echidnoid

Echidnoid is a term sometimes used to refer to the two extant families of monotremes: the Tachyglossidae (echidnas) and the Ornithorhynchidae (platypus). Monotremes are a unique group of mammals distinguished by their reproductive method of laying eggs rather than giving birth to live young. This ancient lineage diverged from other mammals early in their evolutionary history.

The family Tachyglossidae includes four species of echidnas, commonly known as spiny anteaters. These are found in Australia and New Guinea. Echidnas are characterized by their spiny covering, long snout, and powerful claws used for digging. They are insectivorous, feeding primarily on ants and termites.

The family Ornithorhynchidae contains only one living species, the platypus, found in eastern Australia. The platypus is semi-aquatic and possesses a distinctive duck-like bill, webbed feet, and a beaver-like tail. It is also an egg-laying mammal and feeds on invertebrates found in freshwater environments. Both echidnas and the platypus possess mammary glands but lack nipples; milk is secreted through pores onto the mother's skin, and the young lap it up. This shared set of primitive mammalian traits is why the term echidnoid is sometimes applied to them collectively.