Relationships among orders and families of marsupials based on 12S ribosomal DNA sequences and the timing of the marsupial radiation

Part of the mitochondrial 12S ribosomal RNA gene was amplified and sequenced for 26 marsupials. Multiple alignments for these sequences as well as seven additional sequences taken from GenBank were obtained using CLUSTAL. PAUP was used for phylogenetic analysis and to obtain random tree-length distributions. Analyses were performed with and without phylogenetic constraints. Our results clearly show that 12S rDNA contains phylogenetic signal at and above the ordinal level and is thus appropriate for addressing phylogenetic questions deep in the mammalian tree. Standard parsimony analyses provide some support for a clade containing diprotodontians, dasyurids,Dromiciops, andNotoryctes; transversion parsimony analysis suggests the possible inclusion of peramelids as well. Within the Diprotodontia, vombatids and phascolarctids cluster together on transversion parsimony and phalangerids may be associated with this clade. The enigmatic tarsipedids are apparently part of a clade that also contains pseudocheirids, petaurids, and acrobatids. The 12S sequences suggest that the origination of extant marsupial orders peaked 15 million years later than the equivalent taxonomic diversification of extant placental orders and may be entirely post-Cretaceous. Families of diprotodontian marsupials originated during the Eocene and early Oligocene, which is consistent with previous single-copy DNA hybridization results.