Marsupials (Marsupialia) and monotremes (Monotremata) are two of the three extant mammalian lineages (the other being that of the placentals). Currently, 99.9% of the mammalian fauna consists of placentals and marsupials, with the monotremes only contributing 0.1%. However, in the past the relative abundance and diversity of these lineages was very different.
Marsupials are a subgroup belonging to the Metatheria, an assemblage that includes all the extinct mammals that are more closely related to extant marsupials than to placental mammals, who are themselves, a subgroup of the Eutheria. The diversification of these two mammalian groups is believed to have not occurred any later than 125 million years ago, during the Cretaceous.
The monotremes are not a primitive form of mammal but are thought to have branched from an early mammalian animal that was an ancestor to the placentals and monotremes.
Classic views on biogeographic mammalian evolution propose that both the eutherians and metatherians evolved within the northern continents and then proceeded from Asia and North America, to North and South America. A large diversification is thought to have occurred in North America during the Late Cretaceous (100-65 million years ago) and again when the group proceeded south into South America during the Paleocene, an event that the marsupial's current diversity is based on. Approximately 70 species of marsupials remain in South America, the majority of which are opposums, however, many marsupial ancestors also travelled into Australia, through the previously adjoined Antarctica, where the greatest diversity and number of species is seen today (around 200 species).
Images used, with permission, from Wikipedia
Author: Catrin Roberts
Last updated: 21.11.06
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