Platypus is part bird, mammal and reptile.
Scientists Say they have mapped the genetic makeup of the platypus — one of nature's
strangest animals with a bill like a duck's, a mammal's fur and snake-like venom.....The
platypus is classed as a mammal because it has fur and feeds its young with milk. It flaps
a beaver-like tail. But it also has bird and reptile features — a duck-like bill and
webbed feet, and lives mostly underwater".
Of course it is a mammal. It is a warm blooded animal with fur. That alone tells you it is a mammal. Though, a Platypus is one of only a few mammals that lay eggs instead of having live young. There are 4 species of Echidna that also lay eggs. The Platypus and Echidnas are the only Monotremes that live today. Monotremes are egg laying mammals. The eggs are leathery, like reptile eggs. The male Platypus also has a spur on the hind foot that contains venom for defense if a predator grabs it from behind. If you want to learn more about these animals then go to http://en.wikipedia.org/wiki/Platypus.