Monotremes are mammals, yet do not share the property of giving live birth.
And many decidedly non-mammal animals do give live birth. Wikipedia lists “scorpions, some sharks, some snakes, [...] velvet worms [and] certain lizards”.
And many decidedly non-mammal animals do give live birth. Wikipedia lists “scorpions, some sharks, some snakes, [...] velvet worms [and] certain lizards”.