Living and fossil marsupials

There are about 265 living species of marsupial mammals, the majority in Australasia, about 60 in South America, and a handful in Central and North America (Macdonald 2001). They are distinguishable from the placental mammals by many characters, but most profoundly by their mode of reproduction. Compared to the placentals, there is only a relatively brief intrauterine period, during which the embryo exchanges nutrients and gases with the mother via a simple, non-invasive yolk sac placenta. There is no development of the complex, highly invasive chorio-allantoic placenta found in the placentals with the partial exception of the bandicoots in which there is a small, short-lived, but true chorio-allantoic placenta. The marsupial neonate is born at a very immature stage, and most of the total maternal provision comes via lactation. In the majority of cases the young are carried in a pouch, although there are exceptions to this. Whether pouched or not, the young attach themselves continuously to the teat for an extended period of time. There has been much discussion about whether the marsupial mode of reproduction is ancestral to that of the placental mammals, or whether it represents an independent, parallel acquisition of viviparity. Lillegraven (1979), Lillegraven et al. (1987), and Szalay (1994), for example, regarded the marsupial mode as primitive and inefficient compared to the placental mode, and that it was failure of the marsupials to evolve a mechanism to prevent immunological rejection of the embryo by the mother that prevented any extension of the gestation period. Placentals, they argued, solved the problem by evolving the trophoblast layer of embryonic cells that performs the function of preventing the maternal antibodies from damaging the embryo. Conversely, several authors such as Parker (1977) have argued that the marsupial mode is an alternative, but equally well-adapted strategy of reproduction to that of placentals. It is one of low investment but low risk, and is therefore suitable for a more unpredictable environment. Tyndale-Biscoe and Renfree (1987) suggested that primitive marsupials and placentals had quite similar reproduction, with relatively immature neonates and a relatively long lactation period. Subsequent specialisation in the two groups went in different directions.