Definition of monotreme in US English:



  • 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.’
    • ‘Many of the comprehensive, algorithm-based analyses place multituberculates within the crown group of living mammals, in some cases tentatively united with monotremes.’
    • ‘Molecular evidence now suggests that the marsupials and the monotremes branched off much earlier than the placentals did and that they are therefore the closer relatives.’
    • ‘The Australian fossil record of monotremes also includes some quite good Miocene and Pleistocene fossils of giant echidnas.’
    • ‘Placental and marsupial mammals are more closely related to one another than to the third living group of mammals, the monotremes.’


Mid 19th century: from mono- ‘single’ + Greek trēma ‘hole’.