The platypus has many features not found in other mammals. It is part of a group of mammals known as monotremes, which lay eggs and have specialized mouthparts. They branched off earlier in the evolutionary tree than marsupials and placental mammals, and they retained more reptilian features, such as a lower body temperature.
Why is the platypus a mammal?
How are mammals distinct from other animals?
An animal is considered a mammal if it can produce milk. Other features unique to mammals include hair or fur (chemically different from hairlike structures on non-mammals); the malleus, incus, and stapes in the ear; and a diaphragm separating the heart and lungs from the abdomen. Also, mammals lack nuclei in mature red blood cells.
