The 21 Basic Mammal Groups
Monotremes (Order Monotremata)
Hands-down the most bizarre mammals on the face of the earth, monotremes —consisting of one species of platypus and four species of echidna — lay soft-shelled eggs, rather than giving birth to live young. And that's not the end of the monotreme weirdness: these mammals are also equipped with cloacas (a single orifice for urinating, defecating, and reproducing), they're completely toothless as adults, and they have a talent for electroreception (sensing faint electrical currents from a distance). According to current thinking, monotremes evolved from a Mesozoic ancestor that predated the split between placental and marsupial mammals, hence their extreme weirdness.
The 21 Basic Mammal Group caches were placed for HNY21.