a sharp one-generation dividing line,
Question: What would be the gradual steps between egg birth and live birth?
Question: Are there any examples of answers to the first question today?
It seems that marsupials are a different thing altogether, not something in between egg laying and live birth (no modern mammal's ancestors aren't believed to have marsupial-style birth in their evolutionary history, right?)?