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