Don: Yael, did you hear that the long-beaked echidna, which scientists thought was      extinct, has been found again in New Guinea?

Yael: Long-beaked echidna? Is that some kind of bird?

D: No Yael, it's one of only five living species of Monotremes. Monotremes are an      order of mammals that lay eggs rather than giving birth to live young, like all other      mammals do. Modern monotremes are toothless, have leathery or bird-like beaks,      and like birds and reptiles, they have only a single opening through which they lay      eggs and eliminate waste.

Y: Wait, so why are they classified as mammals?

D: Like other mammals, monotremes are warm-blooded. They have hair on their bodies      and produce milk to feed their young.

Y: They lay eggs and also nurse their young?

D: Yep. In echidnas, the female lays eggs into a pouch of skin on her stomach, where      she carries them until they hatch. A female platypus lays her eggs in a burrow, and      curls around them until they hatch. Although they have mammary glands, monotremes      do not have nipples like other mammals. Instead, milk is released through pores in      the skin and the young suckle from these "milk patches" until they are mature enough      to fend for themselves.

Y: Fascinating! Where do monotremes fit in the evolutionary tree of life?

D: The current thought is that monotremes come from a lineage of mammals that      branched off the mammalian family tree over two-hundred million years ago, and      have been evolving independently from the rest of the earth's mammals ever since.      Scientists are currently working on sequencing the platypus genome, which could      shed more light on the evolutionary history of these mystifying animals.  

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