Marsupials – an introduction

Marsupials

Australia is famous for its marsupials, but we still have much to find out about this fascinating group of mammals.

The ARC Centre for Kangaroo Genomics uses a mid sized species, the tammar wallaby as a representative marsupial for its studies, since it is breeds well in captivity and its physiology, ecology and behaviour are well understood.

Marsupials are named for their pouch (marsupios: Ancient Greek for purse), athough not all marsupials have a pouch. They are distinguished from other mammals – the monotremes and the eutherian mammals by their reproduction. They are a diverse group and have occupied many of the ecological niches occupied in other continents by eutherian mammals. One of the largest groups of marsupials is the kangaroo family, the “macropodids” (named from the Latin macro=big and pod=foot). The kangaroo family includes about 50 species, ranging from the diminutive rat-kangaroos (not to be confused with the kangaroo rats, which are rodents) to the majestic red kangaroo.

Phylogeny of the main mammal groups

Phylogeny of the main mammal groups

The three groups of modern-day mammals arose from a common therian ancestor. It is likely that the montremes became distinct about 170 million years ago. Marsupials evolved as a distinct line sometime between 100 and 150 million years ago. The poor fossil record makes it difficult to set the date precisely. Eutherian mammals (often called placental mammals, though marsupials have placentas too) evolved rapidly between about 100 mya and 60 mya to give rise to the main groups we see today, such as the rodents, carnivores, hooved mammals and primates.

Marsupial Evolutionary tree

The evolution of the different groups of marsupials is given on this diagram.

phylogeny_of_marsupials

This figure shows the evolution of the marsupials. The current South American groups are shown in green type, the Australian groups are in black. The branching tree on the left indicates the approximate time when the various lineages diverged in geological time (top) or millions of years (bottom). The meteorite impact that wiped out the dinosaurs happened about 65 million years ago; most of the radiation of marsupials occurred after this time. (timing based on data from Nilsson et al (2004) Gene 340:189-196)