The platypus lays eggs, makes milk, and carries venom

The platypus is a mammal that lays eggs, feeds its young on milk secreted through the skin, and, in the male, has a venomous spur.

Monotremes — the platypus and the echidnas — split from the other mammals deep in the Mesozoic. They keep traits the rest of us dropped. The young hatch from eggs. Milk is produced by glands that open onto the skin, with no nipple. The male platypus has a spur on the hind leg connected to a venom gland, used in fights with other males, painful and occasionally dangerous to a person.

The 2008 genome added another oddity: ten sex chromosomes, a chain of five X and five Y in the male, which pair in a complicated order at meiosis. Electroreception, a bill that senses the electric field of prey, is a fourth fact of the same animal. None of this is intermediate in the old sense of 'on the way to becoming a proper mammal.' It is a surviving branch with its own inventions.