The anatomy of the monotreme reproductive system reflects its reptilian origins, but shows features typical of mammals 7, as well as unique specialized characteristics. Platypus milk changes in protein composition during lactation (as it does in marsupials, but not in most eutherians 5). 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. Mammal lineages are in red diapsid reptiles, shown as archosaurs (birds, crocodilians and dinosaurs), are in blue and lepidosaurs (snakes, lizards and relatives) are in green. Geological eras and periods with relative times (Myr ago) are indicated on the left. Therian mammals with common characters split into marsupials and eutherians around 148 Myr ago 2 (dark-red text). Monotremes diverged from the therian mammal lineage ~166 Myr ago 2 and developed a unique suite of characters (dark-red text). These small early mammals developed hair, homeothermy and lactation (red lines). 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.Įmergence of traits along the mammalian lineageĪmniotes split into the sauropsids (leading to birds and reptiles) and synapsids (leading to mammal-like reptiles). 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. Jones, 28 Brett Nixon, 28 Jean-Louis Dacheux, 29 Hitoshi Niwa, 30 Yoko Sekita, 30 Xiaoqiu Huang, 31 Alexander Stark, 32 Pouya Kheradpour, 32 Manolis Kellis, 32 Paul Flicek, 3 Yuan Chen, 3 Caleb Webber, 4 Ross Hardison, 7 Joanne Nelson, 1 Kym Hallsworth-Pepin, 1 Kim Delehaunty, 1 Chris Markovic, 1 Pat Minx, 1 Yucheng Feng, 1 Colin Kremitzki, 1 Makedonka Mitreva, 1 Jarret Glasscock, 1 Todd Wylie, 1 Patricia Wohldmann, 1 Prathapan Thiru, 1 Michael N. Ray, 24 Michael Kube, 25 Richard Reinhardt, 25 Thomas H. Hannon, 21 Enkhjargal Tsend-Ayush, 5 Daniel McMillan, 2 Rosalind Attenborough, 2 Willem Rens, 9 Malcolm Ferguson-Smith, 9 Christophe M. Murchison, 21 Ravi Sachidanandam, 21 Carly Smith, 21 Gregory J. Vargas Jentzsch, 19 Angelika Merkel, 19 Juergen Schmitz, 20 Anja Zemann, 20 Gennady Churakov, 20 Jan Ole Kriegs, 20 Juergen Brosius, 20 Elizabeth P. Gemmell, 19 Emmanuel Buschiazzo, 19 Iris M. Smit, 15 Andrew Pask, 16 Peter Temple-Smith, 16, 17 Mark A. Wakefield, 12 Tsviya Olender, 13 Doron Lancet, 13 Gavin A.
Deakin, 2 Amber Alsop, 2 Katherine Thompson, 2 Patrick Kirby, 2 Anthony T. Eichler, 11 Lin Chen, 11 Ze Cheng, 11 Janine E. Puente, 10 Carlos López-Otín, 10 Gonzalo R. Waters, 2 Frédéric Veyrunes, 2, 9 Lucinda Fulton, 1 Bob Fulton, 1 Tina Graves, 1 John Wallis, 1 Xose S. Chinwalla, 1 Shiaw-Pyng Yang, 1 Andreas Heger, 4 Devin P. Ponting, 4 Frank Grützner, 5 Katherine Belov, 6 Webb Miller, 7 Laura Clarke, 8 Asif T. Marshall Graves, 2 Ewan Birney, 3 Chris P.