For about 4 months, when most organ systems differentiate, the young depend on milk sucked directly from the abdominal skin, as females lack nipples. The egg is laid in an earthen nesting burrow after about 21 days and hatches 11 days later 5, 6. In 1884, William Caldwell’s concise telegram to the British Association announced “Monotremes oviparous, ovum meroblastic”, not holoblastic as in the other two mammalian groups 3, 4. The most extraordinary and controversial aspect of platypus biology was initially whether or not they lay eggs like birds and reptiles. Estimates of the monotreme–theria divergence time range between 160 and 210 Myr ago here we will use 166 Myr ago, recently estimated from fossil and molecular data 2. The divergence of monotremes and therians falls into the large gap in the amniote phylogeny between the eutherian radiation about 90 million years (Myr) ago and the divergence of mammals from the sauropsid lineage around 315 Myr ago ( Fig. Traditionally, the Monotremata are considered to belong to the mammalian subclass Prototheria, which diverged from the therapsid line that led to the Theria and subsequently split into the marsupials (Marsupialia) and eutherians (Placentalia). The platypus was placed with the echidnas into a new taxon called the Monotremata (meaning ‘single hole’ because of their common external opening for urogenital and digestive systems). Some initially considered it to be a true mammal despite its duck-bill and webbed feet. The platypus ( Ornithorhynchus anatinus) has always elicited excitement and controversy in the zoological world 1.
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