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Monotremes are mammals that lay eggs (they are part of Prototheria) instead of giving birth to live young like marsupials and placental mammals. The only surviving examples of monotremes are all indigenous to Australia and New Guinea, although there is evidence that they were once more widespread. The existing monotreme species are the platypus and six species of echidnas (or spiny anteaters). There is currently some debate regarding monotreme taxonomy.

The word monotreme comes from the Greek μονός, monos ("single") and τρῆμα, trema ("hole"), referring to the cloaca.

The fossil record of monotremes is relatively sparse. The first Mesozoic monotreme to be discovered was Steropodon galmani from Lightning Ridge in eastern Australia dating back to the Early Cretaceous. Although biochemical and anatomical evidence suggests that the monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose, only a handful of monotreme fossils are known from before the Miocene epoch. The known Mesozoic monotremes are Steropodon, Kollikodon, and Teinolophos, all from Australian deposits in the Cretaceous, suggesting monotremes had already diversified by that time. A platypus tooth has been found in the Palaeocene of Argentina, so Michael Benton suggests in Vertebrate Paleontology monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and some subsequently migrated across Antarctica to reach South America, both of which were still united with Australia at that time. However, a number of genetic studies suggest a much earlier origin in the Late Triassic.

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