The animal that baffled scientists

This animal, called ornithorhynchidae, lives in northern Australia. It is not classified as a bird because it crawls on its stomach. It is not a member of the fish family because it lives on land. It is not classified as a reptile because it has a beak like a duck's beak. It is not a terrestrial animal because it can live underwater. It is not classified as a mammal because it lays eggs. It is not classified as a bird because it breastfeeds its young.