They are strange creatures indeed, but I think my link should clear things up. They are not the only mammals to lay eggs. In fact, they are a member of one of three orders in the mammalian class: Eutheria (birth through placental development), Metatheria (marsupials), and Monotremata, or mammals that lay eggs. Platypuses are also in the family Ornithorhynchidae, which comprises the modern playtpus and all of its extinct relatives, and the genus Ornithorhynchus.

It's just that their closest relatives go all the way back to their order. If our order was as spread apart as theirs and as rare as theirs, then our closest relatives could be hippos or dogs or rats or even bats. However, our species has much closer relatives than does the platypus; we have other primates.

Go the platypus ....what an awsome lil aussie mammal!!....I think,there is no other animal like it in the world.

just because platpuses do not have all the main features that a normal mammal has does that mean that it should be deprived of its rights? besides which group would it go into otherwise?

Lets sum this up, yes. They breathe air. Their mammals. Thought you guys went through second grade.