A primitive mammal that lays large yolky eggs and has a common opening for the urogenital and digestive systems. Monotremes are now restricted to Australia and New Guinea, and comprise the platypus and the echidnas.
- Order Monotremata and subclass Prototheria: two families.
- There are only three living monotremes, the duck-billed platypus and two species of echidna, or ‘spiny anteaters’, such as the one shown at right.
- Placental and marsupial mammals are more closely related to one another than to the third living group of mammals, the monotremes.
- The Australian fossil record of monotremes also includes some quite good Miocene and Pleistocene fossils of giant echidnas.
Mid 19th century: from mono- 'single' + Greek trēma 'hole'.
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