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RE: Tree of Life: The Fascinating Journey of Placental Evolution
Two things define them specifically I think: Nurture with milk (the egg laying monotremes still do this in a different way) and hair/fur. There are bound to be exceptions to all of these things though, probably! Generally you should tick a few boxes to be classified
Yeah Yeah... You should tick a few boxes to be classified as a member of a group and only the egg laying factor is not enough to disqualify them from being classified as mammals...
Thanks for the clarification...