Introduction

Molecular evidence collected over the last decade suggests that the ancestral marsupials parted company from other early mammals during the Jurassic at least 150 million years ago (Woodburne et al. 2003; Nilsson et al. 2010). Well-preserved and clearly differentiated fossils of ancestral marsupials and eutherian mammals aged around 125 Ma support an early separation date (Ji et al. 2002; Luo et al. 2003), and the recent discovery of an early eutherian dated at 160 Ma (Luo et al. 2011) confirms the deep antiquity of the marsupial–eutherian split. Originating most probably in the great northern landmass, Laurasia, perhaps in modern-day China where the oldest fossils have been found, the ancestral marsupials subsequently dispersed to most of the accessible parts of the Earth. Their hegemony diminished in most regions with the ascendancy of other mammals, but marsupials still retain strongholds in the Americas, especially South America, and in the Australasian region. There is some agreement that early marsupials arrived in the Australian part of the southern supercontinent, Gondwana, around 65 Ma, and then underwent a spectacular evolutionary radiation that led to seven orders and at least 31 families (Long et al. 2002; Archer and Kirsch 2006). This explosion in marsupial diversity was so great that the celebrated mammalogist Ellis Troughton was moved to describe the Australian radiation of this group as ‘the greatest phylogenetic deployment of a single mammalian Order that the World can ever know’ (1959: 69).