Molecular phylogenetics and the origins of placental mammals

The precise hierarchy of ancient divergence events that led to the present assemblage of modern placental mammals has been an area of controversy amon g morphologists, palaeontologists and molecular evolutionists. Here we addr ess the potential weaknesses of limited character and taxon sampling in a c omprehensive molecular phylogenetic analysis of 64 species sampled across a ll extant orders of placental mammals. We examined sequence variation in 18 homologous gene segments (including nearly 10,000 base pairs) that were se lected for maximal phylogenetic informativeness in resolving the hierarchy of early mammalian divergence. Phylogenetic analyses identify four primary superordinal clades: (I) Afrotheria (elephants, manatees, hyraxes, tenrecs, aardvark and elephant shrews); (II) Xenarthra (sloths, anteaters and armad illos); (III) Glires (rodents and lagomorphs), as a sister taxon to primate s, flying lemurs and tree shrews; and (IV) the remaining orders of placenta l mammals (cetaceans, artiodactyls, perissodactyls, carnivores, pangolins, bats and core insectivores). Our results provide new insight into the patte rn of the early placental mammal radiation.