Monotremes are our most ancient mammal group and possess many unique traits and features that have allowed them to survive and thrive here in Australia. Monotremes are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals, and marsupials. Wikipedia Get to know our egg-laying mammals, echidnas and platypus, and discover what reptile-like features they have that other mammals don’t.