Monotremata: Morphology

In some ways, monotremes are very primitive for mammals because, like reptiles and birds, they lay eggs rather than having live birth. In a number of other respects, monotremes are rather derived, having highly modified snouts or beaks, and modern adult monotremes have no teeth. Like other mammals, however, monotremes have a single bone in their lower jaw, three inner ear bones, high metabolic rates, hair, and they produce milk to nourish the young.

Pictured are the skulls of a platypus (on the left) and an echidna, viewed from the underside. Notice that neither of these adult skulls has teeth, though the platypus has hardened dermal tissue that functions in the place of teeth.