Monotreme
Monotremes (from the Greek monos 'single' + trema 'hole', referring to the
cloaca) are
mammals that lay eggs instead of giving birth to live young like
marsupials (
Metatheria) and
placental mammals (
Eutheria).They are conventionally treated as comprising a single order Monotremata, though a recent classification proposes to divide them into the orders
Platypoda (the
platypus) and
Tachyglossa (the
echidnas). The entire grouping is also traditionally placed into a subclass
Prototheria, which was extended to include several fossil orders but these are no longer seen as constituting a natural group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a
clade termed
Australosphenida.
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Monotremata
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