The name Monotremata means “one hole,” a reference to the single anal opening (cloaca) which these mammals share with reptiles, amphibians, and fishes.
The order Monotremata includes just three species, the platypus and the short and long-nosed echidnas. Home base for monotremes is Australia. They are also found on some nearby islands except for the long-nosed echidna, which is found only on the island of New Guinea.
Monotremes are the world’s only egg-laying mammals. Or are they? Some scientists have suggested that they’re living fossils that only resemble mammals. They thought monotremes might actually be primitive reptiles known as therapsid reptiles.
Other scientists suggested that monotremes are living fossils AND mammals. They thought they could be related to multiberculates, a group of plant-eating mammals that evolved towards the end of the Age of Dinosaurs and became extinct early in the Age of Mammals.
Monotremes have “marsupium” bones, the bones that gave marsupials their name. But scientists decided monotremes were just too unique to find a home even among marsupials. So they created a new order, Monotremata. Ironically, this name comes from a reptilian feature—the single anal opening.
Thus, we have three major groups of living mammals—eutherians (a group that includes everything from cats, horses, and humans to bats and whales), marsupials (most of which live in Australia), and the egg-laying, toothless monotremes.
Toothless? That’s right, monotremes do not have any teeth. Well, platypuses do when they’re young, but they don’t even penetrate the gums. Fossils reveal that at least some prehistoric monotremes did have teeth.
