Australia: The Land Where Time Began

A biography of the Australian continent 

Defining characters of marsupials

The character that was used by the first person to determine characterises that differentiated them from the placentals, de Blainville in 1916, was the anatomy of the female reproductive tract of marsupials, a character that clearly differentiated them from the placentals. The vagina, uterus and oviduct are all paired in female marsupials, as opposed to the situation in placentals, where there is a single vagina and uterus, with paired oviducts. His name for marsupials was the Didelphia (2 uteri), the placentals being the Monodelphia. When he studied the monotremes he named them Ornithodelphia (bird-uterus), as their reproductive tract is more like that found in birds and reptiles, with a single opening from the gut, bladder and gonads. The division of the mammals he devised has remained unchanged, but the Ornithodelphia became the Monotremata, the Didelphia are the Marsupiala and the Monodelphia are the Placentalia. A problem with this arrangement was that in the case of the pouch, not all female marsupials have a well-developed pouch. The female echidna, a monotreme, only develops a pouch during lactation. All marsupials have a placenta at the stage of development in the uterus. Some marsupial species have a complex placenta that can be connected intimately with the uterus.

Huxley (1880) reduced the confusion caused by the non-exclusivity of these terms by introducing the terms prototheria for the monotremes, metatheria for the marsupials and eutheria for the placental mammals, based on the evolutionary view of the monotremes being the earliest mammals, followed by the marsupials, and the eutheria, the ultimate mammals, including humans.

1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
2. M.Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
3. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.