The duck-billed platypus, short-beaked echidna, and long-beaked echidnas (3 species) are the living descendants of one of the oldest lineages of mammals. They differ from all of the other mammals as the only ones that lay eggs and hatch their young outside of their bodies. Besides laying eggs, they share some physical characteristics with reptiles, as well as many characteristics in common with the other mammals.

Echidnas are also known as spiny anteaters. They have a short coat of dark fur, but also an array of sharp spines sticking out beyond the fur. They have long noses and long, sticky tongues used to snare insects or worms. They also have well-developed front claws to dig out their prey. The noses, tongues and claws are reminiscent of anteaters, though the anteaters are only distant cousins of the echidnas.